Rozprawy doktorskie na temat „Soft matter rheology”

Many different physical systems, such as granular materials, colloids, foams and emulsions exhibit a jamming transition where the system changes from a liquid-like flowing state to a solid jammed state as the packing fraction increases. These systems are often modeled using soft-core particles with repulsive contact forces. In this thesis we explore several different dynamical models for these kinds of systems, and see how they affect the behavior around the jamming transition. We investigate the effect of different types of dissipative forces on the rheology, and study how different methods of preparing a particle configuration affect their probability to jam when quenched. We study the rheology of sheared systems close to the jamming transition. It has been proposed that the athermal jamming transition is controlled by a critical point, point J, with certain scaling properties. We investigate this using multivariable scaling analysis based on renormalization group theory to explore the scaling properties of the transition and determine the position of point J and some of the critical exponents.

La fracture des matériaux, omniprésente aussi bien en science des matériaux qu’en géologie, implique souvent des événements soudains et imprévisibles, sans précurseurs détectables macroscopiquement. Une compréhension approfondie des mécanismes microscopiques conduisant in fine à la rupture est requise, mais les expériences restent rares. La détection de la dynamique microscopique dans les échantillons cisaillés est expérimentalement très difficile, car elle nécessite de combiner sensibilité mécanique, qualité optique et exigences strictes sur l’encombrement. Dans ce travail, nous présentons l'une des premières tentatives réussies de mesure des précurseurs microscopiques de fracture dans des matériaux mous modèles, grâce à des mesures de la plasticité microscopique à l'aide d'un nouvel instrument, couplant une cellule de cisaillement à contrainte contrôlée à un appareil de diffusion de lumière statique et dynamique (DLS) à petits angles. Dans un premier temps, nous montrons théoriquement, numériquement et expérimentalement comment la DLS, une technique très puissante difficile à utiliser pour un échantillon sous cisaillement, peut être utilisée comme outil de mesure de la dynamique microscopique dans les systèmes mous sous cisaillement. Pour un solide parfait et un fluide visqueux simple, le champ de déplacement résultant d'une déformation de cisaillement est purement affine. Nous montrons comment les déplacements affines et non affines, qui sont présents dans de nombreuses situations d’intérêt (matériaux élastiquement hétérogènes ou en raison de réarrangements plastiques) peuvent être évalués séparément par DLS et discutons de l'effet des non-idéalités dans des expériences typiques.Ce travail est centré sur un gel colloïdal fractal modèle, dont nous caractérisons la rhéologie linéaire en loi de puissance. Nous montrons que celle-ci est décrite par un modèle phénoménologique Fractional Maxwell (FMM), et discutons la relation possible entre FMM et la structure microscopique du gel.Sous une contrainte de cisaillement constante (expérience de fluage), le gel colloïdal présente une déformation rapide élastique suivie d'un fluage lent en loi de puissance, puis, après plusieurs heures par une accélération du taux de cisaillement, entraînant la rupture retardée du gel. Nos expériences montrent que le premier régime en loi de puissance, bien décrit par la viscoélasticité linéaire, correspond à l'échelle microscopique à une dynamique partiellement nonaffine, mais entièrement réversible. Lorsque la viscoélasticité dévie de la linéarité, une accélération nette, localisée dans le temps de la dynamique non-affine, est observée. Ces réarrangements rapides précèdent la fracture macroscopique du gel de plusieurs heures: ce sont des précurseurs dynamiques de la fracture qui permettent de prédire l’évolution du gel bien avant toute mesure rhéologique.Pour obtenir une image plus complète de la fracture, nous étudions l'apparition de l'irréversibilité lors d’une perturbation cyclique répétée plusieurs fois (expérience de fatigue). En suivant l'évolution stroboscopique du système en fonction de la déformation cumulée, on constate que, au-delà du régime linéaire, le taux de relaxation augmente brusquement, signature de plasticité. Si la contrainte appliquée est suffisamment grande, le gel à long terme montre une rupture retardée, en analogie avec celle observée en fluage. Les différences et similitudes entre les deux mécanismes de fracture sont discutées.Enfin, la généralité des résultats obtenus sur les gels colloïdaux est vérifiée en étudiant comme second système modèle un verre colloïdal, dont la mise en écoulement sous contrainte oscillante est un processus progressif, pour lequel deux modes de relaxation contribuent à la dynamique observée. Les analogies qualitatives trouvées avec des systèmes similaires (par ex. des émulsions concentrées) suggèrent qu'une image unifiée pourrait être obtenue, motivant des recherches futures
Material failure is ubiquitous, with implications from geology to everyday life and material science. It often involves sudden, unpredictable events, with little or no macroscopically detectable precursors. A deeper understanding of the microscopic mechanisms eventually leading to failure is clearly required, but experiments remain scarce. The detection of microscopic dynamics in samples under shear is experimentally very challenging, because it requires to combine the highest mechanical sensitivity to strict requirements on the geometry of the whole setup and on the quality of the optical interfaces. In this work we present one of the first successful attempts to measure microscopic failure precursors in model soft solids. Here, microscopic plasticity under shear is observed using a novel setup, coupling a custom-made stress controlled shear cell to small angle static and dynamic light scattering (DLS).DLS is a very powerful technique, but its application to materials under shear is not trivial. In a first step we show a theoretical, numerical and experimental investigation of how DLS may be used as a tool to measure the microscopic dynamics in soft systems under shear. In ideal solids and simple viscous fluids, the displacement field resulting from an applied shear deformation is purely affine. Additional non-affine displacements arise in many situations of great interest, for example in elastically heterogeneous materials or due to plastic rearrangements. We show how affine and non-affine displacements can be separately resolved by DLS, and discuss the effect of several non-idealities in typical experiments.As a model system, this work mainly focuses on a fractal colloidal gel. We thoroughly characterize the linear power-law rheology of the gel, we show that it is very accurately described by the phenomenological Fractional Maxwell (FM) model, and we discuss the possible relationship between the FM model and the microscopic structure of the gel.Under a constant shear stress (creep experiment), the colloidal gel exhibits a fast, elastic deformation followed by a slow sublinear power-law creep, which is eventually interrupted after several hours by an upturn in the shear rate, leading to the delayed failure of the material. Our experiments show that the first power-law regime, nicely described by linear viscoelasticity, corresponds at the microscopic scale to partially nonaffine, yet fully reversible dynamics. Upon deviation from the linear viscoelasticity, a sharp acceleration, localized in time of the nonaffine dynamics is observed. These faster rearrangements precede the macroscopic failure of the gel by thousands of seconds: they thus are dynamic precursors of failure that allow one to predict the fate of the gel well before any rheological measurement.To obtain a more comprehensive picture of material failure, we next address the onset of irreversibility under a cyclic perturbation repeated many times (fatigue experiment). By following the stroboscopic evolution of the system as a function of the cumulated deformation, we observe that as soon as the shear amplitude is increased beyond the linear regime the relaxation rate increases abruptly, indicating that irreversible plasticity is at play. If a large enough stress amplitude is applied, the system on the long run displays delayed fatigue failure, with reminiscences of the one observed in creep. Differences and similarities between the two failure mechanisms are discussed.Finally, the generality of the results obtained on colloidal gels is checked by investigating as second model system a soft colloidal glass. In this case, our experiments indicate that oscillatory yielding is a gradual process, where two relaxation modes contribute to the observed dynamics. Qualitative analogies found with similar systems (e.g. concentrated emulsions) suggest that a general picture might be obtained with our study, which motivates ongoing and future investigations

