Optical Tweezers-based velocimetry: A method to measure microscale unsteady flows

Motility and transport of fluids by micro-organisms relies on the generation of oscillatory flows generated by flagella or cilia at high frequencies. To understand the underlying physics governing the motility of ciliated microorganisms and ciliary synchronization, it is essential to accurately measure and characterize the unsteady flow fields they generate. Time resolved measurements of unsteady flows of small amplitudes and high frequencies are challenging with PIV or PTV, and measurements have often been limited to average flows. The limitation of particle based methods lies in the fact that, on these scales, the particle motion is indistinguishable from Brownian motion. Here, we present a velocimetry method based on optical tweezers that allow us to measure oscillatory flows with high accuracy and resolution. In this method, we measure the nanometer-scale displacements of a bead trapped inside the optical tweezers, and analyze the position measurements to extract the actual flow velocity. In addition, as an example, we measure the unsteady oscillatory flow around the bi-flagellated green alga Chlamydomonas reinhardtii, and compare with numerical predictions.