Research Interests

Electrostatics in Soft Matter

Experiments in the labs of Dennis Discher and Tobias Baumgart have demonstrated phase separation in micelles and bilayers that form from a blend of two types of block copolymer - one charged and one neutral. The main driving force for this effect is believed to be an effective attraction between the charged polymers, induced by the divalent calcium ions present in the system. Together with Andrea Liu I developed a simple model for the physics of this phenomenon.

In a related project in collaboration with David Christian and Paul Janmey, we have studied phase separation in charged-neutral lipid mixtures in a monolayer, again driven by addition of salts containing (divalent) calcium. Using a coarse-grained molecular dynamics simulation we have demonstrated that this effect can indeed be treated as purely electrostatic. We are currently exploring the phase behavior as a function of the lipid charge and salt conditions.

More generally, I am interested in novel ways of efficiently treating electrostatics in soft matter systems, especially when if comes to effects that defy mean-field treatment, such as the counterion-mediated attractions mentioned above.

Jamming in Granular Media

Jammed granular media are disordered solids that consist of many particles (marbles, sand, rice) that typically need to be pressed together to be rigid, because of the lack of attractions between such particles. Smooth spheres are an interesting type of such material, because the transition to a loose, non-rigid material, as the pressure is lowered, has many features of a critical phase transition. Besides, they provide a model for emulsions and wet foams.

Granular media close this unjamming transition are very unusual solids from the point of view of elasticity theory: Deformations are strongly non-affine, in such a way that the grains are predominantly sliding past each other instead of following the displacement field that a continuum material would display. See publication 4 on my list for more information on how to analyze this non-affinity and on the diverging length scale that we identified in this system. I am still working with Wim van Saarloos and Martin van Hecke on several aspects of this system.