From active stresses and forces to self propulsion of droplets

TL;DR

This paper models the self-propulsion of spherical droplets driven by active surface or body forces, focusing on hydrodynamics and trajectories derived from active stresses, relevant for understanding microswimmer behavior.

Contribution

It introduces a hydrodynamic framework for droplet self-propulsion based on active stresses, emphasizing force and stress distributions over velocity field approaches.

Findings

Calculated internal and external flow fields for active droplets.

Derived trajectories from active force and stress distributions.

Demonstrated how internal active stresses influence droplet motion.

Abstract

We study the self-propulsion of spherical droplets as simplified hydrodynamic models of swimming microorganisms or artificial microswimmers. In contrast to approaches, which start from active velocity fields produced by the system, we consider active surface force or body force densities or active stresses as the origin of autonomous swimming. For negligible Reynolds number and given activity we first calculate the external and the internal ow fields as well as the center of mass velocity and an angular velocity of the droplet at fixed time. To construct trajectories from single time snapshots, the evolution of active forces or stresses must be determined in the laboratory frame. Here, we consider the case of active matter, which is carried by a continuously distributed, rigid but sparse (cyto)-sceleton that is immersed in the droplet's interior. We calculate examples of trajectories of a droplet and its sceleton from force densities or stresses, which may be explicitely time dependent in a frame fixed within the sceleton