Letteratura scientifica selezionata sul tema "Electrorotation de Quincke"

Particle rotation leads to a steady-state which is different from the equilibrium state in the absence of rotational motion. The change of the polarization of the particle due to the rotational motion is called the dynamic electrorheological effect (DER). There are three cases to be considered: rotating particles in a dc field, particle rotation due to a rotating field and spontaneous rotation of particle in dc field (Quincke rotation). In the DER of rotating particles, the particle rotational motion generally reduces the interparticle force between the particles. The effect becomes pronounced when the frequency is on the order of the relaxation rate of the surface charges. In the electrorotation of particles, the mutual interaction between approaching particles will change the electrorotation spectrum significantly. The electrorotation spectrum depends strongly on the medium conductivity as well as the conductivity contrast between the particle and the medium. In the collective behaviors of Quincke rotors, the mutual interactions between the individual rotors lead to the assembly of chain-like structures which make an angle with the applied field. This has an implication of a new class of material.

A dielectric drop suspended in an immiscible dielectric fluid of higher conductivity can spontaneously generate the so-called Quincke rotation (a rotating activity that a weakly conducting drop/solid particle displays in an electric field) subjected to sufficiently strong electric field strength. The steady tilt has been extensively studied and is well elucidated now. However, the unsteady electrorotation of drop remains a largely unclear, complex issue. Motivated by this, we examine the unsteady drop electrorotation in this work with the required integrated convective bulk charge transport effect. First, for the steady rotation, the transient evolution to a steady droplet tilt from the symmetric Taylor state is analyzed in-depth. Here we discover several new phenomena, including the evolving equatorial charge jets. For unsteady rotation, based on a drop's interfacial charge variation, deformation, and tilt angle, the study reports the growth of three distinct rotating patterns in the viscosity ratio range [Formula: see text] and electric field strength [Formula: see text] at a fixed conductivity ratio Q ( = [Formula: see text]) = 0.026 and permittivity ratio S (=[Formula: see text]) = 0.566. A low-viscosity drop ([Formula: see text]) exhibits only the periodic rotation. For the viscosity ratio [Formula: see text], the increased electric intensity creates two new unsteady rotation modes: the pseudo-periodic tumbling and the irregular one. For [Formula: see text], the periodic mode remains absent; instead, the drop displays the electric intensity-dependent tumbling and irregular rotation patterns. Our study shows that the rotation reduces a drop's transitory interfacial charge. At this stage, the drop rotation behavior is controlled by competing charge convection due to fluid flow and charge supply by conduction. The resulting varying electric Reynolds number [Formula: see text] (the time ratio of charge relaxation and charge convection) explains the created different rotation mechanisms. For [Formula: see text], owing to lacking enough interfacial charge to sustain rotation, the drop's transition to a temporary non-rotating Taylor state occurs until the interface recharges. The resultant mechanism supports the periodic batch-type rotation for a low-viscosity drop and the irregular rotation for a high-viscosity drop in a higher electric field. In contrast, for [Formula: see text], the drop timely acquires sufficient charge to support continuous tumbling electrorotation.

Weakly conducting dielectric liquid drops suspended in another dielectric liquid and subject to an applied uniform electric field exhibit a wide range of dynamical behaviours contingent on field strength and material properties. These phenomena are best described by the Melcher–Taylor leaky dielectric model, which hypothesizes charge accumulation on the drop–fluid interface and prescribes a balance between charge relaxation, the jump in ohmic currents from the bulk and charge convection by the interfacial fluid flow. Most previous numerical simulations based on this model have either neglected interfacial charge convection or restricted themselves to axisymmetric drops. In this work, we develop a three-dimensional boundary element method for the complete leaky dielectric model to systematically study the deformation and dynamics of liquid drops in electric fields. The inclusion of charge convection in our simulations permits us to investigate drops in the Quincke regime, in which experiments have demonstrated a symmetry-breaking bifurcation leading to steady electrorotation. Our simulation results show excellent agreement with existing experimental data and small-deformation theories.

