Hauptseminar Soft Matter SS 2019/Transport of poly electrolytes and colloids in electric fields

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Transport of (poly-)electrolytes and colloids in electric fields
Henrik Jäger
Patrick Kreissl


Many polymers, including biomelcules like e. g. DNA, contain numerous electrolyte groups. In aqueous solution, these groups can dissociate, making the polymer a charged polyelectrolyte. Under the influence of an electric field this can lead to motion of the polyelectrolyte relative to the fluid – a transport mechanism know as electrophoresis.

This talk will provide an overview of how electrophoretic systems can be modeled using computer simulations. As a theoretical basis the description of electroosmotic flow (EOF) at charged walls will be presented as well as its analytical solution in the thin-Debye-Layer (Smoluchowski) limit. Then the Standard Electrokinetic Model by O'Brian and White is introduced and it is demonstrated how this perturbation theory approach can be used to solve the case of a spherical particle, also showing how mobility changes with the size of the particles. However, the full system of electrokinetic equations can also be soved without the thin-Debye-layer approximation, using computer simulations. This can be done using, e. g., the finite element method (FEM) or molecular dynamics (MD) simulations, where a combination of explicit electrostatics, hydrodynamics via the lattice-Boltzmann (LB) method, and a representation of the particle using multiple fluid-coupling points ("raspberry approach") is applied. With slight modifications, this MD approach can also be used to model charged polyelectrolytes, demonstrating for instance how their transport properties change with polymer chain length.