Osmotic Compressibility of Colloidal Crystals and Suspensions Measured by Dielectrophoresis and Fluorescence Microscopy

Osmotic compressibility is the relative volume change upon the change in pressure. For colloids, it is measured traditionally by scattering methods, which limit to only dilute solutions below 1.0 wt%. Between 1.0 and 2.0 wt%, highly repulsive colloids can form Wigner crystals. There is no experimental study to date of the compressibility near such phase transition. We present a new method for measuring compressibility of colloids using fluorescence microscopy and non-invasive dielectrophoretic force field generated from radiofrequency electric field. With known force field and spatial distribution of particle concentration determined from fluorescence microscopy, we can use Einstein’s osmotic equilibrium theory to calculate the osmotic pressure and hence construct the equation of state and determine the compressibility. With this method, we can determine the compressibility for colloids from the disordered phase to crystalline phase by either changing salt or particle concentration. To validate our method, we compare our results with charge renormalization theory by Alexander et. al. (J. Chem. Phys. 1984, 80, 5776) and computer simulations.