A touch of non-linearity: active matter in fluids at intermediate Reynolds
The complexity of emergent active-matter behavior has been demonstrated at many length-scales in both biological and artificial systems. However, a whole region of parameter space, that is mesoscale active matter, i.e. active matter of inertial particles in fluids at intermediate Reynolds numbers (Re), remains largely unexplored. The intermediate regime covers at least three orders of magnitude in Re (1-1000), opening up numerous possibilities for materials science, and describing a plethora of organisms, that we can study as model systems. In this talk, I will show how we are building a framework to study mesoscale active matter in fluids, starting with a classification of model inertial swimmers. I will present experiments and simulations of a reciprocal self-propelled swimmer made out of two unequal spheres. I will show what happens at the onset of inertia, where there is a transition from rest to swimming, and then demonstrate how this simple object actually switches direction as Re increases! The switch is a result of the nonlinearities that add up over a cycle at intermediate Re. I will discuss the relation with the pusher-numbers puller models in Stokes flows and the next steps for exploring collective behavior.