Laboratoire Matière et Systèmes Complexes

Motility-Induced Solidification in roller Flocks

David Martin

Abstract: 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. 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.

a- active solid propagating upstream in an experimental racetrack. b to f- close-up pictures of the different phases observed in the racetrack. The graphs below represent the density Φ and order parameter W as a function of the curvilinear coordinate for each corresponding phase.

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)

Friday July 12 at 12pm in room 454A