Comportement rhéologique dépendant du temps dans les verres colloïdaux de particules déformablesNous étudions des verres colloïdaux constitués de suspensions denses de particules déformables dans un fluide suspendant. Des exemples pratiques comprennent beaucoup de matériaux pâteux tels que les encres solides, les produits de soins personnels et certaines préparations alimentaires. Ces matériaux se comportent comme des fluides à seuil d’écoulement qui répondent de façon élastique à des petites déformations mais s’écoulent au-delà d’une contrainte seuil. Beaucoup de travaux expérimentaux et théoriques ont étudié leurs propriétés rhéologiques à l’état stationnaire. Les connaissances sur la réponse transitoire lors de la mise en écoulement ou au contraire lors de l’arrêt sont beaucoup moins avancées.Nous abordons ce problème dans le cas de suspensions très concentrées de microgels polyélectrolytes dont la microstructure est bien caractérisée. Sous écoulement ces verres colloïdaux accumulent des contraintes internes d’origine élastique qui relaxent très lentement de sorte qu’ils conservent au repos la mémoire de la direction dans laquelle ils ont été cisaillés. Nous proposons plusieurs protocoles de préparation qui minimisent l’effet de ces contraintes résiduelles et permettent de réaliser des expériences dénuées de biais directionnels. Les contraintes transitoires associées à la mise en écoulement sont analysées à l’aide d’expériences systématiques, de simulations, et d’un modèle phénoménologique, à la fois pour des microgels répulsifs et associatifs. Un dernier chapitre est consacré aux propriétés viscoélastiques à haute fréquence des suspensions. Nos résultats démontrent le rôle clé de la compétition entre l’élasticité due aux interactions de contact et les forces de viscosité dans le fluide suspendant. Ceci nous permet de proposer un cadre conceptuel qui rationalise les comportements dépendants du temps des verres colloïdaux de particules déformables
Time-dependent rheology of soft particle glassesSoft particle glasses are jammed suspensions of soft and deformable particles dispersed in a viscous fluid. Common examples include pasty materials such as solid inks, personal care products, and foods. They behave as yield stress fluids, which respond elastically to small perturbations, but deform irreversibly and flow when they are subjected to large enough stresses. Many experimental and theoretical studies have focused on the steady shear rheology of these materials. However much less is known about their behavior in transient situations like flow cessation or startup flow.Here we investigate the time-dependent rheology of jammed suspensions made of well-characterized polyelectrolyte microgels. Upon flow cessation, these materials store residual stresses that relax very slowly and are responsible for long-lived directional memory and aging. We design several experimental protocols that minimize residual stresses and memory making it possible to perform startup experiments without directional bias. The behavior of the stress growth function is then analyzed and discussed using systematic experiments, simulations, and a phenomenological model for microgels with both repulsive interactions and short-range associations. A final chapter is devoted to the high-frequency linear viscoelasticity of the suspensions. Overall, our results demonstrate the key role played by the competition between elastic contact forces and viscous forces, thus providing a unifying framework to rationalize the time-dependent rheology of soft particle glasses

The goal of this thesis is to study the role of single particle elasticity in the overall behavior of particulate systems. For this purpose, we use microgel particles, which are crosslinked polymer networks immersed in a solvent. In these systems, the amount of cross-linker determines their elasticity and ultimately the stiffness of the particle. For a system of hard spheres, the phase behavior is solely determined by the volume fraction occupied by the particles. Based on the volume fraction, liquid, crystal and glassy phases are observed. Interestingly, microgel particles display a richer and fascinating set of different behaviors depending on the particle stiffness. Previous results obtained in our group show that for highly cross-linked microgels, the glass phase disappears and there are only liquid and crystalline phases. By contrast, preliminary measurements indicate that for ultrasoft microgel particles the system does not show any signature of crystalline or glassy phases. The system seems to remain liquid irrespective of volume fractions. In this Thesis, we will address this striking result using light scattering as well as rheology, in order to access both static and dynamic properties in a wide range of length and time scales. In addition, we will also perform additional studies using very stiff microgels and use their swelling capabilities to change the volume fraction. We will use hydrostatic pressure to change the miscibility of the polymer network and thus change the microgel size; the use of this external variable allows fast equilibration times and homogeneous changes throughout the sample. By using neutron scattering techniques, we study the structural and dynamical properties of the system in its different phases involved.

Dans ce manuscrit, nous mesurons la réponse mécanique à l’échelle nanométrique de divers systèmes issus de la matière molle en utilisant un microscope à force atomique basé sur un diapason à quartz. Utilisé comme un nano-rhéomètre, cet instrument permet une mesure quantitative des propriétés viscoélastiques des matériaux et des processus frictionnels et dissipatifs aux nanoéchelles. Nous montrons d’abord que les liquides ioniques confinés aux nanoéchelles peuvent subir un changement dramatique de leurs propriétés mécaniques, suggérant une solidification capillaire. Cette transition est favorisée par la nature métallique des interfaces confinantes, montrant la présence d’effets électrostatiques subtils dans ces électrolytes denses. Nous étudions ensuite les mécanismes de plasticité à l’échelle atomique en mesurant la réponse viscoélastique de jonctions d’or de quelques atomes de diamètre. Nous mettons en évidence une transition sous cisaillement entre un régime élastique, puis plastique, jusqu’à la liquéfaction complète de la jonction. Nous caractérisons ainsi de manière fine les mécanismes de plasticité dans ces systèmes moléculaires. Finalement, nous montrons les effets profonds que les interactions à l’échelle nanométrique peuvent avoir sur le comportement macroscopique de la matière molle. Nous mesurons le profil frictionnel entre paires de particules de suspensions de PVC et de maïzena. Nos mesures mettent en lumière le rôle dominant des interactions locales entre particules dans la rhéologie non-newtonienne des suspensions
In this manuscript, we use a tuning fork based atomic force microscope to measure the mechanical response of various soft matter systems at the nanoscale. This instrument is used as a nano-rheometer, allowing quantitative measurements of viscoelastic material properties, and unprecedented characterization of friction and dissipation at the nanoscale. First, we show that ionic liquids can undergo a dramatic change in their mechanical properties when confined at the nanoscale, pointing to a capillary freezing transition. This transition is favored by the metallic nature of the confining substrates, suggesting the occurrence of subtle electrostatic effects in those dense electrolytes. Second, we probe plasticity at the individual atomic level, by measuring the viscoelastic rheological response of gold junctions of few atoms diameter. For increasing shear, we uncover a transition from a purely elastic regime to a plastic flow regime, up to the complete shear-induced melting of the junction. Our measurements give unprecedented insights on the plastic mechanisms at play in those molecular systems. Finally, we show that nanoscale interactions can have profound effects on the macroscopic behavior of soft materials. Focusing on the nonnewtonian flow behavior of concentrated suspensions of particles, we measure the nanoscale frictional force profile between pairs of particles of PVC and cornstarch suspensions. Our measurements highlight the dominant role of local interparticle interactions on the macroscale rheology of suspensions

Cette thèse est une étude expérimentale des écoulements forcés de fluides sous leur propre poids à travers un orifice. La première partie traite des écoulements de fluides newtoniens. Une étude récente a montré que pour un écoulement de fluide peu visqueux à travers un orifice de taille comparable à la longueur capillaire, l'effet du mouillage était important. Il impacte le débit et la forme du jet. Nous avons développé une méthode originale utilisant d'une résine photosensible comme revêtement de surface afin de varier de façon continue les conditions de mouillage de la surface extérieure de la plaque de fond du réservoir. Ainsi, nous avons montré que le paramètre contrôlant du débit est très probablement l'angle de contact statique formé par le fluide sur le matériau constituant cette surface. Nous avons établi les conditions d'existence de la déformation du jet et montré que cette perturbation est induite par un couplage entre le mouillage et la génération de turbulence à l'intérieur de l'orifice, liée au phénomène de vena contracta. La seconde partie de cette thèse porte sur les écoulements de fluides viscoélastiques. Les expériences menées révèlent que ces solutions présentent un débit oscillant dévoilant ainsi une dynamique très riche. Contrairement aux fluides newtoniens, l'écoulement est piloté par le comportement du fluide complexe à l'intérieur de la cuve, et non au niveau de l'orifice. Deux méthodes de visualisation : par biréfringence sous écoulement et par PIV, ont été mises au point pour explorer la dynamique des écoulements de solutions de micelles géantes à l'intérieur de la cuve. Ces expériences ont permis de montrer que le cisaillement du fluide dans la cuve était très localisé et que la zone de cisaillement se déplace tout au long de la vidange. Nous avons également pu mesurer le champ de vitesse au sein de la cuve. Ces mesures ont pu démontrer la complexité de la dynamique du fluide, qui présente notamment des recirculations autour de la zone de cisaillement
This thesis is an experimental work on gravity induced flows through an orifice.The first part of this manuscript deals with flows of Newtonian fluids. A recent study showed that for flows of low-viscosity fluids through an orifice the size of which is close to the capillary length, the effect of wetting is significant. Wetting conditions impact both flow rate and jet shape. During this thesis, an original method has been developed using a photosensitive resin as surface coating to vary the wetting conditions of the outer surface of the tank bottom plate continuously. Thanks to this method, it was shown that most likely the parameter monitoring the output flow rate is the static contact angle that the outgoing fluid forms on the material of this surface. It was established the conditions for the existence of a jet deformation and showed that this disturbance originates from the coupling between wetting and generation of turbulence within the orifice related to the phenomenon of vena contracta.The second part of this manuscript deals with flows of viscoelastic fluids. Drainage experiments carried out with these solutions reveal that they exhibit oscillating flow rate showing a very rich dynamic. Contrary to Newtonian fluids, in this case flows are controlled by the behaviour of the complex fluid inside the tank, and not at the outlet orifice. Two methods of visualization, flow- induced birefringence and PIV, were developed to explore the flow dynamics of wormlike micelle solutions inside the tank. These experiments show shear localisation within the wormlike micelle solutions. It is also observed that this shear zone moves inside the tank throughout the drainage process. The velocity field within the tank was measured as well, it demonstrates the complexity of the dynamics of these solutions, showing recirculation around the shear zone

