Mixing by Swimming Algae

TL;DR

This study demonstrates how swimming microalgae significantly enhance microscale mixing of passive particles through their flagellar motion, with implications for biological and environmental processes.

Contribution

It provides experimental evidence of microscale mixing enhancement by swimming algae and analyzes the flow dynamics around them, highlighting flagellar beating effects.

Findings

Enhanced mixing with increased algae concentration

Tracer displacement PDFs develop exponential tails

Flagellar beating creates flows beyond Brownian motion

Abstract

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