Droplet collective propulsion is a crucial technology for microscale engineering applications. Despite great progress, current approaches to droplet manipulation still face many challenges. Here, a novel strategy for the collective propulsion of droplet pairs is proposed, which is based on two fundamental dynamics phenomena: i) the Quincke rotation; ii) the dynamics of vortex pairs. In this work, a two-dimensional (2D) numerical computation is performed to study the effect of viscosity ratio (λ = μi/μo ≤ 60, “i” and “o” indicate the drop and bulk phase) and electric field strength (E0*≤ 6.78) on the collectively propelling performance and reveal the propelled mechanisms of the droplet pair with fixed conductivity ratio Q (=σi/σo) = 0.01 and permittivity ratio S (=εi/εo) = 0.5. The novel approach to spontaneous propulsion proposed in this work achieves the remote manipulation of droplets without limiting the translation distance. The translation velocity can reach 2.0 mm/s for the examined cased in this work. In addition, the findings indicate that two factors determine the collective propulsion of droplet pairs: the strength of the Quincke vortex (Γ*) and the front vortex pair, which appears at the front end of the droplet pair and essentially counteracts the propulsion. For 5.0 < λ < 10, a weaker front vortex pair is generated. The increase in λ augments the strength of the Quincke vortex and in turn accelerates the collective propulsion. As 10 < λ < 28, the increasing λ results in a stronger front vortex pair and thus weakens the performance. As λ > 28, the direction of translation is reversed and the front vortex pair becomes weaker until it disappears completely at λ = 50. Thus, the increase in λ improves the collectively propelled performance in λ > 28. In addition, the effect of E0* on the collective propulsion is examined with varied λ (=8, 15, 50) and the fixed Q = 0.01, S = 0.5. The stronger E0* can lead to a faster translation. However, when the drop pair with the higher viscosity (λ = 50) is exposed to a stronger electric field (E0* = 5.08), two drops undergo irregular electrorotation (the direction of rotation changes alternately). The alternating up/down translation cannot produce the directional translation.

On définit la matière active comme une assemblée de particules capables de transformer à leur échelle l'énergie en mouvement. Les exemples de matière active sont nombreux dans la nature, allant de la colonie de bactérie au troupeau de zèbres en passant par les bancs de poissons et les foules humaines. Malgré ce mouvement perpétuel des individus, il est possible dans certains cas d'observer une séparation de phase, c'est-à-dire la formation de zones définies de densités différentes. Ceci peut s'expliquer par la détection de quorum : les particules tiennent compte de leurs voisines pour ajuster leur activité. Depuis une dizaine d'années, l'ensemble des briques élémentaires de la matière molle (polymères, colloïdes, ...) ont été motorisées pour fabriquer de la matière active en laboratoire. Cependant aucune forme de détection du quorum synthétique n'a été rapportée jusqu'à aujourd'hui. Dans cette thèse, nous présentons les premiers résultats permettant de montrer la possibilité de créer une forme simple de détection de quorum en laboratoire. Pour cela nous avons choisi comme élément de base un bâtonnet colloïdal. Nous présentons d'abord une analyse théorique expliquant le comportement de bâtonnets actifs. Cette analyse est une extension aux particules anisotropes du phénomène d'électrorotation de Quincke, déjà utilisé pour rendre des sphères actives. Elle permet de mettre en lumière le comportement plus riche des bâtonnets. Puis nous détaillons la démarche expérimentale pour la mise en œuvre concrète de la motorisation de ces colloïdes actifs, qui est au cœur de ces travaux de thèse. Enfin, nous rapportons les résultats obtenus, qui indiquent une première réalisation expérimentale de détection de quorum artificielle, avec notamment l'observation et la caractérisation d'une séparation de phase induite par la motilité conditionnelle des particules
Active matter is defined as an assembly of particles capable of transforming energy into movement on their own scale. There are many examples of active matter in nature, from a colony of bacteria to a flock of zebras, from school of fishes to human crowds. Despite this perpetual movement of individuals, it is possible in some cases to observe phase separation, i.e. the formation of defined zones of different densities. This can be explained by the detection of quorum: particles take account of their neighbors to adjust their activity. Over the last ten years or so, all the building blocks of soft matter (polymers, colloids, etc.) have been motorized to produce active materials in the laboratory. However, no form of synthetic quorum sensing has yet been reported. In this thesis, we present the first results demonstrating the possibility of creating a simple form of quorum sensing in the laboratory. For this purpose, we have chosen a colloidal rod as the basic element. We first present a theoretical analysis explaining the behavior of active rods. This analysis is an extension to anisotropic particles of Quincke's electrorotation phenomenon, already used to render spheres active. It sheds light on the richer behavior of rods. We then detail the experimental approach for the concrete implementation of motorization of these active colloids, which is at the heart of this thesis work. Finally, we report on the results obtained, which indicate a first experimental realization of artificial quorum sensing, including the observation and characterization of a phase separation induced by conditional particle motility