The Interaction of Acoustical Waves with Concentration Gradients

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The interaction of ultrasonic waves with concentration gradients in solutions has been studied quantitatively with schlieren techniques having a resolution of the order of 10−4 cm. Although the gradients have been produced electrochemically at anodes and cathodes in copper sulfate solutions, the conclusions have been extended to concentration gradients in solutions in general. Moderately intense ultrasonic waves (e.g., 1 watt/cm2) are more effective than conventional agitation in disrupting these concentration gradients primarily because of the micro agitation associated with cavitation at or near the interface. Streaming effects in the ultrasonic field augment cavitation. The effective dimensions of the concentration gradients are reduced to a value approaching 10−4 cm as compared with 10−3 cm for ordinary directional flow. This accounts in good part for the increments produced by ultrasonic waves in the rates of various physical and chemical processes involving two or more phases. In a standing wave of low intensity with the wave front perpendicular to the phase boundary, the concentration gradients become localized at the displacement nodes or loops depending on whether there is a deficit or excess of solute at the interface.