Abstract: Eric Dufresne

Electrostatics Meets Entropy

                     Physics and Astronomy Colloquium  Friday, November 4, 2011                        2:30 pm Thompson Physical Laboratory 205

Electrostatic forces drive systems with free charges toward charge neutrality. However, entropy can drive persistent charge separation over nanometer and micrometer length scales.  This is typically manifested as charged molecules or interfaces dressed with a diffuse layer of counterions.  Such charge separation underlies the structure of polymers and proteins, the stability of suspensions and even our nervous system’s ability to process information.

We study electrostatic interactions over short length scales, deep inside the diffuse counterion  cloud.  Our model system consists of micron-sized plastic particles floating in oil.  The surfaces of  these particles spontaneously charge at room temperature under appropriate solvent conditions. We  exploit optical forces and thermal fluctuations to measure femtoNewton scale repulsive electrostatic  forces between the particles. Interestingly, the repulsion between any pair of particles can be  strongly reduced when other particles are nearby.   This many-body effect is not accounted for by the  usual suspects: nonlinearity in the Poisson-Boltzmann equation or counter-ion correlations.    Instead, we can quantitatively predict interparticle forces in a variety of geometries by  simply assuming that the surface charge densities adjust to keep the surface electrostatic potentials  at a fixed value.  Finally, I will describe the curious balance of electrostatic, chemical and entropic  forces that leads to constant surface potentials and discuss implications for the structure and stability of systems with large numbers of particles.