The experimental apparatus, as shown in Fig. 2, are composed of an ultrasonic generator, a transparent test cell, a high-speed digital video camera system, an LED light source, and a data acquisition system. The power of ultrasonic generator varies from 0 to 540 W, corresponding to the ultrasonic intensity range of 0–6.26 W cm−2. In general, there is no cavitation effect within this JDTic intensity range. For the ultrasonic vibrator employed in this work, three frequencies are available (20, 28 and 40 kHz) and acoustic intensity maintains constant when the frequency changes. To realize the idea, the acoustic intensity in different frequencies was tested by an acoustometer and was adjusted to an identical value by changing the input power. The test cell is symbiosis made of Perspex and the ultrasonic vibrator is attached to one side of the test cell. Ultrasonic waves generated by the vibrator propagate forward, hit the other side of the test cell, and then reflect. Two waves superpose together to yield a horizontal standing wave field in the test cell. The high-speed digital video camera (NAC Hotshot 1280) is equipped with a lens (Mitutoyo 5× objective), which can accurately capture the motion of micron-sized droplets. An LED is used for lighting without changing the oil temperature in the cell.