The motion of particles in a fluid within an intense ultrasound beam is called what?

• A. Transient cavitation
• B. Thermal interaction
• C. Acoustic streaming
• D. Rarefaction

Answer: (C)

Acoustic streaming is the steady motion of fluid produced by an intense ultrasound beam. While the ultrasound wave causes particles to oscillate rapidly back and forth, nonlinear interactions and the gradual loss of energy (attenuation) impart a small, time-averaged force to the fluid. This creates a net flow, or streaming, rather than just rapid oscillation. This concept is distinct from transient cavitation, which involves bubble formation and collapse; thermal interaction, which relates to heating from energy absorption; and rarefaction, which is simply the low-pressure phase of the wave. In short, acoustic streaming describes the continuous, directional movement of fluid driven by the ultrasound field.