Modeling And Simulation Of Biologically Active Suspensions
Thursday, October 11, 3:30PM – 5PM
Active particle suspensions, of which a bath of swimming bacteria is a paradigmatic example, are characterized by complex dynamics involving strong fluctuations and large-scale correlated motions. These motions, which result from the many-body interactions between particles, are biologically relevant as they impact mean particle transport, mixing and diffusion, with possible consequences for nutrient uptake and the spreading of bacterial infections. In this work, we use a combination of theory and simulations to analyze these effects. First, a kinetic theory is presented and applied to elucidate the dynamics and pattern formation arising from mean-field interactions. Based on this model, the stability of both aligned and isotropic suspensions is investigated. In isotropic suspensions, a new instability for the active particle stress is found to exist, in which shear stresses are eigenmodes and grow exponentially at low wavenumbers, resulting in large-scale fluctuations in suspensions of rear-actuated swimmers, or pushers, when the product of the linear system size with the suspension volume fraction exceeds a given threshold; no such instability is predicted for head-actuated swimmers, or pullers. We confirm and extend the predictions from the kinetic model using large-scale direct numerical simulations based on a slender-body model for interacting self-propelled particles, accelerated by an efficient smooth particle-mesh Ewald algorithm for the calculation of hydrodynamic interactions. These simulations confirm the existence of a transition to large-scale correlated motions in suspensions of pushers above a critical volume fraction and system size, which is seen most clearly in particle velocity and passive tracer statistics. Extensions of this work to model chemotactic interactions with an external oxygen field as well as steric interactions at high volume concentrations are also discussed.
David Saintillan is an Assistant Professor of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign. He received a B.S. in Engineering from Ecole Polytechnique, France, in 2001, and an M.S. and a Ph.D. in Mechanical Engineering from Stanford University in 2003 and 2006. He then worked as a Junior Research Scientist at the Courant Institute of Mathematical Sciences of New York University before joining the University of Illinois in 2008. He was the recipient of the Andreas Acrivos Dissertation Award in Fluid Dynamics of the American Physical Society in 2007, of the Pi Tau Sigma Gold Medal in Mechanical Engineering in 2011, and of an NSF CAREER award in 2012. His research focuses on modeling and simulations of complex fluids, biophysical fluid dynamics, and electrokinetic phenomena.
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