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

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Date

2019-05-31

Time

14:00

Topic

Transport of (poly-)electrolytes and colloids in electric fields

Speaker

Tutor

Patrick Kreissl

Contents

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 (Smoluchowsky) 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 used. With slight modifications, this MD approach can be used to also model charged polyelectrolytes, demonstrating for instance how their transport properties change with polymer chain length.

References

• R. W. O'Brien, L. R. White.
  Electrophoretic Mobility of a spherical colloidal Particle.
  Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics 74(2):1607, 1978.
  [PDF] (3.2 MB) [DOI]
• Vladimir Lobaskin, Burkhard Dünweg, Martin Medebach, Thomas Palberg, Christian Holm.
  Electrophoresis of Colloidal Dispersions in the Low-Salt Regime.
  Physical Review Letters 98:176105, 2007.
  [PDF] (169 KB) [DOI]
• J. L. Viovy.
  Electrophoresis of DNA and other polyelectrolytes: Physical mechanisms.
  Reviews of Modern Physics 72(3):813–872, 2000.
  [PDF] (1.3 MB)
• Kai Grass, Christian Holm.
  Mesoscale modelling of polyelectrolyte electrophoresis.
  Faraday Discussions 144:57–70, 2010.
  [PDF] (984 KB) [DOI]