Direct Measurement of Inertial Impact and Propulsive Force in a Eukaryotic Swimmer

TL;DR

This study directly measures inertial impact and propulsive forces in a eukaryotic microswimmer, revealing high-frequency ciliary pulses and demonstrating how inertia filters motor signals, challenging assumptions about kinematics and motor dynamics.

Contribution

It provides the first direct measurements separating inertial impact from propulsive force in a microswimmer, highlighting the role of inertia in active matter at intermediate Reynolds numbers.

Findings

Discovered a 30 Hz propulsive pulse linked to ciliary action.

Inertia filters high-frequency motor signals, resulting in smooth swimming.

Kinematics are not a direct indicator of motor dynamics in non-Stokes regimes.

Abstract

The transduction of force into motion for microswimmers at intermediate Reynolds numbers ($Re \sim 1$), where inertia becomes relevant, is a fundamental problem in active matter. Using the multicellular alga \textit{Volvox} as a model physical system, we perform the first direct measurements that deconvolve a swimmer's inertial impact force from its motor's propulsive force. We discover a $\sim$30 Hz propulsive pulse, the mechanical signature of collective ciliary action. This high-frequency motor output drives a fluctuating velocity in the low-$Re$ \textit{V. carteri}, but is mechanically filtered by the inertia of the larger \textit{V. ferrisii}, resulting in a smooth swimming trajectory. Our work demonstrates that for swimmers beyond the Stokes regime, kinematics are not a direct proxy for the underlying motor dynamics, a foundational assumption in the study of microscopic motility.