Dans ce manuscrit, j'ai brièvement décrit mes activités scientifiques depuis la soutenance de ma thèse de doctorat en physique en fin 2003. J'ai prêté une attention particulière aux recherches axées sur la rhéologie et la capture de particules des suspensions magnétiques. Une grande partie de mes recherches a été focalisée sur l'effet de la forme des particules magnétiques et de l'orientation du champ magnétique sur la contrainte seuil apparaissant dans les suspensions magnétiques suite à l'agrégation de ces particules induite par le champ appliqué. En comparant les microfibres magnétiques aux microsphères magnétiques, il a été montré que la contrainte seuil des suspensions de fibres était d'environ trois fois supérieure à celle des suspensions de sphères soumises aux mêmes conditions. Un tel effet de forme a été expliqué par (1) une perméabilité magnétique plus élevée des agrégats composés de fibres ; (2) une friction solide plus forte entre les fibres. En ce qui concerne l'effet d'orientation du champ, nous avons confirmé expérimentalement un comportement à seuil important dans le cas où le champ magnétique est orienté le long de l'écoulement ou de la vorticité. Un tel effet magnétorhéologique " longitudinal " a été expliqué par des fluctuations stochastiques des agrégats autour de leur orientation d'équilibre; ces fluctuations étant causées par des interactions magnétiques entre agrégats. Une autre partie des études a été focalisée sur la séparation magnétique des nanoparticules en vue de leur application dans la séparation de biomolécules ou l'extraction de micropolluants. On a notamment étudié l'effet de la séparation de phase sur la capture de nanoparticules par des collecteurs magnétiques (microsphères aimantées par un champ magnétique externe). Il a été montré que la séparation de phase augmente considérablement l'efficacité de capture et que ce processus est régi par trois nombres sans dimensions : la concentration en nanoparticules, le paramètre d'interaction dipolaire et le nombre de Mason qui représente le rapport des forces hydrodynamiques sur les forces magnétiques exercées sur les nanoparticules.

From a physical perspective biological cells consist of active soft matter that exist in a thermodynamic state far from equilibrium. Not only in muscles but also during cell proliferation, wound healing, embryonic development, and many other physiological tasks, generation of forces on the scale of whole cells is required. To date, cellular contractions have been ascribed to adhesion dependent processes such as myosin driven stress fiber formation and the development of focal adhesion complexes. In this thesis it is shown for the first time that contractions can occur independently of focal adhesions in single suspended cells. To measure mechanical properties of suspended cells the Optical Stretcher – a dualbeam laser trap – was used with phase contrast video microscopy which allowed to extract the deformation of the cell for every single frame. For fluorescence imaging confocal laser scanning microscopy was employed. The ratio of the fluorescence of a temperature sensitive and a temperature insensitive rhodamine dye was utilized to determine the temperatures inside the optical trap during and after Optical Stretching. The rise in temperature at a measuring power of 0.7W turned out to be enough to open a temperature sensitive ion channel transfected into an epithelial cell line. In this way a massive Ca2+ influx was triggered during the Optical Stretcher experiment. A new setup combining Optical Stretching and confocal laser scanning microscopy allowed fluorescence imaging of these Ca2+ signals while the cells were deformed by optically induced surface forces, showing that the Ca2+ influx could be manipulated with adequate drugs. This model system was then employed to investigate the influence of Ca2+ on the observed contractions, revealing that they are partially triggered by Ca2+. A phenomenological mathematical model based on the fundamental constitutive equation for linear viscoelastic materials extended by a term accounting for active contractions allowed to quantify the activity of the measured cells. The skewness and the median of the strain distributions were shown to depend on the activity of the cells. The introduced model reveals that even in measurements, that seemingly are describable by passive viscoelasticity, active contractililty might be superimposed. Ignoring this effect will lead to erroneous material properties and misinterpretation of the data. Taken together, the findings presented in this thesis demonstrate that active processes are an essential part of cellular mechanics and cells can contract even independently of adhesions. The results provide a method that allows to quantify active contractions of suspended cells. As the proposed model is not based on specific assumptions on force generating processes, it paves the way for a thorough investigation of different influences, such as cytoskeletal structures and intra-cellular signaling processes, to cellular contractions. The results present an important contribution for better mechanical classification of cells in future research with possible implications for medical diagnosis and therapy.

Ce manuscript d'Habilitation à Diriger des Recherches présente mes travaux de recherche pour la période 2000-2011, pendant laquelle je me suis intéressée à la description et à la compréhension de la dynamique d'écoulement des mousses aqueuses ainsi qu' à l'écoulement de liquide et de particules dans un tel milieu. Une brève introduction aux mousses aqueuses dé finit le matériau et ses propriétés caractéristiques ainsi qu'un vocabulaire parfois spéci fique à la communauté "mousse". Les problèmatiques et leur contexte sont présentés plus précisément au début de chaque chapitre ou sous-chapitre.

Im Mittelpunkt dieser Arbeit steht die Verwendung verschiedener Fluoreszenzfarbstoffe in Form photostabiler Perylenbisimide sowie etallischer Nanopartikel zur Untersuchung von Polymeren und nanoskopischen Flüssigkeitsfilmen. Einzelmoleküluntersuchungen zeigen, dass eine chemische Modifizierung der Farbstoffe durch löslichkeitserhöhende Seitengruppen, Molekülkonformationen mit stark variierenden Emissionswellenlängen je nach Seitengruppen-orientierung zur Folge hat. Zeitabhängige Fluoreszenzmessungen an einzelnen Molekülen ermöglichen eine direkte Beobachtung von Übergängen zwischen diesen molekularen Konformationen deren Dynamik vorwiegend durch die Eigenschaften der umgebenden Polymermatrix bestimmt wird. Die gewonnenen Ergebnisse lassen somit Rückschlüsse auf die nanoskopische Umgebung des Moleküls zu. Es werden diskrete Zustände innerhalb der Molekülumgebung sowie eine erhöhte Konformationsdynamik im Falle von alkylsubstituierten Perylenbisimiden beobachtet. Darüber hinaus werden die nanoskopischen Auswirkungen von makroskopischen, mechanischen Deformationen auf amorphe Polymerfilme mikrorheologisch mit Hilfe von stäbchenförmigen Perylenbisimiden studiert. Die gewonnenen Einzelmoleküldaten ermöglichen die Berechnung der lokalen, mikroskopischen Deformation sowie der Orientierung der Sondenmoleküle, welche gut mit einem Model für stäbchenförmige Objekte in einem uniaxial deformierten Kontinuum übereinstimmt. In weiteren Experimenten gelingt der Nachweis ultradünner Wasserfilme auf SiO2-Oberflächen durch Messung der Diffusion von Silbernanopartikeln. Die verwendeten Nanopartikel weisen hierbei eine monodisperse Größenverteilung im Bereich von einem Nanometer als Resultat ihrer Synthese in Y-Zeolith-Kristallen auf. Die Untersuchungen ergeben eine Filmdickenabhängigkeit des Diffusionsverhaltens sowie einen starken Einfluss durch Oberflächensilanisierung bzw. Hydroxylierung.

Diese Arbeit behandelt die Beschreibung des Fließens und des Blockierens von granularer Materie. Granulare Materie kann einen Verfestigungsübergang durchlaufen. Dieser wird Jamming genannt und ist maßgeblich durch vorliegende Spannungen sowie die Packungsdichte der Körner, welche das Granulat bilden, bestimmt. Die Rheologie dichter granularer Medien ist zusätzlich zu Spannung und Packungsdichte stark durch Reibung zwischen den Körnern beeinflusst. Wir zeigen mittels numerischer Simulationen und analytischer Betrachtungen, wie Reibung Jamming qualitativ verändert. Reibungsfreies Jamming ist ein kontinuierlicher Phasenübergang mit einem kritischen Punkt bei verschwindender Spannung. Reibungsbehaftetes Jamming ist ein diskontinuierlicher Phasenübergang mit einem kritischen Punkt bei endlicher Spannung. Der kritische Punkt bei endlicher Spannung führt zu bemerkenswertem Verhalten: Oberhalb der kri- tischen Packungsdichte gibt es ein Intervall an Packungsdichten, innerhalb dessen große oder kleine Spannungen zum Fließen führen, mittlere Spannungen hingegen führen zum Blockieren des Mediums. Das Fließverhalten nahe Jamming ist stark durch die Systemgröße beeinflusst: Es gibt eine kritische Systemgröße, oberhalb derer zeitabhängiger Fluss entsteht. Dieser zeitabhängige Fluss wird durch die Ausbildung von großskaligen Strukturen im Spannungsfeld erklärt. Sowohl die großskaligen Strukuren als auch der damit einhergehende zeitabhängige Fluss sind neuartige Phänomene im Fluss von trockenen Granulaten und durch Rei- bung hervorgerufen.

