Swimmer-tracer scattering at low Reynolds number

Abstract

Understanding the stochastic dynamics of tracer particles in active fluids is important for identifying the physical properties of flow generating objects such as colloids, bacteria or algae. Here, we study both analytically and numerically the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields. Specifically, we compare the tracer motion induced by an externally driven colloid with the one generated by various self-motile, multi-sphere swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories. The specific topological structure of these loops is determined by the hydrodynamic properties of the microswimmer. Quantitative estimates for typical experimental conditions imply that the loops survive on average even if Brownian motion effects are taken into account.

The efficiency of self-phoretic propulsion mechanisms with surface reaction heterogeneity

“…This can significantly impact the ability of the swimmer to mix its environment. In addition, the strongly quadrupolar flow field of the self-electrophoretic delta-type swimmer could also lead to enhanced stirring of the fluid [50].…”

Lattice-Boltzmann simulations of microswimmer-tracer interactions

“…We do so by comparing to the corresponding exact solutions of the Stokes equation for the same problem, as established in Ref. [8].…”

“…One set of problems that has attracted particular interest over the past decade is the enhanced diffusion of nonswimming (passive) tracer particles suspended in a bacterial or algal bath [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], compared to that expected from thermal fluctuations alone. This phenomenon has been extensively analyzed theoretically and rationalized in terms of characteristic hydrodynamic scattering events between the tracer and the swimmer flow field [13].…”

“…Since a real microswimmer will have a finite separation between the force points, the description of a microswimmer flow field as that of a point stresslet is only valid at distances appreciably larger than the typical size of the swimmer. Nevertheless, this minimal stresslet-based model has proven accurate in numerically describing collective phenomena in microswimmer suspensions [8,14,17], while still being simple enough to provide some analytical tractability [25][26][27].…”

Generalized Navier-Stokes equations for active suspensions

“…If the swimmer concentration is sufficiently high, their collective dynamics can induce rich non-equilibrium flow patterns in the ambient fluid [8][9][10][11][12][13][14][15][16], thereby causing significant changes in the transport properties [17][18][19][20] and rheological response [21][22][23][24] of the solvent medium. An intriguing, seemingly generic feature of dense active suspensions is the emergence of a characteristic topological defect or vortex distance [5-7, 12, 25, 26], thought to arise from the competition between selfpropulsion, steric and hydrodynamic interactions [27].…”