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In this fluid dynamics video, we demonstrate the microscale mixingenhancement of passive tracer particles in suspensions of swimming microalgae,Chlamydomonas reinhardtii. These biflagellated, single-celled eukaryotes (10micron diameter) swim with a "breaststroke" pulling motion of their flagella atspeeds of about 100 microns/s and exhibit heterogeneous trajectory shapes.Fluorescent tracer particles (2 micron diameter) allowed us to quantify theenhanced mixing caused by the swimmers, which is relevant to suspension feedingand biogenic mixing. Without swimmers present, tracer particles diffuse slowlydue solely to Brownian motion. As the swimmer concentration is increased, theprobability density functions (PDFs) of tracer displacements develop strongexponential tails, and the Gaussian core broadens. High-speed imaging (500 Hz)of tracer-swimmer interactions demonstrates the importance of flagellar beatingin creating oscillatory flows that exceed Brownian motion out to about 5 cellradii from the swimmers. Finally, we also show evidence of possible cooperativemotion and synchronization between swimming algal cells.

Mixing by Swimming Algae