MEASUREMENT OF ANISOTROGIC BROWNIAN-MOTION NEAR AN INTERFACE BY EVANESCENT LIGHT-SCATTERING SPECTROSCOPY

We performed a dynamic evanescent light-scattering experiment to obser ve the Brownian motion of the sphere particles near the solid-liquid i nterface. An evanescent wave generated in a colloidal solution picks u p information on the dynamics close to the solid interface within the submicrometer penetration depth. Measurement was made for various diam eters of polystyrene spheres in a wide wave-number range of light scat tering. The autocorrelation function obtained for particles smaller th an the penetration length shows nonexponential behavior, which is succ essfully described by considering the complex scattering wave number b rought about by the finite interaction region between the light and th e sphere. The diffusion constant near the interface is smaller than th at for free diffusion in a bulk solution, suggesting the suppression o f Brownian motion due to the hydrodynamic interaction between the sphe re and the solid wall. Furthermore, the Brownian motion was found to b e anisotropic with respect to the directions parallel and perpendicula r to the interface. The two different diffusion constants can be obtai ned uniquely by observation over a wide range of wave number. The diff usion constants thus obtained agree well with the values derived from hydrodynamic theory. [S1063-651X(98)11911-6].