Daphne Klotsa

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

Living matter, such as biological tissue, can be seen as a nonequilibrium hierarchical assembly of assemblies of smaller and smaller active components, where energy is consumed at many scales. The functionality and versatility of such living or “active-matter” systems render it a promising candidate to study and to synthetically design. While many active-matter systems reside in fluids (solution, blood, ocean, air), so far, studies that include hydrodynamic interactions have focussed on microscopic scales in Stokes flows, where the active particles are <100μm and the Reynolds number, Re <<1. At those microscopic scales, viscosity dominates and inertia can be neglected. However, what happens as swimmers slightly increase in size (say ~0.1mm-100cm) or as they form larger aggregates and swarms? The system then enters the intermediate Reynolds regime where both inertia and viscosity play a role, and where nonlinearities in the fluid are introduced. In this talk, I will present a simple model swimmer used to understand the transition from Stokes to intermediate Reynolds numbers, first for a single swimmer, then for pairwise interactions, and finally for collective behavior. We show that, even for a simple model, inertia can induce hydrodynamic interactions that generate novel phase behavior, steady states, and transitions. References: 1) D. Klotsa. As Above, So Below, and also in Between: Mesoscale active matter in fluids. Soft Matter invited Perspective 15, 8946 (2019). 2) T. Dombrowski and D. Klotsa. Kinematics of a simple reciprocal model swimmer at intermediate Reynolds numbers. Phys. Rev. Fluids 5, 063103 (2020). (Editor's suggestion). 3) Dombrowski, T; Jones, S. K.; Katsikis, G.; Bhalla, A. P. S.; Griffith, B. E.; Klotsa, D. Transition in Swimming Direction in a Model Self-Propelled Inertial Swimmer. Phys Rev Fluids 2019, 4, 021101 4) Klotsa, D; Baldwin, K. A; Hill, R. J. A; Bowley, R. M.; Swift, M. R. Propulsion of a Two-Sphere Swimmer. Physical Review Letters 2015, 115, 248102