ASA 125th Meeting Ottawa 1993 May

4pPA9. Ultrasonic four-wave mixing mediated by a suspension of microspheres in water: Comparison between two scattering theories.

Harry J. Simpson

Naval Res. Lab., Code 7136, 4555 Overlook Ave. SW, Washington, DC 20375

Philip L. Marston

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

Two ultrasonic pump waves are used to produce a grating in a suspension of 25-(mu)m-diam latex particles. A higher frequency ultrasonic wave is used to probe the established grating to produce ultrasonic Bragg scattering. The scattering depends strongly on the pump waves and is an unusual class of nonlinearity. A previously summarized model of the interaction [H. J. Simpson and P. L. Marston, J. Acoust. Soc. Am. 90, 2244 (A) (1991) and 91, 2351 (A) (1992)] uses an Epstein layer approximation and effective medium approximation. This previous analysis is compared with a Fourier series approximation of the grating and the resulting predicted scattered wave fields. The Fourier series expansion is only valid when the Bragg condition is satisfied, thus giving no information off the Bragg peak. The two models match at higher pump pressures and predict similar onset pressures, but differ slightly in the medium pressure region. Both results are compared to experimentally measured scattering amplitudes for a range of probe frequencies and pump pressures. [Work supported by ONR.]