Littérature scientifique sur le sujet « Self-diffusion and self-propulsion »

Abstract Depending on their mechanism of self-propulsion, active particles can exhibit time-dependent, often periodic, propulsion velocity. The precise propulsion velocity profile determines their mean square displacement and their effective diffusion coefficient at long times. Here, we demonstrate that any periodic propulsion profile results in a larger diffusion coefficient than the corresponding case with constant propulsion velocity. We investigate, in detail, periodic exponentially decaying velocity pulses, expected in propulsion mechanisms based on sudden absorption of finite amounts of energy. We show, both analytically and with numerical simulations, that in these cases the effective diffusion coefficient can be arbitrarily enhanced with respect to the case with constant velocity equal to the average speed. Our results may help interpret, in a new light observations on the diffusion enhancement of active particles.

Self-propelled nanomotors have shown enormous potential in biomedical applications. Herein, we report on a nanozyme-powered cup-shaped nanomotor for active cellular targeting and synergistic photodynamic/thermal therapy under near-infrared (NIR) laser irradiation. The nanomotor is constructed by the asymmetric decoration of platinum nanoparticles (PtNPs) at the bottom of gold nanocups (GNCs). PtNPs with robust peroxidase- (POD-) like activity are employed not only as propelling elements for nanomotors but also as continuous O2 generators to promote photodynamic therapy via catalyzing endogenous H2O2 decomposition. Owing to the Janus structure, asymmetric propulsion force is generated to trigger the short-ranged directional diffusion, facilitating broader diffusion areas and more efficient cellular searching and uptake. This cascade strategy combines key capabilities, i.e., endogenous substrate-based self-propulsion, active cellular targeting, and enhanced dual-modal therapy, in one multifunctional nanomotor, which is crucial in advancing self-propelled nanomotors towards eventual therapeutic agents.

By means of coordinate transformation and the volume-of-fluid-level set multiphase flow method, a three-dimensional multiphase numerical model is established to simulate a Marangoni self-driven object. The forces on the Marangoni self-driven object are discussed as the driving force, viscous resistance, and pressure resistance. A typical disk-shaped, Marangoni self-driven object driven by the diffusion of camphor from its tail to water is utilized to perform a numerical study. Its motion evolution and force change in the whole process are represented quantitatively alongside the flow field and camphor concentration distribution in the flow domain. Meanwhile, the influence of Marangoni convection, which is induced by camphor diffusion at the moving gas–liquid interface, on surfer motion is surveyed. The results presented in this work can improve understanding of self-driven Marangoni propulsion since self-driven object motion and fluid movement details are difficult to acquire experimentally.

Catalytic micromotors can be used to detect molecules of interest in several ways. The straightforward approach is to use such motors as sensors of their “fuel” (i.e., of the species consumed for self-propulsion). Another way is in the detection of species which are not fuel but still modulate the catalytic processes facilitating self-propulsion. Both of these require analysis of the motion of the micromotors because the speed (or the diffusion coefficient) of the micromotors is the analytical signal. Alternatively, catalytic micromotors can be used as the means to enhance mass transport, and thus increase the probability of specific recognition events in the sample. This latter approach is based on “classic” (e.g., electrochemical) analytical signals and does not require an analysis of the motion of the micromotors. Together with a discussion of the current limitations faced by sensing concepts based on the speed (or diffusion coefficient) of catalytic micromotors, we review the findings of the studies devoted to the analytical performances of catalytic micromotor sensors. We conclude that the qualitative (rather than quantitative) analysis of small samples, in resource poor environments, is the most promising niche for the catalytic micromotors in analytical chemistry.

