DIFFUSING-WAVE SPECTROSCOPY AND INTERFEROMETRY

Diffusing-wave spectroscopy and interferometry, the extensions of dyna mic light scattering to materials which exhibit a very high degree of multiple scattering, have been used to measure the early-time, short l ength scale motion of Brownian particles. The transition from ''ballis tic'' to ''diffusive'' motion is observed. In very dilute samples, thi s transition is described very well by theories which explicitly accou nt for the time-dependent hydrodynamic interaction between a Brownian particle and the surrounding fluid. For particle volume fractions exce eding a few per cent, the data deviate from the theory for dilute susp ensions, but exhibit a remarkable scaling with the suspension viscosit y.