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Background
Ideally, perfectly concave ultrasonic transducers produce linear ultrasonic waves which combine to reinforce each other at one discrete point. In practical situations, however, imperfect curvature and variations in the piezoelectric crystal cause the focal point to be more properly characterized as a focal region. In this region, a host fluid is subjected to considerable forces. Motion induced in fluids due to these ultrasonic forces is known as streaming (See Figure 1). Streaming is used to noninvasively speed the introduction of medicines to the blood stream and to hasten chemical reactions through its stirring effect.
Figure 1. Streaming induced by high intensity ultrasound
When the focal region lies on the surface of a fluid, a fountain can result as the force of the ultrasonic waves pushes the fluid up and out of the way (See Figure 2). Fountain height and activity is closely linked to the transducer's depth and drive frequency.
Figure 2. Fountain created when
High Intensity
Ultrasound is focused at the fluid's surface
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