Phoretic swimmers are a class of artificial active particles that has received significant attention in recent years. By making use of self-generated gradients (e.g. in temperature, electric potential or some chemical product) phoretic swimmers are capable of self-propulsion without the complications of mobile body parts or a controlled external field. Focusing on diffusiophoresis, we quantify in this paper the mechanisms through which phoretic particles may achieve chemotaxis, both at the individual and the non-interacting population level. We first derive a fully analytical law for the instantaneous propulsion and orientation of a phoretic swimmer with general axisymmetric surface properties, in the limit of zero Péclet number and small Damköhler number. We then apply our results to the case of a Janus sphere, one of the most common designs of phoretic swimmers used in experimental studies. We next put forward a novel application of generalised Taylor dispersion theory in order to characterise the long-time behaviour of a population of non-interacting phoretic swimmers. We compare our theoretical results with numerical simulations for the mean drift and anisotropic diffusion of phoretic swimmers in chemical gradients. Our results will help inform the design of phoretic swimmers in future experimental applications.

Propulsion of otherwise passive objects is achieved by mechanisms of active driving. We concentrate on cases in which the direction of active drive is subject to spontaneous symmetry breaking. In our case, this direction will be maintained until a large enough impulse by an additional stochastic force reverses it. Examples may be provided by self-propelled droplets, gliding bacteria stochastically reversing their propulsion direction, or nonpolar vibrated hoppers. The magnitude of active forcing is regarded as constant, and we include the effect of inertial contributions. Interestingly, this situation can formally be mapped to stochastic motion under (dry, solid) Coulomb friction, however, with a negative friction parameter. Diffusion coefficients are calculated by formal mapping to the situation of a quantum-mechanical harmonic oscillator exposed to an additional repulsive delta-potential. Results comprise a ditched or double-peaked velocity distribution and spatial statistics showing outward propagating maxima when starting from initially concentrated arrangements.

Swimming micro-organisms rely on effective mixing strategies to achieve efficient nutrient influx. Recent experiments, probing the mixing capability of unicellular biflagellates, revealed that passive tracer particles exhibit anomalous non-Gaussian diffusion when immersed in a dilute suspension of self-motile Chlamydomonas reinhardtii algae. Qualitatively, this observation can be explained by the fact that the algae induce a fluid flow that may occasionally accelerate the colloidal tracers to relatively large velocities. A satisfactory quantitative theory of enhanced mixing in dilute active suspensions, however, is lacking at present. In particular, it is unclear how non-Gaussian signatures in the tracers' position distribution are linked to the self-propulsion mechanism of a micro-organism. Here, we develop a systematic theoretical description of anomalous tracer diffusion in active suspensions, based on a simplified tracer-swimmer interaction model that captures the typical distance scaling of a microswimmer's flow field. We show that the experimentally observed non-Gaussian tails are generic and arise owing to a combination of truncated Lévy statistics for the velocity field and algebraically decaying time correlations in the fluid. Our analytical considerations are illustrated through extensive simulations, implemented on graphics processing units to achieve the large sample sizes required for analysing the tails of the tracer distributions.

In the real world, active agents interact with surrounding passive objects, thus introducing additional degrees of complexity. The relative contributions of far-field hydrodynamic and near-field contact interactions to the anomalous diffusion of passive particles in suspensions of active swimmers remain a subject of ongoing debate. We constructed a quasi-two-dimensional microswimmer–colloid mixed system by taking advantage of Serratia marcescens’ tendency to become trapped at the air–water interface to investigate the origins of the enhanced diffusion and non-Gaussianity of the displacement distributions of passive colloidal tracers. Our findings reveal that the diffusion behavior of colloidal particles exhibits a strong dependence on bacterial density. At moderate densities, the collective dynamics of bacteria dominate the diffusion of tracer particles. In dilute bacterial suspensions, although there are multiple dynamic types present, near-field contact interactions such as collisions play a major role in the enhancement of colloidal transport and the emergence of non-Gaussian displacement distributions characterized by heavy exponential tails in short times. Despite the distinct types of microorganisms and their diverse self-propulsion mechanisms, a generality in the diffusion behavior of passive colloids and their underlying dynamics is observed.

