The stochastic dynamics and energetics of a microswimmer

Optical tweezers have been employed widely to explore the physical properties of matter. In our experiments, we implement a calibration technique known as the Photon-Momentum Method which allows the direct measurement of forces exerted by a microswimmer without prior knowledge of its refractive index and size. We trap Chlamydomonas reinhardtii using optical tweezers and measure the stochastic forces generated. We study the force dynamics of the swimmer using tools from statistical mechanics to quantify violation of the fluctuation-dissipation theorem and estimate the energy dissipation rate of the non-thermal forces generated by the microswimmer. We find that the average power dissipated by the swimmer to be 5 fW with a maximum power dissipated to be 10 - 15 fW. Interestingly, this power dissipation estimated from optical trapping measurements is in close agreement with fluid dynamics studies of viscous dissipation. Our results suggest that the stochastic energetics of a microswimmer as measured from force fluctuations via optical tweezers is compatible with the fluid dynamics approach. We anticipate that these tools can be extended to dense suspensions of self-propelled particles/swimmers.