Determination of Colloidal Osmotic Equation of State by Dielectrophoresis

Osmotic equation of state [P(N,T)] of a colloidal suspension, where P is the osmotic pressure, N the particle number density and T the absolute temperature, describes both the mechanical properties and phase behavior of a colloidal suspension. As an alternative to the conventional sedimentation or scattering approaches to determine P(N,T), we propose a new approach by dielectrophoresis (DEP). Spatial distributions of the density of fluorescent nanoparticles in a DEP field -- imaged by confocal microscopy -- can be used to determine the DEP force field at low particle concentration, at which the inter-particle interactions are negligible. Using the known force field and Einstein's osmotic equilibrium equation, we calculate P(N,T) from the particle density profiles of interacting, charge-stabilized polystyrene latex particles under different salt concentrations.