DUAL-BEAM INCOHERENT CROSS-CORRELATION SPECTROSCOPY

An incoherent dynamic light-scattering technique is developed to measu re the local velocity and its statistics. By employing two parallel la ser beams of different colors, the technique measures the cross-correl ation function of the scattered intensities from two separate illumina ting volumes. Because there is no phase coherence between the two lase r beams, the measured cross-correlation function is sensitive only to the intensity fluctuations caused by a seed particle that crosses the two beams in succession. The flow velocity is obtained from the measur ed particle transit time. We frame the scattering theory so as to acco unt for the two-beam scattering geometry. Our experiment verifies the calculation and demonstrates applications of the technique. The method has the unique feature of being able to measure simultaneously the lo cal velocity in two opposite directions perpendicular to the incident laser beams. Its advantages are high spatial resolution and accuracy, fast temporal response, and ease of use. The technique is useful in st udies of turbulent flows, sedimentation of heavy particles, and flow p henomena in complex fluids.