Scattering of focused ultrasound in water by spherical micro‐particles

Abstract

The scattering of focused ultrasound in water by three different types of microspheres (∼25‐75 μm diam), with ka < 1, was studied. The three scatterers were a spherical glass bubble, solid polystyrene spheres, and hydrogen bubbles. The measurement arrangement consisted of two transducers (transmitter and receiver) of 0.5‐in. diameter and with focussing lenses mounted on their faces. They were positioned so that their focal regions overlapped and their axes were 60° apart. A swept frequency ultrasonic signal was used, of 30‐μs duration, in which the frequency increased linearly from 3.5‐6.8 MHz. The glass bubble was mounted on a nylon filament and positioned in the overlapping focal regions. The polystyrene spheres were in solution, and reflections were obtained as they drifted through the focal regions. The hydrogen bubbles were generated by electrolysis and reflections obtained as they rose through the focal regions. From the shape of the echo envelope the frequency dependence of the scattered‐pressure amplitude was determined for each scatterer, and significant differences were observed between the echoes from the solid scatterers and the gas bubbles. The experimental results are compared with theoretical predictions.