Manipulation of Colloids by Osmotic Forces
Résumé
Diffusio-phoresis --the motion of a particle or a macromolecule in a solute concentration gradient-- is an interfacially-driven transport phenomenon. Physically, it results from an osmotic pressure imbalance within the thin (ca. 1-10nm) diffuse layer located at the particle surface. Quite remarkably, it appears to be an efficient means of driving and manipulating colloidal particles. In this contribution, we present the results of our experimental study of this largely unexplored phenomenon. The thorough study of the diffusio-phoretic motility of colloids, as well as biological macromolecules, requires the building up of a spatially and temporally controlled solute gradient. Accordingly, a hydrogel microfluidics device was developed to generate a convection-free and spatiotemporal gradient. Particle migration is characterized and found to be in agreement with theoretical predictions. Furthermore, novel patterning mechanisms of particles are unveiled: trapping by rectification of concentration oscillations and particles localization by osmotic shock. They originate in the specific logarithmic sensing of the diffusio-phoretic velocity with electrolyte: non linear and logarithmic. Both observations are rationalized by numerical and theoretical predicitions. Analogies with positioning mechanisms in biology are finally discussed in an iconoclast questioning. Additionally we take advantage of diffusio-phoresis as an energy transduction mechanism to produce self-propelled particles. Inducing solute gradients by an asymmetric catalysis on the colloidal surfaces themselves, the namely Janus particles are chemically powered in the presence of fuel. In other words, they exhibit self-diffusio-phoretic propulsion. Through hydrodynamics and chemical interactions, these active particles are used as toy-models in the study of collective behavior and out-of-equilibrium statistical physics. The dynamics of individuals is carefully characterized as a persistent random walk. It is then connected to the statistical properties of an active suspensions of swimmers under gravity, reminiscent of Perrin's historic experiment. Our study tests the validity of the Fluctuation-Dissipation Theorem allowing the measurement of an effective temperature for the active system. This, in turn, is linked with the microscopic properties of the active suspension defining a Peclet number for the swimmers.
Thèse soutenue en Anglais
Domaines
Physique [physics]
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