The genesis of "critters": a multi-part story

Courant Institute, NYU

Self-assembly in colloidal systems often requires finely tuning the interactions between particles. When colloids are active, or moving due to an external drive, the assembly is even harder to achieve. In this talk we will show that long-lived compact motile structures, called "critters'', can be formed just with hydrodynamic interactions with a wall. They naturally emerge from a fingering instability recently discovered in an experimental system of microrollers near a floor.

We will focus on the genesis of this phenomenon which involves a cascade of instabilities: the formation of a "shock" front due to the nonlocal nature of hydrodynamic interactions, the destabilization of this front which leads to the formation of fingers, which then detach to form moving persistent autonomous clusters called "critters". Combining nonlocal continuum models, linear stability analysis, 3D large scale simulations, and comparison with experiments, we will explain each step in detail and show how the distance of the particles to the nearby floor controls the dynamics of the system.

These critters are a persistent state of the system, move much faster than individual rollers, and quickly respond to a changing drive. The formation of critters is robust to any initial conditions and our experiments suggest that similar structures could be formed in a thermal colloidal system. We believe the critters are a promising tool for microscopic transport, flow, aggregation and mixing.