A touch of non-linearity: mesoscale swimmers and active matter in fluids*

While many active-matter systems reside in fluids (solution, blood, ocean, air), so far, studies that include hydrodynamic interactions have focussed either on microscopic scales in Stokes flows, or on large scales at high Reynolds numbers. What happens in between? In this talk, I will present my group's work on simple model swimmers we use to understand the transition from Stokes to intermediate Reynolds numbers, first for a single swimmer, then for pairwise interactions and finally for collective behavior and active matter. We show that, even for simple models, inertia can induce hydrodynamic interactions that generate novel phase behavior, steady states and transitions.