Passive cell body plays active roles in microalgal swimming via nonreciprocal interactions

TL;DR

This study reveals that the cell body of microalgae actively enhances swimming performance through nonreciprocal hydrodynamic interactions with flagella, challenging the view of the body as merely passive.

Contribution

It demonstrates that body-flagella hydrodynamic interactions significantly improve swimming efficiency and speed, introducing the concept of nonreciprocal effects in microalgal motility.

Findings

Body-flagella hydrodynamic interactions enhance swimming speed.

Optimal body size balances hydrodynamic benefits and viscous drag.

Nonreciprocal interactions arise from differential effects during power and recovery strokes.

Abstract

The cell body of flagellated microalgae is commonly considered to act merely as a passive load during swimming, and a larger body size would simply reduce the speed. In this work, we use numerical simulations based on a boundary element method to investigate the effect of body-flagella hydrodynamic interactions (HIs) on the swimming performance of the biflagellate, \textit{C. reinhardtii}. We find that body-flagella HIs significantly enhance the swimming speed and efficiency. As the body size increases, the competition between the enhanced HIs and the increased viscous drag leads to an optimal body size for swimming. Based on the simplified three-sphere model, we further demonstrate that the enhancement by body-flagella HIs arises from an effective non-reciprocity: the body affects the flagella more strongly during the power stroke, while the flagella affect the body more strongly during the recovery stroke. Our results have implications for both microalgal swimming and laboratory designs of biohybrid microrobots.