‘Soft condensed matter’ is a newly-emerged sub-discipline of physics concerned with the study of systems that are mechanically soft such as colloids, emulsions, surfactants, polymers, liquid crystals, granular media and various biomaterials including DNA and proteins. These materials display a broad range of interesting microstructures and phase behaviours and have a myriad of applications in the materials, food, paint and cosmetic industries as well as medical technologies. Soft condensed matter physics presents new opportunities and challenges for the development of new ideas and concepts in experimental and theoretical physics alike. Because the field overlaps with many different disciplines, the study of soft matter also offers promising developments to other fields of science including chemistry, chemical engineering, materials science, biology, and environmental science. The behaviour of these systems is dominated by one simple fact: they contain mesoscopic structures in the size range 10 nm to 1 µm that are held together by weak entropic forces. The elastic constants of these materials are 109 times smaller than the conventional atomic materials and hence are easily deformable by external stresses, electric or magnetic fields, or even by thermal fluctuations. We have studied two important classes of soft matter systems in this thesis -colloidal suspensions and surfactant systems. The thesis is divided into two main themes: (a) Effects of electric field on the colloidal suspensions, and (b) Effects of shear on surfactant solutions. Motions of colloidal particles under the influence of applied electric field were observed under a microscope and were studied using image analysis and particle tracking. We have also used tracking of thermal fluctuations of colloidal particles embedded in surfactant gels to study microrheology of surfactant solutions. Linear and non-linear rheology of aqueous solutions of cationic cetyltrimethyl ammonium bromide (CTAB) and anionic sodium-3-hydroxynapthalene-2-carboxylate (SHNC) were studied using bulk rheology in a commercial rheometer. Rheological studies of an anionic surfactant sodium dodecyl sulphate (SDS) in the presence of strongly binding counterion p-toluidine hydrochloride (PTHC) has also been done. Chapter 1 starts with a general introduction to soft condensed matter systems and then we proceed to describe two specific class of soft condensed materials which we have studied in this thesis -colloidal suspensions and surfactant/water systems. After describing different types of colloids, we discuss why colloids are suitable as model systems in condensed matter physics. This is followed by a discussion on the chemical structure, phase behaviour and self assembling properties of surfactant molecules in water. We then discuss the inter-macromolecular forces such as van der Waals interaction, the screened Coulomb repulsion, hydrogen bond, hydrophobic and hydration forces and steric repulsion which are the major players in the interaction in soft condensed matter systems. The systems that have been the subject of our experimental studies, viz. polystyrene colloidal suspensions, CTAB+SHNC, SDS+PTHC and CTAT have also been discussed in detail. Then we have given an overview of effects of electric field on the colloidal suspensions. Two types of geometries have been discussed: one in which the field is parallel to the plates and another when the field is perpendicular to the electrodes. Application of colloidal particles in diagnostic tests (Latex Agglutination Tests) has been discussed after this. Some methods used to enhance the sensitivity of LATs have also been reviewed. This is followed by a theoretical background of linear and non-linear rheology. We have also given an introduction to digital video microscopy, its advantages and discussed few quantities like pair correlation function, structure factor which can be extracted using digital video microscopy and particle tracking. The concluding part of this chapter describes the organization of this thesis. Chapter 2 discusses the experimental apparatus and techniques used in our studies. We describe our setup for applying the electric field to the colloidal particles and imaging and tracking their motion. We also discuss the image processing and analyzing methods for extracting the useful quantities from the digitized images. We have described the various components of the MCR-300 stress-controlled rheometer (Paar Physica, Germany) and the AR-1000N stress-controlled rheometer (T. A. Instruments, U. K.) followed by different experimental geometries that we have used for our experiments. Next we have described the various experiments that can be done using a commercial rheometer. Calculation of surface charge of colloidal particles using a conductivity meter has been demonstrated for our colloidal particle suspensions. We also describe the sample preparation methods employed in different experiments. In Chapter 3, we have discussed our study of clustering of colloidal particles under the influence of an ac electric field as a function of frequency. The field was applied in a direction perpendicular to the confining walls. Two regimes are observed, a low frequency regime where the clusters are isotropic with a local triangular order and a new high-frequency regime where the clusters are highly elongated (anisotropic) with no local order. The crossover from one regime to the other occurs at a critical frequency, fc. The formation of elongated clusters seen at high frequencies is explained in terms of rotation of particles due to a phase lag between the polarization of the electric double layer around a particle and the applied electric field that arises because of inhomogeneities of the conducting surface. We have also observed that the threshold field for the cluster formation, Eth, increases with frequency in both the regimes. We did these studies on two different sizes of particles and found that both Eth and fc were lower for the larger particles. Our model based on particle rotation was able to estimate the value of fc correctly for both the sizes of the particles. Chapter 4 describes a method employing an ac electric field applied perpendicular to the confining walls to increase the sensitivity of recognition of ligands by their corresponding receptors grafted on Brownian latex particles. Application of electric field assists the colloidal micro-particles grafted with receptors to come nearer due to electro-hydrodynamic drag. This increase in the local concentration of the latex particles results in improving the chances of ligand-receptor interaction leading to the aggregation of the latex particles. With this technique we have been able to increase the sensitivity of the ligand-receptor recognition by a factor as large as 50. We have demonstrated the utility of our method using streptavidin as the model receptor and biotinylated RNase A as the model ligand. We have also applied our technique to a commercially available kit for rheumatoid factor (RF) with successful results. The same method was also successfully applied for the detection of typhoid whose antibodies were purified and attached to polystyrene particles by our collaborators from DRDE Gwalior. In Chapter 5, we have studied the statics and dynamics of colloidal particles at different applied electric fields from zero to beyond the threshold field. We have taken a series of time-lapsed images and calculated out the pair-correlation function, mean squared displacement, structure factor, non-Gaussian parameter etc. We have studied both mono-dispersed colloidal system and binary colloidal system (mixture of two different sizes of particles). The aggregates formed in the two cases were analysed with the help of Voronoi polygons to quantify the microscopic structure. In mono-dispersed system, the aggregates formed were two-dimensional hexagonal crystals and we have used this system to study the freezing transition in 2-dimension. The properties of the system in the liquid and the crystalline state satisfy various criteria for the 2-d freezing transition. The first maximum of the structure factor at the voltage at which freezing occurs, is 5.5 as has been suggested for the 2-d freezing. This is reflected in the dynamics of the system also, where the ratio D/D0 falls below 10%, in accordance with the LPS (L¨owen, Palberg, Simon) criterion for freezing in 2-d colloidal systems [Phys. Rev. Lett. 70, 1557 (1993)]. However, in the binary colloidal system the clusters formed were not crystalline but more like 2-d dense liquids. A closer inspection of these clusters reveals that the motion of a smaller subset of particles is cooperative and follows string-like paths. The mean square displacement of such a system shows a plateau in the intermediate times which indicates the “caging” of particles by its neighbours. A peak in non-gaussian parameter indicates the presence of dynamical heterogeneities in the system. In Chapter 6, we have described the use of multiple particle tracking to study the microrheology of semidilute solutions of wormlike micelles and compared the results with those from macrorheology experiments done on the same samples. Two concentrations of CTAT (1.3% and 2%) were used. We observed that, in spite of the mesh size being much smaller than the size of the probe particles, the viscoelastic response function calculated using the one-point microrheology does not match with that measured from macrorheology. This can be attributed to the fact that there is another important length scale in the system, the mean micellar length, and it is comparable to the probe particle size. Two-point microrheology was successful in verifying the macrorheology results for CTAT 1.3% but it fails to do so for CTAT 2%. We attribute this to the fact that in a higher viscosity sample (2%), the hydrodynamic force propagate to a lesser distance, thereby limiting the measurable correlation between the particles and precluding the success of two-point microrheology. Chapter 7 describes a rheological study of aqueous solutions of varying concentration of cationic cetyltrimethyl ammonium bromide (CTAB) and anionic sodium-3-hydroxynapthalene-2-carboxylate (SHNC) kept at a fixed molar concentration ratio [CTAB]/[SHNC] = 2. At this molar ratio, the surfactants self-assemble into wormlike micelles which get entangled above the overlap concentration to form viscoelastic gel. The range of the total surfactant concentration φ varies from 1.17% to 5.16% by weight. We found that, plateau modulus, G0, shows a power law dependence on the surfactant concentration, φ, with an exponent 3, which is higher than the expected value of 2.25 observed for the one-component wormlike micelles. Zero shear viscosity, η0, and relaxation time, τR show a maximum at the surfactant concentration, φmax = 1.9% in contrast to a monotonic increase with φ. We propose that this non-monotonic behaviour is due to the unusual dependence of the average micellar length L ¯on φ, showing a maximum in average micellar length L at φmax. This argument provides a strong support to the model of micellar growth in the presence of electrostatic interactions developed by Mackintosh et. al [Europhys. Lett. 12, 697 (1990)]. The presence of electrostatic interactions also appears in the behaviour of the plateau modulus G0 that exhibits a larger φ dependence than in highly screened micelles. In the non-linear flow experiments, a minimum observed in critical shear rate (the shear rate at which shear thinning starts), ˙γc, at φmax strengthens our arguments. In Chapter 8, we describe the phase behaviour and rheology of SDS+PTHC (sodium dodecyl sulphate + p-toluidine hydrochloride) micellar solutions at different molar ratios α=[PTHC]/[SDS]) of the two components. At low values of α, polarizing microscopy observations reveal a transition from an isotropic to a nematic phase of disk-like micelles, whereas a transition to a lamellar phase occurs at higher α values > 0.5, on increasing the surfactant content. Linear rheology of the isotropic micellar solution reveal a viscous behaviour over a large range of surfactant concentrations. Surprisingly, this also extends to the nematic phase of disk-like micelles observed at α =0.2 and φ =0.35. These systems also exhibit a viscoelastic behaviour over a narrow range of surfactant concentration as reported in earlier studies. The extent of the viscoelastic region of the isotropic micellar solution also decreases with increase in α. Frequency sweep curves in this region, scaled on to a master curve is reminiscent of dilute suspensions of hard spheres or rigid Brownian rods. Consistent with the results from oscillatory shear measurements, the f;ow behaviour examined under steady shear is Newtonian over a large range of surfactant content in the isotropic micellar solution. An interesting result in these studies is the non-monotonic behaviour of the viscosity with increase in surfactant concentration. It is likely that the sharp rise in viscosity arises from a jamming effect of the rigid rods. Dynamic light scattering studies suggest that the drop in viscosity is due to the decrease in the length of the micellar aggregates. This is followed by a change in the morphology of the micelles from rods to disks as indicated by the transition to a nematic phase of disk-like micelles or a lamellar phase. A change in the morphology of micellar aggregates with increase in α is expected in mixed surfactant systems with strongly binding counterions. However, the surprising result is the change in morphology of the micellar aggregates with surfactant content. Such a behaviour is seen in mixed surfactant systems for the first time. The thesis concludes with a summary of our main results and a brief discussion of the scope of future work in Chapter 9.

