Electrophoretic Separation of Charged Particles
Electrophoresis is one of the most important methods for separating colloidal particles and biological molecules such as DNA, RNA, proteins, carbohydrates, pharmaceuticals, herbicides, and pesticides %and so-called DNA fingerprinting, in terms of their charge (or size). This method relies on the correlation between the particle drift velocity and the charge (or size). For a high-resolution separation, we need to minimize fluctuations of the drift velocity of particles or molecules. For a high throughput, on the other hand, we need a concentrated solution, in which many-body electrostatic and hydrodynamic interactions may increase velocity fluctuations. Thus, it is crucial to reveal what physical factors destabilize coherent electrophoretic motion of charged particles. However, this is not an easy task due to complex dynamic couplings between particle motion, hydrodynamic flow, and motion of ion clouds. Here we study this fundamental problem using numerical simulations. We reveal that addition of salt screens both electrostatic and hydrodynamic interactions, but in a different manner. This allows us to minimize the fluctuations of the particle drift velocity for a particular salt concentration. This may have a significant impact not only on the basic understanding of dynamics of driven charged colloids, but also on optimization of electrophoretic separation.