An experimental study of rheology and microscopic dynamics of sheared active fluids

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CBET - 1702352PI: Cheng, XiangThis award will provide support to develop two new instruments to study the properties of active fluids. An active fluid is a suspension of particles in liquid that can generate their own motion and propel themselves. There are both synthetic and naturally-occurring active suspensions. This proposal will focus on suspensions of bacterial cells, which exhibit fascinating motions based on the coordinated behavior of many cells. The cells' motions cause these fluids to flow in new and unusual ways. An ultra-fast confocal microscope equipped with a chamber that can generate flow will be built at the University of Minnesota. This instrument will allow investigators to observe cellular motion on the scale of individual cells as well as detect coordinated structures that develop among the cells in flow. A thin-film rheometer will be built at Cal Poly - San Luis Obispo. This instrument will measure bulk properties of the active suspensions. The results of these experiments will help validate computational and theoretical studies of active suspensions. The instruments will also be useful for studying other complex fluids where microscopic arrangements of suspended particles influence the fluid dynamics of the suspension. The investigators will involve undergraduates in the research, including students from traditionally underrepresented groups in science and engineering. In addition, they will create new demonstrations of active suspensions to engage the public at the Minnesota State Fair and at Cal Poly's open house and pre-collegiate symposium. This project will reveal direct correlations between microscopic active particle dynamics and macroscopic flow behaviors of active suspensions. Specifically, the most-widely studied active suspension, that of E. coli, will be investigated. Two fundamental questions will be addressed. First, what is the dynamical origin of the superfluid phase of bacterial suspensions with extremely low or even zero apparent viscosity? Second, how does an externally imposed shear flow influence the dynamics of individual active particles in concentrated bacterial suspensions? Thus, the experiments will address two key questions underlying the predictions of theories of active suspensions. The experiments will unambiguously reveal the microscopic origin of unusual rheology of active fluids and provide a solid experimental foundation for the development of the field. Results from this project are expected to help practitioners find better ways to control transport in biological suspensions, prevent formation of biofilms, control viscosity in suspensions, and improve mixing in microfluidic devices used in bioassays and advanced materials synthesis.