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Microorganism locomotion in viscoelastic fluidsMathematical Biology
Low Reynolds number swimming of microorganisms in Newtonian fluids is an extensively studied classical problem. However, many biological fluids such as mucus are mixtures of water and polymers and are more appropriately described as viscoelastic fluids. Recently, there have been many studies on locomotion in complex fluids. Both experiments and theory have exhibited that viscoelasticity can lead to either an enhancement or retardation of swimming, but a complete understanding of this problem is lacking. A computational model of finite-length undulatory swimmers is used to examine the physical origin of the effect of elasticity on swimming speed. We show that both favorable stroke asymmetry and swimmer elasticity contribute to a speed-up, but a substantial boost results only when these two effects work together. Additionally, we examine a reduced model of an oscillatory bending beam in a viscoelastic fluid, and identify a threshold in amplitude related to the development of large elastic stresses. We relate this transition to previously studied bifurcations in steady extensional flows of complex fluids. This reduced model sheds some light on properties of swimmer gaits that are related to either elastic enhancement or hindrance.
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