On the generation of steady fountains by high intensity focused ultrasonic: flow field and simple formulation

Laboratory experiments were conducted to investigate the formation of fountains at a quiescent water-air interface with high-intensity focused ultrasound. Particle image velocimetry was used to characterize the flow field within and below the vicinity of the ultrasonic focal spot. We studied two types of ultrasonic transducers with different wave frequencies. Three fountain formation regimes were induced by varying the pressure level of the two transducers, namely weak, intermediate, and high-forced fountains. Between different types, the fountain height underwent a step-change in response to increases in acoustic pressure. A force balance using the flow field data shows that the magnitude of axial momentum flux is an order of magnitude lower than that of gravity and surface tension, indicating that the dominant driving force for the fountain generation is the acoustic pressure. We propose a basic model for the stable fountain shape considering the acoustic pressure, gravity, surface tension, and axial momentum. Our model neglects viscous force, which precludes capturing the intermediate fountain surface curvature; however, it predicts the geometry of the weak and intermediate steady fountains.