Stretching field statistics in time-periodic flows with dilute bacterial suspensions

Swimming microorganisms often encounter natural flows in oceans, lakes, rivers, and inside our bodies. These microorganisms create their own velocity fields that interact with (flowing) media. Understanding this interplay is important for our understanding of many natural and industrial processes including algal blooms, human reproduction, and vaccine production. In this talk, we experimentally investigate the effects of bacterial activity on scalar mixing in electromagnetically driven 2D time-periodic flows using dilute suspensions of Escherichia coli. Our results show that increasing bacterial concentration in the medium reduces flow chaoticity in both spatially periodic and random flows. We calculate the flow Lagrangian Coherent Structures (LCS) from experimental velocity fields and show that “fluid stretching” decreases as bacteria concentration is increased; flows with spatially broken symmetry show faster large-scale mixing than symmetric flows, but small-scale mixing is largely unaltered. These findings highlight the role of both structural symmetry and activity on scalar mixing in 2D chaotic flows.