Internal friction controls active ciliary oscillations near the instability threshold.

Area of Science:

• Biophysics
• Cell Biology
• Fluid Dynamics

Background:

• Cilia generate fluid motion via molecular motors, requiring dissipation to stabilize oscillations.
• External fluid friction is traditionally considered the primary dissipation source.
• The precise dissipation mechanism in isolated cilia remains incompletely understood.

Purpose of the Study:

• To investigate the dominant source of dissipation in isolated, actively beating Chlamydomonas cilia.
• To elucidate the physical mechanisms underlying ciliary oscillation instability.
• To develop a general framework for analyzing active filament dynamics.

Main Methods:

• Simultaneous measurement of isolated Chlamydomonas cilium motion and surrounding flow fields.
• Experimental analysis near the ciliary oscillation instability threshold.
• Theoretical modeling of active filaments incorporating experimental insights.

Main Results:

• External fluid friction is negligible compared to internal elastic stress in Chlamydomonas cilia.
• Internal friction is identified as the sole dissipation source for ciliary oscillations.
• Oscillation instability occurs when active stresses exhibit strain softening and shear thinning properties.

Conclusions:

• Ciliary beating relies on an internal friction mechanism, challenging conventional understanding.
• Active stresses with strain softening and shear thinning properties drive oscillation instability.
• The study offers a versatile experimental and theoretical approach for active filament research.