Figure 5.
Air-water interface filmed with (a) the camera lens in air such that the reflections we see in the bottom of the frames are due to an optical reflection of the same jet and (b) the camera lens positioned so that half the objective is facing air and the other half facing water to simultaneously observe effects in water and air. (a) Shows a sequence of images taken at very low intensities (180 W/cm2) with the camera positioned at a slightly downward angle to catch the surface of the water. Atomization does not occur until the drop chain emerges. At some point, the top drop becomes unstable and atomization jets are released. (b) A sequence of images showing the air to water interface at high acoustic intensities of 24,000 W/cm2. Cavitation occurs before atomization or jetting and is faintly visible just to the right of the HIFU arrowhead in the first frame, which is taken 20 μs after the acoustic wave arrives at the water surface. As the ultrasonic pulse continues, the cavitation bubbles just below the water surface are joined by a cavitation bubble cloud further below the surface, possibly due to the standing wave pattern that develops upon the reflection of the acoustic wave. In both cases, the total HIFU-on time was 10 ms. This figure is available in movie form in supplement 1.