Abstract: Active Brownian particles (ABPs) accumulate at boundaries that confine them owing to their persistent self-propulsion.  In this talk I will discuss several examples of boundary accumulation and its impact on active matter behavior.  In a pressure-driven flow in a channel, active particles can swim upstream owing to the confining effect of the channel coupled with the vorticity of the fluid motion.  Two parallel plates immersed in a bath of active particles can experience an attractive force, known either as the Casimir effect or active depletion, because the plate separation limits the extent of the ABP’s random walk.  A spatial variation in the swim speed of active particles can lead to accumulation of particles where the speed is slow and depletion where fast allowing one to ‘paint’ with active particles.  Finally, when active particles cross a boundary in which the resistivity changes, the direction of motion is refracted in a manner akin to ray optics and follows a variant of Snell’s law, allowing one to design lenses to control and focus active particles.

Bio: 

John F. Brady is the Chevron Professor of Chemical Engineering and Professor of Mechanical Engineering at the California Institute of Technology.  He received his BS in chemical engineering from the University of Pennsylvania in 1975, which was followed by a year at Cambridge University as a Churchill Scholar.  He received both an MS and PhD in chemical engineering from Stanford University, the latter in 1981.  Following a postdoctoral year in Paris at ESPCI, he joined the Chemical Engineering department at MIT.  Dr. Brady moved to Caltech in 1985. 

Dr. Brady’s research interests are in the mechanical and transport properties of two-phase materials, especially complex fluids such as biological liquids, colloid dispersions, suspensions, porous media, active matter, etc.  His research combines statistical and continuum mechanics to understand how macroscopic behavior emerges from microscale physics.  He is the co-inventor of the Stokesian Dynamics technique for simulating the behavior of particles dispersed in a viscous fluid under a wide range of conditions.  Recently, Dr. Brady discovered a new organizing principle for active matter known as the swim pressure.

Dr. Brady has been recognized for his work by several awards, including a Presidential Young Investigator Award, the Professional Progress Award of the American Institute of Chemical Engineers, the Bingham Medal of the Society of Rheology and the Fluid Dynamics Prize of the American Physical Society.  Dr. Brady served as an associate editor of the Journal of Fluid Mechanics and editor of the Journal of Rheology.  He is a fellow of the American Physical Society, the American Academy of Arts & Sciences and a member of the National Academy of Engineering and the National Academy of Sciences.