Is fluid friction enough to counteract the active drive in ciliary oscillations?

Active cilia are prototypical engines of motility at the micron scale. They undergo spontaneous oscillations in viscous fluids by continuously consuming chemical energy and dissipating them through mechanical motion. Therefore, stable oscillations require that the active energy input must be balanced by a significant source of dissipation. Conventionally, it stems from the ambient fluid because cilia operate at a low-Reynolds regime. However, we show that external fluid friction is negligibly small to counteract the passive elastic stresses generated within the filament due to the active drive. This counter-intuitive result is borne out of experiments by simultaneously measuring the waveform and flow field of an isolated and reactivated Chlamydomonas cilia, beating near the instability threshold, for one-shot determination of both elastic and frictional components of the system. Consequently, it is the internal friction of cilium that controls the dynamical steady state in ciliary oscillations. We combine these experimental insights with theoretical modeling of active filaments to illustrate that ciliary oscillations indeed exist in the presence of internal friction as the sole source of dissipation.