Visualizing cavitation bubble transport induced by μs timescale acoustic streaming

Focused, short-duration and high-amplitude ultrasound pulses are used in applications ranging from medical histotripsy to recent advances in controlled surface erosion for localized material sampling and for e-waste recycling of gold. These applications are enabled by cavitation (and heating to a lesser degree) generated in the focal zone. When cavitation occurs in free water, acoustic streaming also becomes a significant nonlinear effect to consider. Usually, acoustic streaming is treated as a longer timescale, averaged movement of the liquid. However, with intense and short ultrasound pulses, streaming occurs on the same timescale as the pulses. We show high-speed (1 million fps) footage of a focused 4.24-MHz ultrasound field that causes heating, cavitation, and a turbulent jet flow, in water, during a single ultrasonic ∼200 cycles (∼50 μs) pulse. A shadowgram produced by the heating enables direct visualization of the turbulent flow. Additionally, cavitation bubbles are generated, and these are transported a significant distance away from their origin by this flow. Understanding and accounting for this single ultrasound pulse timescale flow field is important in applications where cavitation is required to be confined within a well-defined zone, such as localized material removal with focused ultrasound. [Work supported by the Research Council of Finland, Grant Numbers 347459 and 349200.]