Laboratoire Matière et Systèmes Complexes

Video Seminar. Monday 29th June 14h00 Contact Michael Berhanu to attend the video seminar.

Motility-Induced Solidification in roller Flocks

David Martin, MSC, Université de Paris

The emergence of collective motion is a well-established phenomenon in living systems, be it among developed organisms such as fish swarms or starling flocks, or among simpler ones such as bacteria. Although the microscopic mechanisms entailing the transition toward collective motion typically depend on the system, some features are universal, and, as such, can be captured by simple microscopic models. The Vicsek model has proven itself such a minimal description, able to predict important features of flocks further confirmed by different experimental realizations(shaken grains, rollers...). Unfortunately, it does not encompass steric effects and hence fails to describe assemblies at higher densities.

A new physics is however expected in such cases, since repulsive forces, by themselves, can trigger phase transitions such as motility-induced phase separation (MIPS). I will present theoretical and experimental results on how the flocking transition interplays with steric interactions at high density. In particular, I will show how active solids emerge in flocking states made by Quincke colloidal rollers. Particles in the solid are effectively arrested, but the solid collectively propagates upstream due to its rich boundary dynamics.

References [1] D. Geyer, D. Martin, J. Tailleur, D. Bartolo, arXiv:1903.01134 (2019) [2] D. Geyer, A. Morin, D. Bartolo, Nature materials 17, 789 (2018)