Monday, November 5, 2007 - 10:30 AM
41e

Non-Boltzmann Distribution Of Polymers And Suspensions In Dissipative Systems: Cross-Stream Migration Vs. Differential Relaxation

Todd M. Squires, Chemical Engineering, University of California, Santa Barbara, University of California, Santa Barbara, CA 93106

Polymers and Brownian rods have been predicted and observed to migrate {\sl across} streamlines in flowing systems, impacting rheological measurements, material processing, and microfluidic systems. In particular, gradients in cross-stream diffusivity evidently give rise to cross-stream migration, in direct contrast with expectations from equilibrium statistical mechanics. Here, we provide a simple, physicially intuitive understanding of the subtle physics that underlies this counter-intuitive effect, and identify the three minimal ingredients: i) a cross-stream diffusivity that depends upon internal degrees of freedom of the suspended species; ii) internal d.o.f. that are driven non-conservatively and inhomogeneously, and iii) a mechanism for relaxation to steady state. Significantly, we argue that some inhomogeneous steady-state distributions that have been observed do not result from directed cross-stream migration; rather, from anisotropies in rates of relaxation. In fact, we show that no such migration occurs in systems without relaxation. We propose and predict analogous behavior in a variety of new systems, including colloidal models, externally-orientable Brownian rods, and externally-triggerable two-state molecules. Finally, we demonstrate that thermal gradients give rise to weak concentration gradients that might otherwise be interpreted in terms of thermophoresis.