Ultrasound Field and Bubbles
The fundamentals of ultrasound in a fluid are presented. Ultrasound propagates in a fluid as a longitudinal wave, where the fluid particles vibrate along the direction of propagation. A sound wave is partially reflected at the boundary between two media with different characteristic impedances. In particular, almost all of a sound wave is reflected at the boundary between water and air and the reflected sound is superposed on the incident wave to produce a standing wave. A fluid medium exhibits elastic nonlinearity. Primarily as a result of this nonlinearity, nonlinear distortion of the sound waveform occurs. A gas bubble in a liquid vibrates in the presence of sound and reradiates the sound, resulting in strong scattering of the sound. In addition, when the bubble resonates with the incident ultrasound, at a frequency specified by certain parameters, e.g., bubble size, the bubble exhibits significant elastic nonlinearity.