‘Soft condensed matter’ is a newly-emerged sub-discipline of physics concerned with the study of systems that are mechanically soft such as colloids, emulsions, surfactants, polymers, liquid crystals, granular media and various biomaterials including DNA and proteins. These materials display a broad range of interesting microstructures and phase behaviours and have a myriad of applications in the materials, food, paint and cosmetic industries as well as medical technologies. Soft condensed matter physics presents new opportunities and challenges for the development of new ideas and concepts in experimental and theoretical physics alike. Because the field overlaps with many different disciplines, the study of soft matter also offers promising developments to other fields of science including chemistry, chemical engineering, materials science, biology, and environmental science. The behaviour of these systems is dominated by one simple fact: they contain mesoscopic structures in the size range 10 nm to 1 µm that are held together by weak entropic forces. The elastic constants of these materials are 109 times smaller than the conventional atomic materials and hence are easily deformable by external stresses, electric or magnetic fields, or even by thermal fluctuations. We have studied two important classes of soft matter systems in this thesis -colloidal suspensions and surfactant systems. The thesis is divided into two main themes: (a) Effects of electric field on the colloidal suspensions, and (b) Effects of shear on surfactant solutions. Motions of colloidal particles under the influence of applied electric field were observed under a microscope and were studied using image analysis and particle tracking. We have also used tracking of thermal fluctuations of colloidal particles embedded in surfactant gels to study microrheology of surfactant solutions. Linear and non-linear rheology of aqueous solutions of cationic cetyltrimethyl ammonium bromide (CTAB) and anionic sodium-3-hydroxynapthalene-2-carboxylate (SHNC) were studied using bulk rheology in a commercial rheometer. Rheological studies of an anionic surfactant sodium dodecyl sulphate (SDS) in the presence of strongly binding counterion p-toluidine hydrochloride (PTHC) has also been done. Chapter 1 starts with a general introduction to soft condensed matter systems and then we proceed to describe two specific class of soft condensed materials which we have studied in this thesis -colloidal suspensions and surfactant/water systems. After describing different types of colloids, we discuss why colloids are suitable as model systems in condensed matter physics. This is followed by a discussion on the chemical structure, phase behaviour and self assembling properties of surfactant molecules in water. We then discuss the inter-macromolecular forces such as van der Waals interaction, the screened Coulomb repulsion, hydrogen bond, hydrophobic and hydration forces and steric repulsion which are the major players in the interaction in soft condensed matter systems. The systems that have been the subject of our experimental studies, viz. polystyrene colloidal suspensions, CTAB+SHNC, SDS+PTHC and CTAT have also been discussed in detail. Then we have given an overview of effects of electric field on the colloidal suspensions. Two types of geometries have been discussed: one in which the field is parallel to the plates and another when the field is perpendicular to the electrodes. Application of colloidal particles in diagnostic tests (Latex Agglutination Tests) has been discussed after this. Some methods used to enhance the sensitivity of LATs have also been reviewed. This is followed by a theoretical background of linear and non-linear rheology. We have also given an introduction to digital video microscopy, its advantages and discussed few quantities like pair correlation function, structure factor which can be extracted using digital video microscopy and particle tracking. The concluding part of this chapter describes the organization of this thesis. Chapter 2 discusses the experimental apparatus and techniques used in our studies. We describe our setup for applying the electric field to the colloidal particles and imaging and tracking their motion. We also discuss the image processing and analyzing methods for extracting the useful quantities from the digitized images. We have described the various components of the MCR-300 stress-controlled rheometer (Paar Physica, Germany) and the AR-1000N stress-controlled rheometer (T. A. Instruments, U. K.) followed by different experimental geometries that we have used for our experiments. Next we have described the various experiments that can be done using a commercial rheometer. Calculation of surface charge of colloidal particles using a conductivity meter has been demonstrated for our colloidal particle suspensions. We also describe the sample preparation methods employed in different experiments. In Chapter 3, we have discussed our study of clustering of colloidal particles under the influence of an ac electric field as a function of frequency. The field was applied in a direction perpendicular to the confining walls. Two regimes are observed, a low frequency regime where the clusters are isotropic with a local triangular order and a new high-frequency regime where the clusters are highly elongated (anisotropic) with no local order. The crossover from one regime to the other occurs at a critical frequency, fc. The formation of elongated clusters seen at high frequencies is explained in terms of rotation of particles due to a phase lag between the polarization of the electric double layer around a particle and the applied electric field that arises because of inhomogeneities of the conducting surface. We have also observed that the threshold field for the cluster formation, Eth, increases with frequency in both the regimes. We did these studies on two different sizes of particles and found that both Eth and fc were lower for the larger particles. Our model based on particle rotation was able to estimate the value of fc correctly for both the sizes of the particles. Chapter 4 describes a method employing an ac electric field applied perpendicular to the confining walls to increase the sensitivity of recognition of ligands by their corresponding receptors grafted on Brownian latex particles. Application of electric field assists the colloidal micro-particles grafted with receptors to come nearer due to electro-hydrodynamic drag. This increase in the local concentration of the latex particles results in improving the chances of ligand-receptor interaction leading to the aggregation of the latex particles. With this technique we have been able to increase the sensitivity of the ligand-receptor recognition by a factor as large as 50. We have demonstrated the utility of our method using streptavidin as the model receptor and biotinylated RNase A as the model ligand. We have also applied our technique to a commercially available kit for rheumatoid factor (RF) with successful results. The same method was also successfully applied for the detection of typhoid whose antibodies were purified and attached to polystyrene particles by our collaborators from DRDE Gwalior. In Chapter 5, we have studied the statics and dynamics of colloidal particles at different applied electric fields from zero to beyond the threshold field. We have taken a series of time-lapsed images and calculated out the pair-correlation function, mean squared displacement, structure factor, non-Gaussian parameter etc. We have studied both mono-dispersed colloidal system and binary colloidal system (mixture of two different sizes of particles). The aggregates formed in the two cases were analysed with the help of Voronoi polygons to quantify the microscopic structure. In mono-dispersed system, the aggregates formed were two-dimensional hexagonal crystals and we have used this system to study the freezing transition in 2-dimension. The properties of the system in the liquid and the crystalline state satisfy various criteria for the 2-d freezing transition. The first maximum of the structure factor at the voltage at which freezing occurs, is 5.5 as has been suggested for the 2-d freezing. This is reflected in the dynamics of the system also, where the ratio D/D0 falls below 10%, in accordance with the LPS (L¨owen, Palberg, Simon) criterion for freezing in 2-d colloidal systems [Phys. Rev. Lett. 70, 1557 (1993)]. However, in the binary colloidal system the clusters formed were not crystalline but more like 2-d dense liquids. A closer inspection of these clusters reveals that the motion of a smaller subset of particles is cooperative and follows string-like paths. The mean square displacement of such a system shows a plateau in the intermediate times which indicates the “caging” of particles by its neighbours. A peak in non-gaussian parameter indicates the presence of dynamical heterogeneities in the system. In Chapter 6, we have described the use of multiple particle tracking to study the microrheology of semidilute solutions of wormlike micelles and compared the results with those from macrorheology experiments done on the same samples. Two concentrations of CTAT (1.3% and 2%) were used. We observed that, in spite of the mesh size being much smaller than the size of the probe particles, the viscoelastic response function calculated using the one-point microrheology does not match with that measured from macrorheology. This can be attributed to the fact that there is another important length scale in the system, the mean micellar length, and it is comparable to the probe particle size. Two-point microrheology was successful in verifying the macrorheology results for CTAT 1.3% but it fails to do so for CTAT 2%. We attribute this to the fact that in a higher viscosity sample (2%), the hydrodynamic force propagate to a lesser distance, thereby limiting the measurable correlation between the particles and precluding the success of two-point microrheology. Chapter 7 describes a rheological study of aqueous solutions of varying concentration of cationic cetyltrimethyl ammonium bromide (CTAB) and anionic sodium-3-hydroxynapthalene-2-carboxylate (SHNC) kept at a fixed molar concentration ratio [CTAB]/[SHNC] = 2. At this molar ratio, the surfactants self-assemble into wormlike micelles which get entangled above the overlap concentration to form viscoelastic gel. The range of the total surfactant concentration φ varies from 1.17% to 5.16% by weight. We found that, plateau modulus, G0, shows a power law dependence on the surfactant concentration, φ, with an exponent 3, which is higher than the expected value of 2.25 observed for the one-component wormlike micelles. Zero shear viscosity, η0, and relaxation time, τR show a maximum at the surfactant concentration, φmax = 1.9% in contrast to a monotonic increase with φ. We propose that this non-monotonic behaviour is due to the unusual dependence of the average micellar length L ¯on φ, showing a maximum in average micellar length L at φmax. This argument provides a strong support to the model of micellar growth in the presence of electrostatic interactions developed by Mackintosh et. al [Europhys. Lett. 12, 697 (1990)]. The presence of electrostatic interactions also appears in the behaviour of the plateau modulus G0 that exhibits a larger φ dependence than in highly screened micelles. In the non-linear flow experiments, a minimum observed in critical shear rate (the shear rate at which shear thinning starts), ˙γc, at φmax strengthens our arguments. In Chapter 8, we describe the phase behaviour and rheology of SDS+PTHC (sodium dodecyl sulphate + p-toluidine hydrochloride) micellar solutions at different molar ratios α=[PTHC]/[SDS]) of the two components. At low values of α, polarizing microscopy observations reveal a transition from an isotropic to a nematic phase of disk-like micelles, whereas a transition to a lamellar phase occurs at higher α values > 0.5, on increasing the surfactant content. Linear rheology of the isotropic micellar solution reveal a viscous behaviour over a large range of surfactant concentrations. Surprisingly, this also extends to the nematic phase of disk-like micelles observed at α =0.2 and φ =0.35. These systems also exhibit a viscoelastic behaviour over a narrow range of surfactant concentration as reported in earlier studies. The extent of the viscoelastic region of the isotropic micellar solution also decreases with increase in α. Frequency sweep curves in this region, scaled on to a master curve is reminiscent of dilute suspensions of hard spheres or rigid Brownian rods. Consistent with the results from oscillatory shear measurements, the f;ow behaviour examined under steady shear is Newtonian over a large range of surfactant content in the isotropic micellar solution. An interesting result in these studies is the non-monotonic behaviour of the viscosity with increase in surfactant concentration. It is likely that the sharp rise in viscosity arises from a jamming effect of the rigid rods. Dynamic light scattering studies suggest that the drop in viscosity is due to the decrease in the length of the micellar aggregates. This is followed by a change in the morphology of the micelles from rods to disks as indicated by the transition to a nematic phase of disk-like micelles or a lamellar phase. A change in the morphology of micellar aggregates with increase in α is expected in mixed surfactant systems with strongly binding counterions. However, the surprising result is the change in morphology of the micellar aggregates with surfactant content. Such a behaviour is seen in mixed surfactant systems for the first time. The thesis concludes with a summary of our main results and a brief discussion of the scope of future work in Chapter 9.

