Electrophoresis of Rigid Rods in Obstacle Arrays
Electrophoresis is the motion of microscopic charged objects in an electric field in a fluid medium. Among many applications, electrophoresis in a nanoporous medium (such as a gel, an entangled polymer solution, or a microfabricated array of obstacles) can be used to separate macromolecules, as well as other objects, such as cells, by size and other characteristics. Electrophoretic motion of flexible polymers in arrays of obstacles (modeling such nanoporous media) has been studied extensively in computer simulations. However, the double-stranded DNA can be stiff on the length scale of a single pore, and there are even stiffer objects of interest, such as rodlike viruses. Using Brownian Dynamics simulations, we have studied electrophoresis of stiff rods in arrays of obstacles. We find a nonmonotonic dependence of the velocity of the rods on their length. As the length increases, the rods first move more slowly as they collide with obstacles more often. However, as the rod length increases further, they orient along the direction of their motion and pass through the obstacle array more easily. A simple scaling theory that we have developed predicts this behaviour.