Effect of shear and activity on scalar mixing in 2D flows

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 focus on understanding how bacterial concentration and flow structure, specifically shear, control dispersion of bacteria and effects on chaoticity of 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. In all cases, large shear zones mediate scale of mixing due to trapping in shear. These findings highlight the role of both structural symmetry, presence of shear and activity on scalar mixing in 2D chaotic flows.