|Phone:||+49 711 685-60458|
|Fax:||+49 711 685-63658|
|Email:||ohickey _at_ icp.uni-stuttgart.de|
Institute for Computational Physics
I am interested in the simulation of electrokinetic phenomena, namely electrophoresis and electroosmotic flow, involving polymers. Particularly I am interested in simulating systems which have yet to be explored using molecular dynamics and the sometimes surprising results of the interplay between electrostatic and hydrodynamic interactions.
Implicit Electrohydrodynamics Using Lattice-Boltzmann and ESPResSo
During my time in Stuttgart in 2009 I developed an implicit scheme for simulating the electrohydrodynamics of polyelectrolytes which was later published in 2010. I have made a number of videos (with much help from Georg Rempfer) which illustrate some of the intricacies of electrokinetics.
Polymers subject to a constant force are said to be sedimenting. The polymers move through the fluid and drag the surrounding fluid along with it. Larger polymers move faster as the force (which is proportional to the length of the polymers) grows faster than the average radius of the polymer coil. This can be clearly seen in the video below.
Comparing Electrophoresis to Sedimentation
If we compare the movement of polymers subject to an electric field (electrophoresis) with polymers subject to a constant force (sedimentation) there are distinct differences. Notably unlike in electrophoresis the sedimenting polymer perturbs the surrounding fluid and essentially drags the fluid within it at roughly the same velocity. This is caused by the drag force of the fluid on the polymer which also in turn deforms the polymer. Since the electrophoresing polymer does not produce drag with the surrounding fluid it does not deform as it moves through the fluid. The difference can be seen in the video below where the upper polymer undergoes electrophoresis while the lower one is sedimenting.
Free Solution Electrophoresis of Polyelectrolytes
This video shows the electrophoresis of charged polymers in free solution (bulk fluid without obstacles or a sieving matrix). The video illustrates two surprising points, namely polymers of different length all move with the same speed and the motion of the polymers do not perturb the surrounding fluid at all.
Polymers Grafted at an End Subject to a Constant Force Versus Constant Electric Field
Since polyelectrolytes subject to an electric field to not perturb the surrounding fluid when undergoing electrophoresis this has led many people to incorrectly assume that hydrodynamic interactions are screened by the addition of the electric field. This is in fact not true and can be seen when the polymer is subject to an additional force (or non-uniform electric field) which causes it to no longer being free draining. The simplest example of this is a polymer which is grafted by one end. If we compare the stretching by an electric field (top) with that by a constant force (bottom) we see that initially as the polymers stretch the one being stretched by a constant force perturbs the surrounding fluid. As the polymers stretch the one being pulled by the force perturbs the fluid less and less while the one being stretched by an electric field creates more and more flow. In the limit that the polymers are stretched in fact it is only the one subject to an electric field which generates flow in the surrounding fluid.
Neutral Polymers Migrating Subject to an Electric Field
The fact that hydrodynamic interactions are not screened during electrophoretic phenomena leads to some counter-intuitive effects. In the case of polyelectrolytes several important effects were pointed out by Long et al. . One prediction was that a net neutral polymer composed of positive and negative blocks will have a nonzero velocity depending on how the charges are distributed. This can be seen in the video below where positive (blue) and negative (red) monomers makeup a net neutral polymer. The polymers with more positive monomers on the end move in the direction of the electric field while the symmetric one remains in place. This has important consequences such as the end effect in end labeled free solution electrophoresis (ELFSE), a technique for the sequencing of DNA.
Polymer Colliding with a Post
One promising method for new DNA sequencing techniques is separating DNA through collisions with posts. While often ignored it has been suggested that hydrodynamic interactions may play a role in such systems . When using the method proposed in our paper we can see that there is indeed significant perturbation of the surrounding fluid and that it differs for the case of polymers being driven by an electric field (top) or a mechanical force/flow field (bottom).
Neutral Cross moves Perpendicular to the Applied Electric Field
Even more surprising than the fact that neutral objects can have non zero velocities when subject to an electric field is that the velocity can actually be perpendicular to the applied field. For certain geometries of positive (green) and negative (red) particles they can produced and electroosmotic flow perpendicular to the applied field due to steric interactions. By pumping the fluid upwards the particles are forced to move downwards as seen here.
- Hickey O.A. and Holm C., Electrophoretic mobility reversal of polyampholytes induced by strong electric fields or confinement, 2013, Journal of Chemical Physics 138 (19), 194905 
- Suo T., Shendruk T.N., Hickey O.A., Slater G.W. and Whitmore M.D., Controlling Grafted Polymers inside Cylindrical Tubes, 2013 46 (3), pp 1221–1230, Macromolecules 
- Hickey O.A., Shendruk T.N., Harden J.L. and Slater G.W., Simulations of Free-Solution Electrophoresis of Polyelectrolytes with a Finite Debye Length Using the Debye-Hückel Approximation, 2012 109 (9), 098302, Physical Review Letters 
- Hickey O.A., Harden J.L. and Slater G.W., Computer simulations of time-dependent suppression of EOF by polymer coatings, 2012, Microfluidics and Nanofluidics 13 (1), pp 91-97 
- Hickey O.A., Modulating Electro-osmotic Flow with Polymer Coatings, Ph.D. thesis, University of Ottawa 2012 
- Shendruk T.N., Hickey O.A., Slater G.W., Harden J.L.. Electrophoresis: When hydrodynamics matter, 2012 17 (2), Current Opinion in Colloid and Interface Science 
- Hickey O.A., Holm C., Harden J.L. and Slater G.W., Influence of Charged Polymer Coatings on Electro-Osmotic Flow: Molecular Dynamics Simulations, 2011 44 (23), Macromolecules, pp 9455–9463 
- Hickey O.A., Harden J.L., Holm C. and Slater G.W., Implicit method for simulating electrohydrodynamics of polyelectrolytes, 2010 105 (14), 148301, Physical Review Letters 
- Slater G.W., Holm C., Chubynsky M.V., de Haan H.W., Dube A., Grass K., Hickey O.A., Kingsburry C., Sean D., Shendruk T.N. and Zhan L., Modeling the separation of macromolecules: a review of current computer simulation methods, 2009, Electrophoresis 20 (5), pp.792-818 
- Hickey O.A., Harden J.L. and Slater G.W., Molecular Dynamics Simulations of Optimal Dynamic Uncharged Polymer Coatings for Quenching Electro-osmotic Flow, 2009, Physical Review Letters 102 (10), 108304 
- Hickey O.A. and Slater G.W., The diffusion coefficient of a polymer in an array of obstacles is a non-monotonic function of the degree of disorder in the medium, 2007, Physics Letters, Section A: General, Atomic and Solid State Physics 364 (6), pp. 448-452 
- Hickey O.A., Mercier, J-F, Gauthier, M.G., Tessier, F., Bekhechi S. and Slater G.W., Effective molecular diffusion coefficient in a two-phase gel medium, 2006, Journal of Chemical Physics 124 (20), 204903