In this thesis, we have studied non-equilibrium phenomena in sheared soft matter systems and in granular active matter. The surfactant systems are driven out of thermal equilibrium by applying external forcing at the boundary. We have used rheology, in-situ x-ray scattering techniques, in-situ optical microscopy to study the flow behavior and the structural changes of surfactant gels and Langmuir monolayers. The flow of soft gel and colloidal glass below their yield stress shows large burst in the shear rate due to internal reorganization as confirmed by in-situ optical microscopy. The statistical properties of the shar rate fluctuations resembles those observed in ground acceleration in earth-quakes, like Omori law, Gutenberg-Richter law and power law distribution of inter-occurrence time. The ordered mesh phase under shear shows isomorphic twinning transition having slightly different lattice parameters, giving rise to the splitting of the Bragg peaks and the six or eight points modulation of the Bragg rings. The rheology of the surfactant’s Langmuir monolayer shows a phase transition at yield strain separating the absorbing to fluctuating steady state. Further, the Grazing Incidence X-ray Diffraction of peptide-lipid monolayer at the air-water interface has been studied to understand the effect of shear on the structural properties of 2D nano-crystallites. In the second part, we have studied the effect of dissenters on the collective behavior of particles which have a tendency to flock. The system of two-step-tapered rods undergoes the flocking phase transition at a threshold area fraction φc ∼ 0.12 having high orientational correlations between the particles. However, the one-step-tapered rods (same as the two-step-tapered rods but without the 1mm middle step) do not flock. We use these one-step-tapered rods as the motile dissenters in the flock-forming granular matter of aligners (the two-step-tapered rods). We mix and disperse them to follow the system in the steady-state and our experiments give a quantitative estimation of dissenter’s effects on the flocking. At the critical fraction of dissenters, f ∼ 0.3, the flocking order of the system gets destroyed completely. The variance of the system’s order parameter shows a maximum near the dissenter fraction f ∼ 0.05, suggesting a finite-size crossover between the ordered and disordered phases.
JRF and SRF fellowships from University Grants Commission (UGC)

