(420h) Low-Frequency Dielectric Response of a Single Particle in Aqueous Suspensions

For a charged colloidal particle in aqueous suspension, an electric double layer composed of absorbed and attracted counterions, forms at the interface between the particle and the liquid. While two mechanisms, diffusion-controlled α-relaxation and surface current-related β-relaxation, have been discussed to understand the physical property of the counterion migration within the double layer in a nonuniform electric field, previous experiments on colloidal ensembles weren't able to decide the competition hence the dominance of the two. In order to solve the debate concerning the two microscopic theories, we investigate systematically the relaxation process of the electric double layer, as a function of particles size, temperature and solvent viscosity. Specifically, we use an optical tweezers-based single-particle dielectrophoretic (DEP) force spectroscopy and measure the dependence of the DEP crossover frequency. This practice avoids the presence of other electrokinetic mechanisms than DEP as complicating factors, which hindered the completeness of interpretations of published results. Finally we compare our experimental data with both predictions of α- and β-relaxations.