Lawrence Berkeley National Laboratory
Many microorganisms and artificial microswimmers (such as catalytic colloids) are able to move under their own power. They achieve this by constantly consuming fuel, forming a non-equilibrium state of matter. This is relevant both to real world applications such as nano-technology, as well as for posing fundamental questions in non-equilibrium statistical physics. Microswimmers are often modeled as active Brownian particles, neglecting hydrodynamic interactions between them. However, real microswimmers, such as ciliated microorganisms, catalytic Janus particles, or active emulsion droplets, employ propulsion mechanisms reliant on hydrodynamics. After giving an overview of different propulsion mechanisms, we to explore the influence of hydrodynamics on the collective behavior microswimmers in a quasi-two-dimensional geometry and on their motion under gravity. A striking consequence of active motion is that for sufficiently strong self-propulsion they phase separate into dense clusters coexisting with a low-density disordered surrounding. Here we examine the influence of hydrodynamic interactions on motility-induced phase separation. For a range of mean densities and Peclet numbers, we observe how the system decomposes into a dilute and a cluster phase, which then coarsens over time. Inspired by experiments and theoretical work, we also examine collective dynamics in the presence of gravity. We observe a rich phenomenology, depending not only on the relative strength of gravity but also on the long-range hydrodynamic interactions between swimmers and with the container's walls.