Swimming efficiency in viscosity gradients

TL;DR

This study investigates how viscosity gradients affect microswimmer efficiency, revealing that propulsion type and shape influence their speed, orientation, and energy expenditure in inhomogeneous fluids.

Contribution

It introduces a model analyzing microswimmer efficiency in viscosity gradients, highlighting the roles of propulsion mode and shape in their dynamics and energy efficiency.

Findings

Pushers are more efficient moving down viscosity gradients.

Pullers are more efficient moving up viscosity gradients.

Shape affects power expenditure and efficiency monotonically with slenderness.

Abstract

In this note, we study the effect of viscosity gradients on the energy dissipated by the motion of microswimmers and the associated efficiency of that motion. Using spheroidal squirmer model swimmers in weak linearly varying viscosity fields, we find that efficiency depends on whether they generate propulsion from the back (pushers) or the front (pullers). Pushers are faster and more efficient when moving down gradients but slower and less efficient moving up viscosity gradients, and the opposite is true for pullers. However, both pushers and pullers display negative viscotaxis, therefore pushers dynamically tend to the most efficient orientation while pullers the least. We also evaluate the effect of shape on power expenditure and efficiency when swimming in viscosity gradients and find that in general the change in both due to gradients monotonically decreases with increasing slenderness. This work shows how shape and gait play an important role in determining dynamics and efficiency in inhomogeneous environments, and demonstrating that both efficiency minimizing and maximizing stable dynamical states are possible.