In day to day life we encounter many different materials which are intermediate between crystalline solids and simple liquids that include paints , glues , suspensions, polymers, surfactants, food and cosmetic products and so on. ‘Soft condensed matter’ is an emerging field of science that aims to generalize the flow and various deformation mechanisms in this apparent diverse class of materials from a ‘mesoscopic’ point of view (important length scales for these systems is usually 10nm-1μm) where the actual atomic and molecular details governed by various quantum mechanical laws are not very important. These soft systems are held together by weaken tropic forces and therefore can be perturbed easily (the typical elastic modulus of these materials is many orders of magnitude lower compared to metallic solids). Moreover, very long relaxation times in these systems(∼10−3 to 1 s) have made them ideal candidates to study non-equilibrium physics. The present Thesis is an endeavor to understand linear and non-linear flow behavior and low Reynolds number instabilities in various soft matter systems like suspensions of flocculated carbon nanotubes and carbon black, surfactant gels, colloidal glasses, Langmuir monolayers etc probed mainly by bulk and interfacial rheology, in-situ light scattering, particle image velocimetry(PIV) techniques and Fourier transform rheology. We also use dynamic light scattering techniques for particle sizing and characterization of Brownian systems. Chapter 1 gives a general introduction to soft condensed matter, particularly, the important length and time scales, various interactions and the rich phase behavior emerged from the delicate balance between energy and entropy in these systems. In this context, We describe the detailed phase behavior of two such systems studied in this thesis. We next describe briefly a few important concepts which motivate the main problems studied in the present thesis like the shear-thickening in suspensions of Brownian and non-Brownian particles, non-equilibrium steady state fluctuation relations in driven systems, elasticity driven instabilities in complex fluids, jamming transitions and aging behavior. This is followed by a discussion of the experimental techniques like linear and nonlinear rheology, including the Fourier transform rheology. Chapter 2 discusses the experimental techniques used by us in detail. We first describe the different components and mode of operations of the MCR-300 stress-controlled rheometer (Paar Physica, Germany) and various experimental geometries. Next we discuss the set up for two dimensional rheological measurements. The homebuilt imaging set up for in-situ polarized light scattering and direct imaging studies is described along with the in-situ particle image velocimetry (PIV) to map out the exact spatially resolved velocity profiles in 2D systems. We give a brief account of the techniques of Fourier transform rheology. At the end of this chapter, we briefly describe the angle resolved dynamic light scattering (DLS) set up (Brookhaven Instruments, USA). In Chapter 3, we study colossal discontinuous shear-thickening transition in confined suspensions of fractal clusters formed by multi-wall carbon nanotubes (MWNT) by rheology and in-situ imaging experiments. Monotonic decrease in viscosity with increasing shear stress, known as shear thinning, is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. In the present experiments we demonstrate a discontinuous shear thickening transition where the viscosity jumps sharply above a critical shear stress by four to six orders of magnitude in flocculated suspensions of MWNT even at very low weight fractions(∼0.5%). Rheo-optical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear thickening phenomena in confined geometries is pertinent for flow controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues. We try to understand the flow of jammed and shear-thickened states under constant applied strain rate by studying the building up and relaxation of individual stress fluctuation events similar to the flow in dense granular materials. We also characterize the metastable shear thickened states by superposing a small sinusoidal stress component on a steady applied stress as well as by studying the a thermal entropy consuming fluctuations which are also observed for other jammed systems under an applied steady shear stress as described in the next chapter. Chapter 4 reports the study of non-equilibrium fluctuations in concentrated gels and glassy systems(in jammed state), the nature of fluctuations and their systemsize dependence in the framework of fluctuation relation and Generalized Gumbel distribution. In the first part, we show that the shear rate at a fixed shear stress in a micellar gel in a jammed state exhibits large fluctuations, showing positive and negative values, with the mean shear rate being positive. The resulting probability distribution functions (PDFs) of the global power flux to the system vary from Gaussian to non-Gaussian, depending on the driving stress and in all cases show similar symmetry properties as predicted by Gallavotti-Cohen steady state fluctuation relation. The fluctuation relation allows us to determine an effective temperature related to the structural constraints of the jammed state. We have measured the stress dependence of the effective temperature. Further, experiments reveal that the effective temperature and the standard deviation of the shear rate fluctuations increase with the decrease of the systemsize. In the second part of this chapter, we report a universal large deviation behavior of spatially averaged global injected power just before the rejuvenation of the jammed state formed by an aging suspension of laponite clay under an applied stress. The probability distribution function (PDF) of these entropy consuming strongly non-Gaussian fluctuations follow an universal large deviation functional form described by the Generalized Gumbel (GG) distribution like many other equilibrium and non-equilibrium systems with high degree of correlations but do not obey Gallavotti-Cohen Steady State Fluctuation Relation (SSFR). However, far from the unjamming transition (for smaller applied stresses) SSFR is satisfied for both Gaussian as well as non-Gaussian PDF. The observed slow variation of the mean shear rate with system size supports a recent theoretical prediction for observing GG distribution. We also establish the universality of the observations reported in this chapter in the light of other jammed systems under shear. We examine in the first part of Chapter 5, the shear-thinning behavior of a two dimensional yield stress bearing monolayer of sorbitan tristearate at air/water interface. The flow curve (stress vs shear rate) consists of a linear region at low shear stresses/shear rates, followed by a stress plateau at higher values. The velocity profile obtained from particle imaging velocimetry indicates that shear banding occurs showing coexistence of fluidized region near the rotor and solid region with vanishing shear-rate away from the rotor. In the fluidized region, the velocity profile which is linear at low shear rates becomes exponential at the onset of shear-thinning, followed by a time varying velocity profile in the plateau region. At low values of constant applied shear rates, the viscosity of the film increases with time, thus showing aging behavior like in soft glassy three-dimensional (3D) systems. Further, at the low values of the applied stress in the yield stress regime, the shear-rate fluctuations in time show both positive and negative values, similar to that observed in sheared 3D jammed systems. By carrying out a statistical analysis of these shear-rate fluctuations, we estimate the effective temperature of the soft glassy monolayer using the Galavatti-Cohen steady state fluctuation relation. In the second part of this chapter, we study in detail the non-linear viscoelastic behavior of Langmuir monolayers. Under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes(γ), the storage modulus (G”) decreases monotonically whereas the loss modulus (G”) exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G” and G” are in the ratio 2:1. The peak in G” is absent at high temperatures and low concentration of sorbitan tristearate. Strain-rate frequency sweep measurements on the monolayers do indicate a strain-rate dependence of the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is very general and exhibits many of the features observed in 3D complex fluids. We report in the first part of Chapter 6 scattering dichroism experiments to quantify the spatio-temporal nematodynamics of shear-thinning worm like micellar gels of surfactant Cetyltrimethylammonium Tosylate (CTAT) in the presence of salt sodium chloride (NaCl) enroute to rheochaos. For shear rates past the plateau onset, we observe a presence of alternating bright and dark‘ intertwined’ birefringent structures along the vorticity direction. The orientational order corresponding to these structures are predominantly oriented at +45deg and−45deg to the flow (v) in the (v,∇v) plane. The orientational dynamics of the nematics especially at the interface between the structures, has a one-to-one correspondence with the temporal behavior of the stress. Experiments show that the spatial motion of the vorticity structures depend on the gap thickness of the Couette cell. We next discuss the random temporal flow behavior of this system at high values of applied shear rate/stress in the framework of elastic turbulence in the second part of this chapter. Here, we study the statistical properties of spatially averaged global injected power fluctuations for the worm-like micellar system described above. At sufficiently high Weissenberg numbers (Wi) the shear rate and hence the injected power p(t) at a constant applied stress shows large irregular fluctuations in time. The nature of the probability distribution function (PDF) of p(t) and the power-law decay of its power spectrum are very similar to that observed in recent studies of elastic turbulence for polymer solutions. Remarkably, these non-Gaussian pdf scan be well described by an universal large deviation functional form given by the Generalized Gumbel (GG) distribution observed in the context of spatially averaged global measures in diverse classes of highly correlated systems. We show by in-situ rheology and polarized light scattering experiments that in the elastic turbulent regime the flow is spatially smooth but random in time, in agreement with a recent hypothesis for elastic turbulence. In Chapter 7, we study the vorticity banding under large amplitude oscillatory shear (LAOS) in a dilute worm-like micellar gel formed by surfactant CTAT by Fourier transform rheology and in-situ polarized light scattering. Under LAOS we found the signature of a non-trivial order-disorder transition of Taylor vortices. In the non-linear regime, higher harmonicde composition of the resulting stress signal reveals that the third harmonic I3 shows a very prominent maximum at the strain value where the number density (nv) of the Taylor vortices is maximum for a wide range of angular frequencies both above and below the linear crossover point. Subsequent increase in applied strain results in distortions of the vortices and a concomitant decrease in nv when I3 also drops very sharply and acts like an order parameter for this order-disorder transition. We further quantify the transition by defining an independent order parameter like quantity from the spatial correlation function of the scattered intensity and equivalently its Fourier transform which essentially captures the non monotonous third harmonic behavior. Lissajous plots indicate an intra-cycle strain hardening for the values of γ corresponding to the peak of I3 similar to that observed for hard-sphere glasses. Our study is an important step forward to correlating the structures developed in the system under LAOS to the appearances of the higher harmonics in the non-linear regime. The Thesis concludes with a summary of the main results and a brief account on the scope of future work as described in Chapter 8.

