Undulatory locomotion of finite filaments: lessons from C. elegans

TL;DR

This study investigates the undulatory locomotion of finite filaments like C. elegans using theoretical and numerical models, revealing that real organisms outperform simple sine wave models in efficiency and distance, highlighting the need for waveform optimization.

Contribution

The paper compares resistive force theory and particle-based simulations to analyze undulatory swimming, and applies these to C. elegans, showing real organisms outperform simple models in efficiency and displacement.

Findings

Particle-based models outperform RFT in predicting optimal gait.

C. elegans exceeds sine wave models in displacement and efficiency.

Real waveforms deviate from simple sine waves, indicating waveform optimization is crucial.

Abstract

Undulatory swimming is a widespread propulsion strategy adopted by many small-scale organisms including various single-cell eukaryotes and nematodes. In this work, we report a comprehensive study of undulatory locomotion of a finite filament using (i) approximate resistive force theory (RFT) assuming a local nature of hydrodynamic interaction between the filament and the surrounding viscous liquid, and (ii) particle-based numerical computations taking into account the intra-filament hydrodynamic interaction. Using the ubiquitous model of a propagating sinusoidal waveform, we identify the limit of applicability of the RFT and determine the optimal propulsion gait in terms of (i) swimming distance per period of undulation and (ii) hydrodynamic propulsion efficiency. The occurrence of the optimal swimming gait maximizing hydrodynamic efficiency at finite wavelength in particle-based computations diverges from the prediction of the RFT. To compare the model swimmer powered by sine wave undulations to biological undulatory swimmers, we apply the particle-based approach to study locomotion of the model organism nematode Caenorhabditis elegans using the swimming gait extracted from experiments. The analysis reveals that even though the amplitude and the wavenumber of undulations are similar to those determined for the best performing sinusoidal swimmer, C. elegans overperforms the latter in terms of both displacement and hydrodynamic efficiency. Further comparison with other undulatory microorganisms reveals that many adopt waveforms with characteristics similar to the optimal model swimmer, yet real swimmers still manage to beat the best performing sine-wave swimmer in terms of distance covered per period. Overall our results underline the importance of further waveform optimization, as periodic undulations adopted by C. elegans and other organisms deviate considerably from a simple sine wave.