"Suspensions of Active Particles"

Conférence "Suspensions of Active Particles" de Jean-Françoiss JOANNY , Collège de France , Paris, organisé par l'Institut Charles Sadron.

Abstract:

We will consider the properties of colloidal particle suspensions which are not at thermodynamic equilibrium, and in which some unexpected effects appear.
In a first part, I will discuss a mixed solution of two types of particles that are in contact with thermostats at different temperatures. This system can be considered as a model for a mixture of passive colloidal particles and bacteria, but there are other examples as chromosomes in the nucleus of cells. The distance distribution between two particles satisfying a Langevin equation at different temperatures is given by Boltzmann’s law with an effective temperature. However, there is transfer of energy from the hot particle to the cold particle. A surprising result is that the equipartition of energy is not satisfied. Kinetic energy of the particles is given by an effective temperature and not by the temperature of the thermostat. I will also briefly discuss the conformation of polymers formed by binding together particles at different temperatures. At finite concentration, there is phase separation between two particles that have different temperatures, even if all their other properties are the same. At low concentration we can build an effective free energy allowing to study the phase diagram. This is no longer the case at higher concentration. I will discuss the case of hard spheres in treating interactions as depletion interactions.
In a second part, I will discuss Ornstein-Uhlenbeck particles which are propelled by a force with a finite memory over time. These particles can also be considered as very simplified models of bacteria. At one dimension we studied their behavior in a harmonic potential. We then used this result to discuss how these particles are pumped by an asymmetrical potential or the depletion forces created by forces created by these particles between two planes, which can be repulsive.

Les personnes souhaitant rencontrer JF Joanny sont priées de prendre contact avec Thierry Charitat (thierry.charitat@ics-cnrs.unistra.fr)