Physics Colloquium - Friday, September 11th, 2009, 4:00 P.M.

Alex Groisman
Department of Physics
University of California, San Diego

Microscopic flows normally have low Reynolds number, Re, and are linear and stable. Therefore, no interesting physics or fluid mechanics is usually anticipated. In this talk, however, I will show how specialized microfluidic devices can be used to observe new flow phenomena in visco-elastic liquids and to study chemical physics of protein molecules. An aqueous solution becomes visco-elastic when a small amount of flexible high-molecular-weight polymers is added to it. Visco-elasticity leads to strong non-linearity in flows at arbitrarily low Re. We used non-linear properties of polymer solutions to build a microfluidic flow controller (non-linear resistor), rectifier (diode), and switchable bi-stable memory element (flip-flop). Studies of conformations of protein molecules are important for understanding their physics and biological function. We built a microfluidic device generating orthogonal gradients of temperature and of concentration of small molecules to study the dependence of protein conformations on the physical and chemical environment. 100×100 point diagrams of conformational states of a fluorescently labeled protein were obtained from single fluorescence micrographs. In another device, a combination of flow acceleration, focusing, and abrupt deceleration was used to study single-molecule kinetics of protein folding on a sub-millisecond scale.