Tanumoy Dhar, University of California, San Diego
Active fluctuations, transport and directed assembly of passive colloids by run-and-tumble microswimmers
The mechanism of colloidal aggregation is of significant interest in a diverse range of physical phenomena. In the classical scenario of Brownian motion, colloidal self-assembly is driven by diffusion of microscopic constituents, for example, a suspension of particles in a fluid. Upon aggregation, these particles collide and stick together, forming clusters. Clusters formed solely due to the Brownian motion of their component often demonstrate a slow development of the microstructure. Recent works have shown that active particles show promise in driving colloidal self-assembly. However, the mechanisms governing the out-of-equilibrium assembly of passive colloids suspended in a bacterial suspension are not yet fully understood. This clustering phenomenon is a direct consequence of the interplay of enhanced colloidal diffusion [1] and active depletion forces [2] induced by the run-and-tumble microswimmers. As a first step, we analyze the dispersion of a passive colloid immersed in a bath of non-interacting and non-Brownian run-and-tumble microswimmers in two dimensions using stochastic simulations and a statistical theory, both based on a minimal model of swimmer-colloid collisions characterized solely by frictionless steric interactions [1]. Long-time stochastic simulations reveal the existence of an effective attractive force between a pair of fixed colloids [2], akin to the depletion force arising in the suspensions of polymer coils. Finally, we adapt our computational model accounting for large-scale particle simulations, where we study the kinetics of aggregation and fragmentation for colloids subject to short-ranged tunable adhesive interactions and explore the phase behavior of the colloid-swimmer mixture. We also account for the curvature in the circular trajectories of the swimming bacteria, for example, E. Coli, which emerges from their hydrodynamic interaction with solid surfaces. The long-time statistics of these colloidal aggregates exhibit slow persistent rotations, which we explain using a semi-analytical model.
References:
- Dhar, T. and Saintillan, D., 2024. Active transport of a passive colloid in a bath of run-and-tumble particles. Scientific Reports, 14(1), p.11844.
- Dhar, T. and Saintillan, D., 2024. Active depletion forces and agglomeration of adhesive colloids by run-and-tumble microswimmers. (In preparation).