Séminaire interne - Suchismita Das (LPENS)

Abstract:
 

Spontaneous collective motion of self-propelled agents is a widespread phenomenon in nature, observed in systems ranging from bird flocks to bacterial colonies and replicated using synthetic constituents. In this talk, I will discuss two instances of collective behavior in systems of active colloids: flocking and wetting transitions.

While flocking was originally conceived to emerge through alignment interactions, we show that it can also emerge from interactions that reorient agents away from each other. Combining simulations, kinetic theory, and experiments, we demonstrate this mechanism of flocking in self-propelled Janus colloids with stronger repulsion on the front than on the rear. Unlike alignment-based interactions, turn-away interactions require particle repulsion for polar order to emerge. These findings could help to reconcile the observations that cells can flock despite turning away from each other via contact inhibition of locomotion. Our work shows that flocking is a very robust phenomenon that can occur even when the orientational interactions would seem to prevent it. In the second part, we explore a minimal model of active colloidal system near solid walls, exhibiting wetting transitions. By tuning the wall-particle interactions along with activity, it could be possible to control the spreading of the aggregates. Our analysis show that the mechanical solid-liquid interfacial tension plays an important role in determining the aggregate state. Detailed knowledge of such wetting transitions can be helpful in various scenarios, including designing surfaces to prevent microbial spreading.