Unsteady feeding and optimal strokes of model ciliates

TL;DR

This paper analytically and computationally investigates how unsteady swimming affects nutrient feeding in a model microorganism, revealing phase delays, mode influences, and optimal stroke strategies for maximizing feeding efficiency.

Contribution

It provides the first analytical link between unsteady swimming and feeding, introduces an adjoint-based optimization for optimal strokes, and shows steady swimming is optimal for feeding under energy constraints.

Findings

Mean feeding rate depends on swimming modes up to order Pe^(3/2).

A phase delay of 1/8th period exists between feeding and swimming.

Optimal unsteady strokes match those optimizing periodic swimming.

Abstract

The flow field created by swimming microorganisms not only enables their locomotion but also leads to advective transport of nutrients. In this paper we address analytically and computationally the link between unsteady feeding and unsteady swimming on a model microorganism, the spherical squirmer, actuating the fluid in a time-periodic manner. We start by performing asymptotic calculations at low P\'eclet number (Pe) on the advection-diffusion problem for the nutrients. We show that the mean rate of feeding as well as its fluctuations in time depend only on the swimming modes of the squirmer up to order Pe^(3/2), even when no swimming occurs on average, while the influence of non-swimming modes comes in only at order Pe^2. We also show that generically we expect a phase delay between feeding and swimming of 1/8th of a period. Numerical computations for illustrative strokes at finite Pe confirm quantitatively our analytical results linking swimming and feeding. We finally derive, and use, an adjoint-based optimization algorithm to determine the optimal unsteady strokes maximizing feeding rate for a fixed energy budget. The overall optimal feeder is always the optimal steady swimmer. Within the set of time-periodic strokes, the optimal feeding strokes are found to be equivalent to those optimizing periodic swimming for all values of the P\'eclet number, and correspond to a regularization of the overall steady optimal.