A water tank, in which a 128-element HIFU spherical array is mounted in the bottom, and another 128-element phased array imaging probe mounted in center of the transducer bowl. The Verasonics Vantage 256 HIFU system (off screen) drives both the HIFU array and the imaging probe. The HIFU focus is near the water surface and the 2 MHz sound atomizes the water, making fog, and ejecting large drops.
@ 0:13 A green laser in the imaging probe housing illuminates the geometrical axis of the HIFU array; a white rectangular frame holds a black thermochromic sheet (liquid crystal changes color with temperature)
The HIFU focus is phased to different locations, visible as light spots on the black background. The laser light provides a stationary reference.
Initially, the focus is steered outside the allowable range, and only “sidelobes” are visible.
After 5 to 10 seconds, the intended focus appears as a single elliptical spot and is steered across the frame
The field of view zooms in for a better look.
@ 0:34 The liquid crystal sheet is removed and the laser now illuminates the water and a bubble cloud (demonstrating cavitation) that forms when the sound pressure is strong enough to create vapor cavities in the focal region. The main cavitation zone is very bright.
Some bubbles are mostly vapor and collapse completely when the field is turned off, but others contain gas (oxygen) that has come out of solution. Strong acoustic radiation force pushes some bubbles out of the focal zone in a stream.
Bubbles change the local acoustic velocity significantly, and thus change the structure of the HIFU field near the focal region: this results in the splitting of the focus into two zones (one at the regular focus, and another one after a gap).
@ 0:54 The view from above the water surface shows the plane illumination of the laser (we were looking edge-on before), and the HIFU focus is near the surface and is displaced in the plane of illumination.
The atomization makes fog that follows the turbulent air flow above the water.
The HIFU is pulsed ON-OFF between different focal locations.
@ 1:41 The “geyser fountain” can be controlled by HIFU beam direction, proximity of the focus to the water surface, and field intensity (acoustic power). Several views of this interesting effect are shown.