Measurement of anisotropic Brownian motion near an interface by evanescent light-scattering spectroscopy

We performed a dynamic evanescent light-scattering experiment to observe the Brownian motion of the sphere particles near the solid-liquid interface. An evanescent wave generated in a colloidal solution picks up information on the dynamics close to the solid interface within the submicrometer penetration depth. Measurement was made for various diameters of polystyrene spheres in a wide wave-number range of light scattering. The autocorrelation function obtained for particles smaller than the penetration length shows nonexponential behavior, which is successfully described by considering the complex scattering wave number brought about by the finite interaction region between the light and the sphere. The diffusion constant near the interface is smaller than that for free diffusion in a bulk solution, suggesting the suppression of Brownian motion due to the hydrodynamic interaction between the sphere and the solid wall. Furthermore, the Brownian motion was found to be anisotropic with respect to the directions parallel and perpendicular to the interface. The two different diffusion constants can be obtained uniquely by observation over a wide range of wave number. The diffusion constants thus obtained agree well with the values derived from hydrodynamic theory.