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.
We developed an automatic measurement system for ultrasonic velocity in the megahertz region particularly designed for materials science. The ultrasonic pulse method is fully supported by digital equipment and the correlation function of the 1st and 2nd pulse echoes is numerically obtained from the ultrasonic velocity precisely determined to a relative accuracy of 10-6. An ultrasonic corner reflector is also employed to realize the high reproducibility of the measurement without adjusting the parallelism of the countering transducer and reflector. The performance of the system is examined by determining the concentration dependence of sound velocity in an electrolyte solution in a dilute range.
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