Shape of optimal active flagella

TL;DR

This paper derives the optimal shape of a flagellum for efficient swimming, showing that natural sperm waveforms are mechanically optimal based on a new energetic cost model considering internal motor power.

Contribution

It introduces a computational optimization framework for flagellar shape, incorporating internal molecular motor power and elastic-viscous effects, to explain observed biological waveforms.

Findings

Optimal flagellar shapes depend on the Sperm number.

Computed shapes match observed sperm waveforms.

Flagella may be evolutionarily optimized for energy efficiency.

Abstract

Many eukaryotic cells use the active waving motion of flexible flagella to self-propel in viscous fluids. However, the criteria governing the selection of particular flagellar waveforms among all possible shapes has proved elusive so far. To address this question, we derive computationally the optimal shape of an internally-forced periodic planar flagellum deforming as a travelling wave. The optimum is here defined as the shape leading to a given swimming speed with minimum energetic cost. To calculate the energetic cost though, we consider the irreversible internal power expanded by the molecular motors forcing the flagellum, only a portion of which ending up dissipated in the fluid. This optimisation approach allows us to derive a family of shapes depending on a single dimensionless number quantifying the relative importance of elastic to viscous effects: the Sperm number. The computed optimal shapes are found to agree with the waveforms observed on spermatozoon of marine organisms, thus suggesting that these eukaryotic flagella might have evolved to be mechanically optimal.