Description:

Microorganisms inhabit and swim in fluids at low Reynolds numbers, a world, though foreign to us, is of utmost importance to many aspects of our daily lives ranging from food production, disease prevention to environmental health. In this talk, I discuss recent works in my group on the fascinating swimming behaviors of two paradigmatic microorganisms, Escherichia coli and Chlamydomonas reinhardtii. First, we study the directed transport and energetics of swimming E. coli navigating through funnel-shaped obstacles---an example of rectification of living active matter. We develop a microscopic parameter-free model and quantify the degree of time irreversibility in bacterial rectification. Our study establishes a generic relationship between time irreversibility, particle fluxes, and extractable work. Second, using high-speed holographic microscopy, we report the first measurement of the time-resolved 3D flow field of a freely-swimming microorganism, C. reinhardti, a unicellular green alga widely adopted as a model for eukaryotic locomotion and flagellar dynamics. We uncover a series of novel flow processes---such as vortex rings, traveling vortices, and topological transformations---that significantly enrich our understanding of low-Reynolds-number fluid mechanics. Access to the complete 3D flow field enables rigorous quantification of the alga's energy expenditure, as well as its swimming and feeding efficiency, substantially improving the precision of these key physiological metrics. Taken together, our study illustrates the extraordinary features evolved in microorganisms enabling their effective navigation in complex environments and provides a guideline in designing artificial microswimmers for biomedical applications.

Xiang Cheng, Professor of of Chemical Engineering and Materials Science, University of Minnesota 

Host: Michelle Driscoll