Abstract:
The interaction of sound with sound in a suspension can be mediated by the suspension's response to acoustic radiation pressure [H.J. Simpson and P.L. Marston, J. Acoust. Soc. Am. 98, 1731--1741 (1995)]. Four wave mixing mediated by a suspension with collinear pump and probe waves is investigated. Radiation pressure from counter propagating pump beams induces a periodic grating in the number density of hollow glass microspheres suspended in a water/sugar mixture designed to make the particles neutrally buoyant. The Bragg scattering amplitude was measured using a single PVDF transducer as both the probe source and receiver that is parallel with the standing pump wave nodal planes. Reflection coefficients are calculated by comparing the reflected and transmitted waves. By increasing the particle concentration, total Bragg reflectivities from the established grating reaching 20% have been seen, which is still in the region of agreement between the Born approximation and transfer matrix theory. The dynamics of both the grating formation and dissolution have been studied, leading to the conclusion that grating dissolution is not dominated by diffusion alone. Bragg scattering by gratings of gas bubbles caused by cavitation was detected. [Work supported by the Office of Naval Research.]