Non-Equilibrium Transport of Biopolymers and Colloids in the Coexistence of Temperature and Concentration Gradients

Abstract

Atoms and molecules move along a gradient of external fields as seen in electrophoresis, which is a motion of charged molecules relative to fluid along an electric field. One unexplored but relevant alternative is thermophoresis, the Ludwig-Soret effect, that makes a solute moves along a temperature gradient. Thermophoresis depletes a polyethylene (PEG) polymer of large concentrations from the hot region and builds a concentration gradient. In such a solution, solutes of small concentration experience thermophoresis and PEG concentration-dependent restoring forces. Under focused laser heating, DNA and RNA as solutes localize as a ring-like structure which diameter monotonically decreases with the size of polymer following a behavior analogous to gel electrophoresis. Thus trapping and separation of biopolymers could be physically feasible in a simple way relying on temperature gradient. Moreover, since this effect relies on the entropic force in a PEG concentration gradient, trapping with little material dependence is feasible for bacteria and motile eukaryotic cells. It may bring new method to control the density of living cells and biopolymers with light.