Shervin Rafatnia

As Shervin Rafatnia is not a member of our working group anymore, the information on this page might be outdated.

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 which consider both the ions and the colloid explicitly, very inefficient, if not impossible. A novel method developed during my PhD work models the colloid as a charged plane and the solvent as a dielectric continuum, whereas the ions are taken into account as charged spheres. The mobility is inferred from equilibrium ion distributions using the SEM. This is justified since the important ion correlations are present in the simulation. Comparison of the results with experimental data reveals that in some cases additional specific adsorption of the counterions to the surface are needed for the mobility reversal to occur.

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 naturally occurring polymer coatings. I studied the electrophoresis of such colloids via MD simulations using the lattice-Boltzmann algorithm to model the solvent. The results showed that when the colloid and the polyelectrolytes are oppositely charged, a mobility reversal is observed with respect to monovalent salt concentration. This is a purely electrokinetic effect, and thus different from that observed in multivalent salt. Moreover, the simulation results were in good agreement with an extended version of the SEM which uses the Darcy-Brinkman formalism to describe the fluid dynamics within the polymer layer.

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).