Polymer dynamics in microfluidic devices: A Brownian dynamics study

Microfluidic processes involve transporting fluids in channels having characteristic dimensions of 10--100 mum. Since most of the components handled in microfluidic processes are biopolymers such as DNA and proteins, understanding polymer dynamics inside microfluidic channels is essential for designing more efficient microfluidic processes. We use Brownian dynamics simulations to study the dynamics of isolated polymer molecules inside microchannels. The polymer is modeled as a set of beads connected by links which can be either springs or rods. Using Brownian dynamics, we have looked at three aspects of polymer dynamics relevant to microfluidics. We studied the stretching and transport of a bead-spring chain in a recirculating electro-osmotic flow which can be generated inside a microchannel and is characterized by an inhomogeneous velocity gradient. This study provides an important extension to previous studies which have looked at polymer dynamics in flows with homogeneous velocity gradients. In another problem, we studied the adsorption of a single polyelectrolyte onto an oppositely charged surface using a bead-rod description for the polymer. We compared adsorption in the absence and presence of a shear flow and found enhanced adsorption in the presence of a shear flow. In contrast to previous simulation studies which have considered only equilibrium aspects of polyelectrolyte adsorption, our study is the first to investigate polyelectrolyte adsorption in the presence of a flow. Using the bead-rod model for a polymer, we also examined the time scales involved in the electrophoresis of a charged polymer through a narrow constriction separating two larger regions. We found that the electrophoresis of the polymer through the constriction is governed by three characteristic time scales. Whereas previous studies have shown the electrophoresis of a polymer to depend on a single time scale, our study demonstrates that more than one time scale can determine its electrophoresis. Our Brownian dynamics studies have contributed to the understanding of polymer dynamics in microchannels and may be useful in designing better microfluidic separation processes. In addition, these studies provide new insights into single polymer dynamics which are relevant to polymer rheology, polyelectrolyte multilayer formation, and DNA dynamics inside biological cells.