Active Colloids

Our first contact with active colloids is due to Denis Bartolo. He had found how to exploit the so-called Quince rotation, an electrokinetic effect known since 1896! It makes it possible to roll an insulating particle in a slightly conductive fluid under the application of a static electric field. The fascinating character of this system is that it makes it possible to set in motion a very large number of colloids, which also travel at speeds of the order of 100 to 1000 times their diameter /s. We have shown that this system exhibits a spectacular transition to collective motion very similar to that observed in the "Ising model of active matter", the Viscek model. Moreover, the transition takes place at a very small surface fraction. It follows that knowledge of electrostatic and hydrodynamic interactions at long distances is sufficient to describe quantitatively our observations. Bricard, A., Caussin, J.-B., Desreumaux, N., Dauchot, O., & Bartolo, D. (2013). Emergence of macroscopic directed motion in populations of motile colloids. Nature, 503(7474), 95–98.

Currently, in collaboration with Marjolaine van der Linden and Dirk Aarts (Oxford), we are interested in a system of Janus colloids, propelled by the effect of an oscillating electric field (self nonlinear di-electrophoresis) Developed by Prof. Sano of the University of Tokyo, and then perfected by Steve Granick. Everything is far from being understood on the mechanisms involved in this system, and it seems very rich in terms of the collective behaviors that it develops spontaneously.