Mimer les propriétés des cellules vivantes dans des protocellules artificielles suscite un intérêt considérable, notamment pour reproduire la motilité et le mouvement directionnel dans des applications de thérapies « intelligentes ». En raison de leur morphologie vésiculaire et de leur stabilité, les polymersomes présentent un grand potentiel pour la délivrance de médicaments, et l'introduction d'une asymétrie est essentielle pour permettre leur auto-propulsion. Bien que plusieurs approches, telles que la séparation de phase au sein de la membrane, aient été utilisées pour créer des polymersomes asymétriques, le choix des polymères appropriés reste un défi. Cette thèse de doctorat vise à concevoir des polymersomes asymétriques, de type Janus, capables de s'auto-propulser grâce à la décomposition enzymatique du glucose. Nous décrivons le développement de vésicules géantes unilamellaires de type Janus (JGUVs) par séparation de phase au sein de la membrane de deux copolymères à blocs distincts composés de blocs hydrophobes chimiquement incompatibles. En utilisant la théorie de Flory-Huggins, nous démontrons que les copolymères peuvent être rationnellement sélectionnés et conçus pour s'auto-assembler en polymersomes asymétriques, avec une séparation de phase modulable selon des paramètres tels que la composition, la masse molaire et la température. Notre méthode prédictive s'est avérée efficace pour les techniques d'auto-assemblage avec et sans solvant, permettant l'élaboration de diagrammes de phase génériques corrélant l'énergie libre de mélange à la morphologie des polymersomes, fournissant ainsi des indications clés pour la conception de JGUVs. Nous montrons également que la présence de solvant lors de la formation des vésicules permet d'étendre la gamme des polymères incompatibles pouvant être utilisés. De plus, nous avons réussi à contrôler, grâce à l'extrusion, la taille des vésicules tout en préservant leur morphologie Janus et avons montré que les JGUVs ainsi obtenus pouvaient être stables pendant plusieurs mois. Enfin, nous avons fonctionnalisé asymétriquement les JGUVs avec l'enzyme glucose oxydase par chimie click, et une étude préliminaire sur leur dynamique en présence de glucose est présentée, fournissant des indications pour leur utilisation comme micromoteurs
Mimicking the properties of living cells in artificial protocells has attracted significant interest, particularly for replicating motility and directional swimming for applications in smart therapeutics. Due to their vesicular and stable morphology, polymersomes hold great promise for drug delivery, and the introduction of asymmetry is crucial to enable self-propulsion. While several approaches, such as phase separation within the membrane, have been used to create asymmetric polymersomes, the selection of appropriate polymers remains a challenge. This PhD thesis aims at designing asymmetric, Janus-like polymersomes capable of self-propulsion, and powered by enzymatic glucose decomposition. We describe the development of Janus Giant Unilamellar Vesicles (JGUVs) through phase separation within the membrane of two distinct block copolymers comprising chemically incompatible hydrophobic blocks. We demonstrate, using the Flory-Huggins theory, that copolymers can be rationally selected and designed to self-assemble into asymmetric polymersomes, with tunable phase separation driven by parameters such as composition, molecular weight, and temperature. Our predictive method proves to be effective for both solvent-free and solvent-switch self-assembly processes, enabling the elaboration of generic phase diagrams correlating mixing free energy with polymersome morphology, providing valuable insights for JGUVs design. We also evidence that the presence of solvent during the vesicle formation broadens the range of incompatible polymers that can be used. Additionally, we successfully control, thanks to extrusion, the vesicle size while preserving their Janus morphology and evidence that the resulting JGUVs could be stable for several months. Furthermore, we asymmetrically functionalized JGUVs with glucose oxidase enzymes via click-chemistry, and a preliminary study on their dynamic behavior in the presence of glucose is presented, looking forward to their potential use as micromotors

We describe the motion of heated particles in a simple liquid, for which we can theoretically derive generalized fluctuation-dissipation relations that hold far from equilibrium, as we demonstrate both experimentally and via molecular-dynamics simulations. Due to persistent laser-light absorption, these particles excite a radially symmetric or asymmetric (Janus particles) temperature profile in the solvent, which affects their random (Brownian) and systematic (self-phoretic) motion. In case of a radially symmetric temperature profile, we show that the particles perform “hot Brownian motion” (HBM), with different effective temperatures pertaining to their various degrees of freedom. We moreover predict and experimentally observe a peculiar dependence of their diffusivity on the particle size. In case of an asymmetric temperature profile, we find a superimposed self-phoretic directed motion. To adjust the importance of this “active” motion relative to the random hot Brownian motion, the shape of the particle is modified by binding DNA molecules and DNA origami to Janus beads. The persistence of the directed transport can thereby greatly be enhanced.

