Shervin Rafatnia

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Shervin Rafatnia
Postdoc
Office:1.035
Phone:+49 711 685-67701
Fax:+49 711 685-63658
Email:shervin _at_ icp.uni-stuttgart.de
Address:Shervin Rafatnia
Institute for Computational Physics
Universität Stuttgart
Allmandring 3
70569 Stuttgart
Germany

Field of Interest:

I am generally interested in softmatter physics and particularly in electrokinetic phenomena of colloidal and polymeric systems. I study the electrophoretic behavior of bare, as well as soft, colloids via MD simulations using the lattice-Boltzmann algorithm to model the fluid.

Electrophoresis is the movement of charged particles in a fluid as a response to an applied electric field and is mainly used as a separation technique. The electrophoretic mobility, defined as the ratio of the drift velocity to the applied field, depends on properties such as shape, size, structure and surface charge density. This makes electrophoresis a useful tool for gaining information about the particle. In order to extract this information, a reliable theoretical model is needed which takes into account all the important factors. The so-called standard electrokinetic model (SEM) is perhaps the best known and most used one, but recent work has shown its inadequacy in explaining some of phenomena observed in experiments. An example is the mobility reversal at high concentrations of multivalent salt caused by the attraction of more counterions to the charged surface than necessary to neutralize it (overcharging). The overcharging itself can be due to ionic correlations which are absent in the underlying Poisson-Boltzmann approach of the SEM.

My aim is to help understand better the electrophoretic behavior of colloids and to find the extent to which the SEM can be applied. One of my focuses is on the mobility reversal at the presence of multivalent salt for colloids which are thousands of times larger than a typical salt ion. This separation in length and time scales makes simulations in which both the ions and the colloid are considered explicitly, very inefficient, if not impossible.

The electrokinetic properties of polyelectrolyte-coated colloids, known as soft colloids, are significantly more complex than those of a bare surface due to the nonuniform charge distribution and the polymers’ hydrodynamic drag. These effects must be taken into account to model the electrophoresis of biological cells, which often have nat- urally occurring polymer coatings.

Publications:

Raafatnia S., Hickey O.A., Holm C., Electrophoresis of a Spherical Polyelectrolyte-Grafted Colloid in Monovalent Salt Solutions: Comparison of Molecular Dynamics Simulations with Theory and Numerical Calculations, Macromolecules 2015, 48, 775−787.

Raafatnia S., Hickey O.A., Holm C., Mobility reversal of polyelectrolyte-grafted colloids in monovalent salt solution, Physical Review Letters 113, 238301 (2014).

Raafatnia S., Hickey O.A., Sega M., Holm C., Computing the Electrophoretic Mobility of Large Spherical Colloids by Combining Explicit Ion Simulations with the Standard Electrokinetic Model, Langmuir 2014, 0743-7463.

Semenov I., Raafatnia S., Sega M., Lobaskin V., Holm C, Kremer F., Electrophoretic mobility and charge inversion of a colloidal particle studied by single-colloid electrophoresis and molecular dynamics simulations, Physical Review E 87, 022302 (2013).