In day to day life we encounter many different materials which are intermediate between crystalline solids and simple liquids that include paints , glues , suspensions, polymers, surfactants, food and cosmetic products and so on. ‘Soft condensed matter’ is an emerging field of science that aims to generalize the flow and various deformation mechanisms in this apparent diverse class of materials from a ‘mesoscopic’ point of view (important length scales for these systems is usually 10nm-1μm) where the actual atomic and molecular details governed by various quantum mechanical laws are not very important. These soft systems are held together by weaken tropic forces and therefore can be perturbed easily (the typical elastic modulus of these materials is many orders of magnitude lower compared to metallic solids). Moreover, very long relaxation times in these systems(∼10−3 to 1 s) have made them ideal candidates to study non-equilibrium physics. The present Thesis is an endeavor to understand linear and non-linear flow behavior and low Reynolds number instabilities in various soft matter systems like suspensions of flocculated carbon nanotubes and carbon black, surfactant gels, colloidal glasses, Langmuir monolayers etc probed mainly by bulk and interfacial rheology, in-situ light scattering, particle image velocimetry(PIV) techniques and Fourier transform rheology. We also use dynamic light scattering techniques for particle sizing and characterization of Brownian systems. Chapter 1 gives a general introduction to soft condensed matter, particularly, the important length and time scales, various interactions and the rich phase behavior emerged from the delicate balance between energy and entropy in these systems. In this context, We describe the detailed phase behavior of two such systems studied in this thesis. We next describe briefly a few important concepts which motivate the main problems studied in the present thesis like the shear-thickening in suspensions of Brownian and non-Brownian particles, non-equilibrium steady state fluctuation relations in driven systems, elasticity driven instabilities in complex fluids, jamming transitions and aging behavior. This is followed by a discussion of the experimental techniques like linear and nonlinear rheology, including the Fourier transform rheology. Chapter 2 discusses the experimental techniques used by us in detail. We first describe the different components and mode of operations of the MCR-300 stress-controlled rheometer (Paar Physica, Germany) and various experimental geometries. Next we discuss the set up for two dimensional rheological measurements. The homebuilt imaging set up for in-situ polarized light scattering and direct imaging studies is described along with the in-situ particle image velocimetry (PIV) to map out the exact spatially resolved velocity profiles in 2D systems. We give a brief account of the techniques of Fourier transform rheology. At the end of this chapter, we briefly describe the angle resolved dynamic light scattering (DLS) set up (Brookhaven Instruments, USA). In Chapter 3, we study colossal discontinuous shear-thickening transition in confined suspensions of fractal clusters formed by multi-wall carbon nanotubes (MWNT) by rheology and in-situ imaging experiments. Monotonic decrease in viscosity with increasing shear stress, known as shear thinning, is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. In the present experiments we demonstrate a discontinuous shear thickening transition where the viscosity jumps sharply above a critical shear stress by four to six orders of magnitude in flocculated suspensions of MWNT even at very low weight fractions(∼0.5%). Rheo-optical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear thickening phenomena in confined geometries is pertinent for flow controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues. We try to understand the flow of jammed and shear-thickened states under constant applied strain rate by studying the building up and relaxation of individual stress fluctuation events similar to the flow in dense granular materials. We also characterize the metastable shear thickened states by superposing a small sinusoidal stress component on a steady applied stress as well as by studying the a thermal entropy consuming fluctuations which are also observed for other jammed systems under an applied steady shear stress as described in the next chapter. Chapter 4 reports the study of non-equilibrium fluctuations in concentrated gels and glassy systems(in jammed state), the nature of fluctuations and their systemsize dependence in the framework of fluctuation relation and Generalized Gumbel distribution. In the first part, we show that the shear rate at a fixed shear stress in a micellar gel in a jammed state exhibits large fluctuations, showing positive and negative values, with the mean shear rate being positive. The resulting probability distribution functions (PDFs) of the global power flux to the system vary from Gaussian to non-Gaussian, depending on the driving stress and in all cases show similar symmetry properties as predicted by Gallavotti-Cohen steady state fluctuation relation. The fluctuation relation allows us to determine an effective temperature related to the structural constraints of the jammed state. We have measured the stress dependence of the effective temperature. Further, experiments reveal that the effective temperature and the standard deviation of the shear rate fluctuations increase with the decrease of the systemsize. In the second part of this chapter, we report a universal large deviation behavior of spatially averaged global injected power just before the rejuvenation of the jammed state formed by an aging suspension of laponite clay under an applied stress. The probability distribution function (PDF) of these entropy consuming strongly non-Gaussian fluctuations follow an universal large deviation functional form described by the Generalized Gumbel (GG) distribution like many other equilibrium and non-equilibrium systems with high degree of correlations but do not obey Gallavotti-Cohen Steady State Fluctuation Relation (SSFR). However, far from the unjamming transition (for smaller applied stresses) SSFR is satisfied for both Gaussian as well as non-Gaussian PDF. The observed slow variation of the mean shear rate with system size supports a recent theoretical prediction for observing GG distribution. We also establish the universality of the observations reported in this chapter in the light of other jammed systems under shear. We examine in the first part of Chapter 5, the shear-thinning behavior of a two dimensional yield stress bearing monolayer of sorbitan tristearate at air/water interface. The flow curve (stress vs shear rate) consists of a linear region at low shear stresses/shear rates, followed by a stress plateau at higher values. The velocity profile obtained from particle imaging velocimetry indicates that shear banding occurs showing coexistence of fluidized region near the rotor and solid region with vanishing shear-rate away from the rotor. In the fluidized region, the velocity profile which is linear at low shear rates becomes exponential at the onset of shear-thinning, followed by a time varying velocity profile in the plateau region. At low values of constant applied shear rates, the viscosity of the film increases with time, thus showing aging behavior like in soft glassy three-dimensional (3D) systems. Further, at the low values of the applied stress in the yield stress regime, the shear-rate fluctuations in time show both positive and negative values, similar to that observed in sheared 3D jammed systems. By carrying out a statistical analysis of these shear-rate fluctuations, we estimate the effective temperature of the soft glassy monolayer using the Galavatti-Cohen steady state fluctuation relation. In the second part of this chapter, we study in detail the non-linear viscoelastic behavior of Langmuir monolayers. Under oscillatory shear usually observed in many 3D metastable complex fluids with large structural relaxation times. At large strain amplitudes(γ), the storage modulus (G”) decreases monotonically whereas the loss modulus (G”) exhibits a peak above a critical strain amplitude before it decreases at higher strain amplitudes. The power law decay exponents of G” and G” are in the ratio 2:1. The peak in G” is absent at high temperatures and low concentration of sorbitan tristearate. Strain-rate frequency sweep measurements on the monolayers do indicate a strain-rate dependence of the structural relaxation time. The present study on sorbitan tristearate monolayers clearly indicates that the nonlinear viscoelastic behavior in 2D Langmuir monolayers is very general and exhibits many of the features observed in 3D complex fluids. We report in the first part of Chapter 6 scattering dichroism experiments to quantify the spatio-temporal nematodynamics of shear-thinning worm like micellar gels of surfactant Cetyltrimethylammonium Tosylate (CTAT) in the presence of salt sodium chloride (NaCl) enroute to rheochaos. For shear rates past the plateau onset, we observe a presence of alternating bright and dark‘ intertwined’ birefringent structures along the vorticity direction. The orientational order corresponding to these structures are predominantly oriented at +45deg and−45deg to the flow (v) in the (v,∇v) plane. The orientational dynamics of the nematics especially at the interface between the structures, has a one-to-one correspondence with the temporal behavior of the stress. Experiments show that the spatial motion of the vorticity structures depend on the gap thickness of the Couette cell. We next discuss the random temporal flow behavior of this system at high values of applied shear rate/stress in the framework of elastic turbulence in the second part of this chapter. Here, we study the statistical properties of spatially averaged global injected power fluctuations for the worm-like micellar system described above. At sufficiently high Weissenberg numbers (Wi) the shear rate and hence the injected power p(t) at a constant applied stress shows large irregular fluctuations in time. The nature of the probability distribution function (PDF) of p(t) and the power-law decay of its power spectrum are very similar to that observed in recent studies of elastic turbulence for polymer solutions. Remarkably, these non-Gaussian pdf scan be well described by an universal large deviation functional form given by the Generalized Gumbel (GG) distribution observed in the context of spatially averaged global measures in diverse classes of highly correlated systems. We show by in-situ rheology and polarized light scattering experiments that in the elastic turbulent regime the flow is spatially smooth but random in time, in agreement with a recent hypothesis for elastic turbulence. In Chapter 7, we study the vorticity banding under large amplitude oscillatory shear (LAOS) in a dilute worm-like micellar gel formed by surfactant CTAT by Fourier transform rheology and in-situ polarized light scattering. Under LAOS we found the signature of a non-trivial order-disorder transition of Taylor vortices. In the non-linear regime, higher harmonicde composition of the resulting stress signal reveals that the third harmonic I3 shows a very prominent maximum at the strain value where the number density (nv) of the Taylor vortices is maximum for a wide range of angular frequencies both above and below the linear crossover point. Subsequent increase in applied strain results in distortions of the vortices and a concomitant decrease in nv when I3 also drops very sharply and acts like an order parameter for this order-disorder transition. We further quantify the transition by defining an independent order parameter like quantity from the spatial correlation function of the scattered intensity and equivalently its Fourier transform which essentially captures the non monotonous third harmonic behavior. Lissajous plots indicate an intra-cycle strain hardening for the values of γ corresponding to the peak of I3 similar to that observed for hard-sphere glasses. Our study is an important step forward to correlating the structures developed in the system under LAOS to the appearances of the higher harmonics in the non-linear regime. The Thesis concludes with a summary of the main results and a brief account on the scope of future work as described in Chapter 8.