Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions

Abstract

Using a fluid-particle dynamics approach, we numerically study the effects of hydrodynamic interactions on the collective dynamics of active suspensions within a simple model for bacterial motility: each microorganism is modeled as a stroke-averaged dumbbell swimmer with prescribed dipolar force pairs. Using both simulations and qualitative arguments, we show that, when the separation between swimmers is comparable to their size, the swimmers' motions are strongly affected by activity-induced hydrodynamic forces. To further understand these effects, we investigate semi-dilute suspensions of swimmers in the presence of thermal fluctuations. A direct comparison between simulations with and without hydrodynamic interactions shows these to enhance the dynamic clustering at a relatively small volume fraction; with our chosen model the key ingredient for this clustering behavior is hydrodynamic trapping of one swimmer by another, induced by the active forces. Furthermore, the density dependence of the motility (of both the translational and rotational motions) exhibits distinctly different behaviors with and without hydrodynamic interactions; we argue that this is linked to the clustering tendency. Our study illustrates the fact that hydrodynamic interactions not only affect kinetic pathways in active suspensions, but also cause major changes in their steady state properties.

Pressure of a gas of underdamped active dumbbells

“…1. It consists of N identical self-propelled dumbbells [5,[20][21][22][23][24][25][26] . A mobile wall of thickness e 2 separates this area into two non-communicating chambers.…”

Hydrodynamic interaction of swimming organisms in an inertial regime

“…Two swimming bacteria, which use rotating helices to propel their cell bodies, considerably change their orientations and avoid each other [21]. When considering the details of the swimming stroke, the swimmer-swimmer interaction is complex depending on their relative displacement, orientation and phase, leading to different scattering angles [22,23] and various types of motion, such as attraction, repulsion, or oscillation [24]. Another interesting phenomenon caused by the hydrodynamic interaction between two microswimmers is the enhancement of the swimming efficiency by synchronizing the phase of two adjacent flagella [25].…”

Recovery of mechanical pressure in a gas of underdamped active dumbbells with Brownian noise

“…1. It consists of N identical self-propelled dumbbells [5,19,[21][22][23][24][25][26] moving in a 2-dimensional space and enclosed between fixed walls with gross size…”