Dans cette thèse, nous étudions la diffusion d'auto-assemblages hybrides à base des virus en forme de filament et des nanoparticules d'or. Dans ce but, nous utilisons des bactériophages qui ont été génétiquement modifiés afin de posséder des groupements cystéines exposés à leur extrémité proximale. La présence des ponts disulfure à l'une de leurs pointes leur permet ainsi de se lier aux nanoparticules métalliques par une liaison covalente de coordination. La morphologie des différents colloïdes hybrides formés dépend de l'excès molaire initial entre virus et nanoparticules. Lorsque les deux éléments sont placés dans la même proportion, leur interaction conduit à la formation de particules en forme d'allumettes (« matchstick-like ») qui consiste en un virus de 1 _m long attaché par sa pointe à une seule nanoparticule d'or. Cependant, si les virus sont en grand excès, cela se traduit par la formation de structures colloïdales en étoile (« colloidal stars ») formées par plusieurs virus attachés à une seule nanoparticule d'or. La dynamique brownienne de ces structures en régime dilué et dense a été étudiée pardiffusion dynamique de la lumière et par suivi temporel des particules individuelles enmicroscopie optique. L'un des avantages majeurs de l'utilisation combinée de ces deux techniques est la possibilité d'étudier séparément les différents éléments qui composent les colloïdes hybrides. Plus précisément, la forte diffusion de la lumière associée aux nanoparticules d'or permet la détermination des propriétés dynamiques de la structure en observant principalement la nanoparticule liée, alors que le marquage du virus avec des colorants fluorescents permet la mesure de leurs coefficients de diffusion par microscopie optique. Nos résultats sur la diffusion des particules « matchstick-like » en fonction de la taille des nanoparticules soulignent la flexibilité de la liaison virus-nanoparticule. Étant donné que ces structures sont intrinsèquement asymétriques, nous avons aussi évalué la possibilité d'induireune autopropulsion par effet thermophorétique afin d'obtenir des colloïdes hybrides actifs. Nous avons également étudié quantitativement l'auto-organisation et la diffusion des « colloidal stars » en fonction de leur fraction volumique. Lorsque la concentration augmente, on observe un arrêt progressif de leur dynamique liée à l'interdigitation des virus qui suggère un état vitreux en régime dense. Dans la dernière partie de cette thèse, nous formulons de nouveaux mutants de bactériophages qui ont la propriété de se lier à des nanoparticules métalliques aux deux extrémités. Ceci conduit au développement de nouvelles superstructures hybrides avec un design plus versatile
In this thesis, we report on the study of the self-diffusion of hybrid gold-virus self-assemblies formed by spherical gold nanoparticles and rod-like viruses. For this purpose, we use genetically modified mutants of filamentous bacteriophages which possess disulfide groups (Cys-Cys) exposed at their proximal end. The presence of a disulfide bridge allows them to bind to metal nanoparticles by one of their tips forming a weak covalent bond. The control of the resulting self-assembled structures is achieved by tuning the molar excess of viruses with respect to nanoparticles. When both components are set in similar proportion, their interaction leads to the formation of matchstick-like particles composed by a 1 µm long virus attached by its tip to a single gold nanobead. However, if the viruses are in high excess, the resulting structures are colloidal stars formed by multiple viruses attached to a single gold nanobead.The Brownian dynamics of these structures is characterized in dilute and dense regimes both by Dynamic Light Scattering (DLS) and single particle tracking through optical microscopy. An advantage of using both techniques resides in the possibility to study separately the different components forming the hybrid particles. Specifically, the high scattering signal coming from the gold nanoparticle facilitates the determination of the dynamic properties of the structure by observing mostly the bounded bead, whereas the labeling with fluorescent dyes allows the direct determination of the virus diffusion coefficient by microscopy.Our findings on the self-diffusion of the matchstick-like particles as a function of the nanoparticle size evidence the flexibility of the virus-bead link. Considering the intrinsic asymmetry of the matchstick-like structure, the possibility to induce self-propulsion has been investigated in order to get active hybrid particles by overcoming their Brownian motion thanks to light or chemical fuels.We quantitatively study the self-organization and diffusion of the colloidal stars as a function of the volume fraction. When the later increases, a progressive dynamical arrest related to the interdigitation of the star viral arms is observed suggesting a glassy state in the dense regime.In the last part of this thesis, we construct optimized new mutants of filamentous bacteriophages which are engineered to bind metal nanoparticles to both tips. This results in the development of novel hybrid superstructures with more versatile design

