Electrophoresis: When hydrodynamics matter

The combination of hydrodynamic and electrostatic interactions leads to non-trivial effects that can be observed in various electrophoretic and electro-osmosis systems. In this article, we focus our attention on problems involving polyelectrolytes. First, we examine the free-draining behavior of polyelectrolytes such as DNA, a remarkable phenomenon that makes it impossible to use free-solution electrophoresis to fractionate nucleic acids. We show that the common assumption that hydrodynamic interactions are screened and therefore irrelevant in this system is wrong, and that one must be very careful when dealing with electro-hydrodynamics, especially when mechanical forces are also present. In the limit of small forces, one can superimpose the mechanical and hydrodynamic flow fields and make predictions that are often in excellent agreement with experiments. For DNA, the full electro- and hydrodynamics can then be reduced to the conformationally dependent superposition of a polymer sedimenting through a fluid and a polyelectrolyte being electrophoresed. This superposition or Electro-hydrodynamic Equivalence Principle has been used to explain a variety of problems and to propose methods that can allow the electrophoretic separation of DNA.