Tank-treading as a means of propulsion in viscous shear flows

TL;DR

This paper explores how vesicles can control their shape and orientation in viscous shear flows through membrane inhomogeneity, enabling propulsion and migration by adjusting membrane stiffness in response to external gradients.

Contribution

It provides an analytical model showing how membrane inhomogeneity influences vesicle shape, orientation, and migration in shear flows, introducing a novel propulsion mechanism.

Findings

Vesicle shape and orientation depend on membrane stiffness distribution.

Membrane inhomogeneity can enable transverse migration in shear flows.

A simple model demonstrates migration driven by membrane stiffening or softening.

Abstract

The use of tank-treading as a means of propulsion for microswimmers in viscous shear flows is taken into exam. We discuss the possibility that a vesicle be able to control the drift in an external shear flow, by varying locally the bending rigidity of its own membrane. By analytical calculation in the quasi-spherical limit, the stationary shape and the orientation of the tank-treading vesicle in the external flow, are determined, working to lowest order in the membrane inhomogeneity. The membrane inhomogeneity acts in the shape evolution equation as an additional force term, that can be used to balance the effect of the hydrodynamic stresses, thus allowing the vesicle to assume shapes and orientations that would otherwise be forbidden. The vesicle shapes and orientations required for migration transverse to the flow, together with the bending rigidity profiles that would lead to such shapes and orientations, are determined. A simple model is presented, in which a vesicle is able to migrate up or down the gradient of a concentration field, by stiffening or softening of its membrane, in response to the variations in the concentration level experienced during tank-treading.