Hydrodynamics is Needed to Explain Propulsion in Chemophoretic Colloidal Rafts

Active particles driven by a chemical reaction are the subject of intense research to date due to their rich physics, being intrinsically far from equilibrium, and their multiple technological applications. Recent attention in the field is now shifting towards exploring the fascinating dynamics of mixture of active and passive systems. Here we realize active colloidal rafts, composed of a single catalytic particle encircled by several shells of passive microspheres assembled via light activated, chemophoretic flow. We show that considering only diffusiophoresis can explain the cluster kinetics but not the cluster propulsion behavior. Thus, using the Lorenz reciprocal theorem, we show that propulsion emerges by considering hydrodynamics via the diffusioosmotic answer of the substrate to the generated chemophoretic flow. While diffusioosmotic flows are often relegate to a secondary role, our work demonstrates their importance to understand the rich physics of active catalytic systems

Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks

International audienceSolid undeformable particles surrounded by a liquid medium or interface may propel themselves by altering their local environment. Such nonmechanical swimming is at work in autophoretic swimmers, whose self-generated field gradient induces a slip velocity on their surface, and in interfacial swimmers, which exploit unbalance in surface tension. In both classes of systems, swimmers with intrinsic asymmetry have received the most attention but self-propulsion is also possible for particles that are perfectly isotropic. The underlying symmetry-breaking instability has been established theoretically for autophoretic systems but has yet to be observed experimentally for solid particles. For interfacial swimmers, several experimental works point to such a mechanism, but its understanding has remained incomplete. The goal of this work is to fill this gap. Building on an earlier proposal, we first develop a point-source model that may be applied generically to interfacial or phoretic swimmers. Using this approximate but unifying picture, we show that they operate in very different regimes and obtain analytical predictions for the propulsion velocity and its dependence on swimmer size and asymmetry. Next, we present experiments on interfacial camphor disks showing that they indeed self-propel in an advection-dominated regime where intrinsic asymmetry is irrelevant and that the swimming velocity increases sublinearly with size. Finally, we discuss the merits and limitations of the point-source model in light of the experiments and point out its broader relevance

Dynamique des nageurs interfaciaux

The self-propelled objects are systems using the energy from their environment or carrying their fuel, to transport themselves. They are ubiquitous since our environment is full of mechanical machines or even living beings. Medical issues such as the controlled delivery of medicine, or issues related to the active matter, such as the spontaneous organisation of different scales objects, have led to intense research activity to elaborate new strategies allowing the self-propelling for various sizes systems. Among these strategies, this thesis focuses on a specific class, the interfacial swimmers. The term “swimmer" refers here to the fluid environment in which the moving being operates, more accurately, to the interface between two fluids as underlined by the term “interfacial". This interface is characterised by surface energy, the surface tension, which can be considered as a linear force. To propel themself, the interfacial swimmer induces a surface tension gradient by releasing a chemical specie, a surfactant, or some heat. The classic case is a swimmer propelling at the water-air interface by discharging a surfactant. Even though this type of propulsion was described in 1686, the same century as the vapour machine, the understanding is still low, quantitatively but also qualitatively. Thus, the behaviour prediction as a function of the object characteristic - for instance, the shape - remain partial. Will we obtain a motion? Will it be a translation or a rotation? In which direction? This thesis tries to address those questions by considering the dynamics of a symmetrical object: a disk. To do so, we have developed a complete approach: experimental, theoretical and even numerical, through the elaboration of toy models and simulations implementationLes objets autopropulsés, capables de se mouvoir en exploitant une source d'énergie embarquée ou présente dans leur milieu, sont courants dans notre environnement de par les machines mécaniques ou même les êtres vivants qui le peuplent. Pourtant, des problématiques médicales telle que la délivrance contrôlée de médicaments, ou liées à la matière active, telle que la structuration ou l'organisation spontanée d'objets à différentes échelles, ont amené une intense activité sur l'élaboration de nouvelles stratégies permettant l'auto-propulsion d'objets de taille variées. Parmi ces stratégies, cette thèse s'intéresse à une classe spécifique que sont les nageurs interfaciaux. Le terme “nageur” renvoie ici au milieu fluide dans lequel évolue le mobile, et plus exactement à l'interface entre deux fluides comme souligné par le terme “interfacial”. Cette interface est caractérisée par une énergie surfacique, la tension de surface, qui peut aussi être considérée comme une force linéaire. Pour se propulser les nageurs interfaciaux génèrent un gradient de tension de surface par libération d'une espèce chimique, le tensioactif, ou de chaleur. Le cas classique étant un nageur se propulsant à l'interface eau-air en relarguant un tensioactif. Paradoxalement, bien que cette forme de propulsion ait été identifiée en 1686, au même siècle que la machine à vapeur, ses caractéristiques n'en restent pas moins partiellement comprises quantitativement mais aussi qualitativement. Ainsi la simple prédiction du comportement d'un objet en fonction de ses caractéristiques - de forme par exemple - reste très partielle. Obtiendra-t-on un mouvement ? Sera-t-il essentiellement translationnel ou rotationnel ? Dans quelle direction ?Ce sont ces questions simples que cette thèse a tenté d'aborder en considérant la dynamique d'un objet parfaitement symétrique : un disque. Pour ce faire, nous avons développé une approche complète du problème en combinant des considérations aussi bien expérimentales que théoriques et même numériques via l'élaboration de modèles-jouets et l'implémentation de simulation

Spontaneous spinning of a dichloromethane drop on an aqueous surfactant solution

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