We describe the motion of heated particles in a simple liquid, for which we can theoretically derive generalized fluctuation-dissipation relations that hold far from equilibrium, as we demonstrate both experimentally and via molecular-dynamics simulations. Due to persistent laser-light absorption, these particles excite a radially symmetric or asymmetric (Janus particles) temperature profile in the solvent, which affects their random (Brownian) and systematic (self-phoretic) motion. In case of a radially symmetric temperature profile, we show that the particles perform “hot Brownian motion” (HBM), with different effective temperatures pertaining to their various degrees of freedom. We moreover predict and experimentally observe a peculiar dependence of their diffusivity on the particle size. In case of an asymmetric temperature profile, we find a superimposed self-phoretic directed motion. To adjust the importance of this “active” motion relative to the random hot Brownian motion, the shape of the particle is modified by binding DNA molecules and DNA origami to Janus beads. The persistence of the directed transport can thereby greatly be enhanced.

Permanent magnet brushless motor drives (BLMD) are extensively used in electric vehicle (EV) propulsion systems because of their high power and torque to weight ratio, virtually maintenance free operation with precision control of torque, speed and position. An accurate dynamical parameter identification strategy is an essential feature in the adaptive control of such BLMD-EV systems where sensorless current feedback is employed for reliable torque control, with multi-modal penalty cost surfaces, in EV high performance tracking and target ranging. Application of the classical Powell Conjugate Direction optimization method is first discussed and its inaccuracy in dynamical parameter identification is illustrated for multimodal cost surfaces. This is used for comparison with the more accurate Fast Simulated Annealing/Diffusion (FSD) method, presented here, in terms of the returned parameter estimates. Details of the FSD development and application to the BLMD parameter estimation problem based on the minimum quantized parameter step sizes from noise considerations are provided. The accuracy of global parameter convergence estimates returned, cost function evaluation and the algorithm run time are presented. Validation of the FSD identification strategy is provided by excellent correlation of BLMD model simulation trace coherence with experimental test data at the optimal estimates and from cost surface simulation.

Thermal transpiration based propulsion is studied. Thermal transpiration describes flowing of the gas through a narrow channel with an imposed temperature gradient. As gas flows from the cold to hot side in the chamber, a pressure gradient is created across the channel induced by the temperature gradient. Between the two sides of the chamber an aerogel substance, which functions as an excellent insulator, is used as a thermal transpiration membrane and allows gas diffuse to the hot chamber. The induced pressure gradient is the driving factor in the propulsion of air, or any gas, into the chamber and through the porous membrane. The use of a porous substance such as aerogel as the transpiration membrane and a pressure gradient served as the two requirements in order to successfully achieve thermal transpiration. The gas diffusion through the aerogel transpiration membrane indicates that the average pore size of the aerogel must be comparable with the free path of the molecules. This concept can be taken further if the outlet chamber served as a combustion reactor. The flowing gas is motivated by the heat produced from the combustion process. Along with the exceptionally low thermal conductivity of the aerogel, the gas flow permits the propulsion device to be self-sustaining. The implications of providing a self-sustaining heat source signify that no external electrical heating is required. The effectiveness of this device can be measured as a function of the porous size of the membrane and the temperature difference applied to the system and pressure gradient created.