Chiral active systems near a substrate: Emergent damping length controlled by fluid friction

Abstract

Chiral active fluids show the emergence of a turbulent behaviour characterised by multiple dynamic vortices whose maximum size varies for each experimental system, depending on conditions not yet identified. We propose and develop an approach to model the effect of friction close to a surface in a particle based hydrodynamic simulation method in two dimensions, in which the friction coefficient can be related to the system parameters and to the emergence of a damping length. This length is system dependent, limits the size of the emergent vortices, and influences other relevant system properties such as the actuated velocity, rotational diffusion, or the cutoff of the energy spectra. Comparison of simulation and experimental results of a large ensemble of rotating colloids sedimented on a surface shows a good agreement, which demonstrates the predictive capabilities of the approach, which can be applied to a wider class of quasi-two-dimensional systems with friction. The dynamics of chiral active fluids is characterised by a multitude of interacting dynamic vortices whose maximum size varies for each system. Here we show how the friction induced by the substrate is related to a damping length which is ultimately responsible of limiting the maximum size of the vortices.