Hydrodynamic genesis of colloidal creatures

When colloidal particles are rotating adjacent to a nearby floor, strong advective flows are generated around them, even quite far away. When a group of these microrollers is driven, the strong hydrodynamic coupling between particles leads to formation of new structures. Our experimental observation show that a suspension of microrollers undergoes a cascade of instabilities: An initially uniform front of microrollers evolves first into a shock-like structure, which then quickly becomes unstable, emitting fingers of a welldefined wavelength; then the fingertips pinch off to form compact motile structures translating at high speed. These colloidal creatures are self-sustained and form a stable state of the system. Combining experiments, large scale numerical simulations and continuum models, we detail the mechanisms involved at each step. We demonstrate that the whole process is primarily controlled by a geometric parameter: The height of the particles above the floor. We also explain the predominant role of hydrodynamic collective effects in the development of these colloidal creatures. To conclude, we show how to use these creatures for particle transport and flow generation in complex environments.