Abstract

The collective dynamics of swimming microorganisms is often dictated by long-ranged hydrodynamic interactions (HIs). One example is the collective motion of swimming, rear-actuated (“pusher”) bacteria that interact through their long-ranged dipolar flow fields to create a state of so-called bacterial turbulence with chaotic, collective swimming with long-ranged correlations. This phenomenology contrasts with the behaviour of front-actuated (“puller”) organisms such as certain algae, that do not exhibit any collective motion in unbounded bulk systems. For pushers, this state of collective motion is widely understood as a manifestation of a hydrodynamic instability due to the mutual alignment of pusher microswimmers due to HIs. Unlike this idealised bulk setting, many experimental realisations of microswimmers instead involves swimming close to a solid surface or air-liquid interface, which effectively confines the swimmers’ motion to a 2D plane. As I will show in this talk, this restriction of the dynamics qualitatively changes the collective motion compared to bulk systems. For pushers, the long-ranged hydrodynamic instability leading to bacterial turbulence in 3D is instead rendered short-ranged, with a collective state characterised by vortices of the order of the swimmer size. Additionally, we demonstrate a previously unknown density instability of confined puller microswimmers, which has no counterpart in unbounded systems. This instability is driven by swimmer advection and leads to phase separation into dense puller clusters for high enough densities. Our results thus highlight that accounting the experimental geometry can have a crucial impact on collective phenomena in active matter dominated by hydrodynamic interactions.