Levich Institute Seminar Announcement, 10/01/2013
Tuesday, 10/01/2013
2:00 PM
Steinman Hall, Room #312
(Chemical Engineering Conference Room)

Professor Patrick Underhill
Rensselaer Polytechnic Institute
Department of Chemical and Biological Engineering

"Active Matter: Suspensions of Self-Propelled Particles"


There is currently much interest in a new class of materials called Active Matter. These are materials that are continually pushed away from equilibrium at each point within the material, instead of from the exterior. Since active materials are always far from equilibrium, they are able to have material properties and assembly not restricted by equilibrium statistical mechanics. This includes enhanced fluctuations and mixing, negative intrinsic viscosity, and non-uniform concentration in confined geometries. One important class of active materials is a suspension of self-propelled particles, which are colloidal suspensions in which the colloids propel themselves through the fluid. Examples of this type are suspensions of swimming microorganisms or synthetic colloids that are engineered to use chemical reactions to move. In this talk, I will focus on two aspects of active suspensions: (1) how self-driven flow can be used to coordinate the motion of large groups and (2) how an external flow alters the dynamics in confined geometries.

One mechanism by which active suspensions can produce unique responses is through coordinating their behavior using the flows they create while swimming. A number of researchers have used direct simulations or continuum theories to examine how these collective behaviors can occur due to hydrodynamic interactions. In all of these studies, the organisms are suspended in a Newtonian fluid. However, many suspensions occur in non-Newtonian environments such as saliva, mucus, or polymeric solutions added to engineer the response. We have developed a theory to include this non-Newtonian suspending fluid and used it to understand how multiple competing timescales in the system determine the response of the system. In the second part of the talk, I will discuss the results of computer simulations of swimming organisms in confined geometries and how an external flow alters the dynamics. In experiments and simulations without flow, it has been shown that swimming microorganisms accumulate near surfaces even in absence of chemical gradients or surface interactions. Using simulations, we show how this accumulation is reduced by an external flow and determine the key dimensionless group that alters the response. We also identify a dip in the swimmer concentration in the channel center for Poiseuille flow, and determine the mechanism for the dip. Finally, I will discuss the dynamics of the organisms, which impacts the beginning of biofilm formation and Taylor dispersion.


I received a BS in Chemical Engineering and a BS in Physics from Washington University in 2001. I received my PhD in Chemical Engineering from MIT in 2006 working for Patrick Doyle and did postdoctoral research under Michael Graham at UW-Madison from 2006-2008, then began as an Assistant Professor of Chemical and Biological Engineering at RPI in 2008.


I use a combination of experiments, theory, and computer simulations to study complex fluids. Recently we have been focused on combinations of polymer solutions, colloidal suspensions, and active matter.

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