# Internal friction controls active ciliary oscillations near the instability threshold

## TL;DR

This study demonstrates that internal friction, rather than external fluid resistance, is the primary dissipation mechanism driving ciliary oscillations near the instability threshold, revealing a new understanding of active filament dynamics.

## Contribution

It shows that internal elastic stress dominates dissipation in ciliary beating, contrasting with previous assumptions of external fluid friction being primary.

## Key findings

- Internal friction surpasses external fluid friction in ciliary oscillations.
- Active stresses leading to oscillations are strain softening and shear thinning.
- A combined experimental and theoretical approach elucidates ciliary instability mechanisms.

## Abstract

Ciliary oscillations driven by molecular motors cause fluid motion at micron scale. Stable oscillations require a substantial source of dissipation to balance the energy input of motors. Conventionally, it stems from external fluid. We show, in contrast, that external fluid friction is negligible compared to internal elastic stress through a simultaneous measurement of motion and flow field of an isolated and active Chlamydomonas cilium beating near the instability threshold. Consequently, internal friction emerges as the sole source of dissipation for ciliary oscillations. We combine these experimental insights with theoretical modeling of active filaments to show that an instability to oscillations takes place when active stresses are strain softening and shear thinning. Together, our results reveal a counterintuitive mechanism of ciliary beating and provide a general experimental and theoretical methodology to analyze other active filaments, both biological and synthetic ones.