D. L. Geng

Vertical vibration and shape oscillation of acoustically levitated water drops

We present the vertical harmonic vibration of levitated water drops within ultrasound field. The restoring force to maintain such a vibration mode is provided by the resultant force of acoustic radiation force and drop gravity. Experiments reveal that the vibration frequency increases with the aspect ratio for drops with the same volume, which agrees with the theoretical prediction for those cases of nearly equiaxed drops. During the vertical vibration, the floating drops undergo the second order shape oscillation. The shape oscillation frequency is determined to be twice the vibration frequency.

Surface waves on floating liquids induced by ultrasound field

We demonstrate a kind of wave pattern on the surface of floating liquids in a modulated ultrasound field. The waves are related to the liquid/solid phase transformation process. The nucleation sites of the eutectics locate at the center of these waves, and the eutectic growth direction is parallel to the propagation direction of the waves. It is revealed that such wave phenomenon can be ascribed to the interaction between ultrasound and eutectic growth at the liquid/solid interface. This result may provide a potential method for fabricating wave patterned surfaces on eutectic alloys.

Dendrite growth within supercooled liquid tungsten and tungsten-tantalum isomorphous alloys

The dendrite growth in both supercooled liquid pure W and binary W-Ta isomorphous alloys has been observed and measured by an electrostatic levitation technique. The liquid W and W-x%Ta (xu2009=u200925, 50, 75) alloys were substantially supercooled by up to 733u2009K (0.2u2009Tm) and 773u2009K (0.23TL), respectively. The measured density and the ratio of specific heat to emissivity displayed a linearly increasing tendency versus supercooling. The thermal dendrites in supercooled liquid tungsten achieved a maximum growing velocity of 41.3u2009m·s−1, and the concurrent recalescence process exhibited Johnson-Mehl-Avrami type kinetics. Liquid W-Ta alloys showed stronger supercoolability but a lower maximum dendrite growth velocity of only 35.2u2009m·s−1. The dendritic growth kinetics was always characterized by a power function relation to liquid supercooling. The microstructure of equiaxed grains transforms to the well-developed dendrites with the increase of supercooling. The grain refinement effect resulting from dendrite fragmentati...

The near-field acoustic levitation of high-mass rotors.

Here we demonstrate that spherical rotors with 40 mm diameter and 0-1 kg mass can be suspended more than tens of micrometers away from an ultrasonically vibrating concave surface by near-field acoustic radiation force. Their rotating speeds exceed 3000 rpm. An acoustic model has been developed to evaluate the near-field acoustic radiation force and the resonant frequencies of levitation system. This technique has potential application in developing acoustic gyroscope.

Surface wave patterns on acoustically levitated viscous liquid alloys

We demonstrate two different kinds of surface wave patterns on viscous liquid alloys, which are melted and solidified under acoustic levitation condition. These patterns are consistent with the morphologies of standing capillary waves and ensembles of oscillons, respectively. The rapid solidification of two-dimensional liquid alloy surfaces may hold them down.

Dynamics of water drop detachment from a superhydrophobic surface induced by an ultrasonic field

We present the dynamics of sessile water drops during their detachment from a superhydrophobic surface induced by ultrasound. The superhydrophobic surface not only serves as a reflector of the ultrasound emitted from the source but also reduces the adhesive force between the drop and the solid surface. The drop is subject to an acoustic radiation force in the ultrasonic field due to the nonlinear effect of the latter. By shifting the reflector upward to approach the first resonance distance, the sessile drop is first elongated in the vertical direction, with its contact line and contact angle decreasing, and finally detaches from the superhydrophobic surface when the acoustic radiation force overcomes the sum of the gravitational and adhesive forces. The acoustic radiation pressure and acoustic radiation force are calculated by solving the acoustic field with the finite element method. The results indicate that the distribution of acoustic radiation pressure provides the upward force to make the drop detach. After its detachment from the reflector, the drop undergoes vertical vibration accompanied by shape oscillations. Oscillations of a water drop that is pinned on the reflector are also demonstrated.We present the dynamics of sessile water drops during their detachment from a superhydrophobic surface induced by ultrasound. The superhydrophobic surface not only serves as a reflector of the ultrasound emitted from the source but also reduces the adhesive force between the drop and the solid surface. The drop is subject to an acoustic radiation force in the ultrasonic field due to the nonlinear effect of the latter. By shifting the reflector upward to approach the first resonance distance, the sessile drop is first elongated in the vertical direction, with its contact line and contact angle decreasing, and finally detaches from the superhydrophobic surface when the acoustic radiation force overcomes the sum of the gravitational and adhesive forces. The acoustic radiation pressure and acoustic radiation force are calculated by solving the acoustic field with the finite element method. The results indicate that the distribution of acoustic radiation pressure provides the upward force to make the drop deta...