Thèses

Jeudi 27 Octobre 2016 à 10h00.

Dynamical aspects of active colloids: from dilute to dense systems

''Collective motion are present at every scales and in very various biological systems. For example one can observe flocks of birds, schools of fishes, or swarms of insects. To be able to describe and understand these collective effects, it is necessary to have experimental abiotic model systems. In this PhD we present an experimental system made of Janus colloids of gold and platinum. When putted in an hydrogen peroxide bath, they set in motion, consuming fuel. This system is fully out of equilibrium because energy is consumed at the scale of individuals. It notably presents collective motion with the apparition of clusters, dynamical aggregates of active colloids at intermediate densities. In this PhD we studied this experimental system in detail, and for various activities and densities. We first characterized the size distribution and kinetics of clusters, and compared it to simple analytic models. Then we took a more thermodynamic approach, measuring for the first time an equation of state in an experimental active system, using sedimentation experiments. We notably show that at intermediate densities, active colloids can be described as hot adhesive hard spheres. Finally we studied dense assemblies of colloids, forming an "active glass". We highlighted that activity has a peculiar effect on the mobility of the system: at first activity freezes the system, but in second time it fluidizes it, increasing the mobility of the particles. This strange "back and forth" behavior cannot be correlated with changes on the structure of the system.''