Drop Dynamics And Solidification Mechanism Of Water And Aqueous Solution Under Acoustic Levitation

As an important technique for containerless processing, acoustic levitation can be applied to various solid and liquid regardless of their conductivity, and therefore is a preferential technique for the investigations of drop dynamics. Furthermore, it can avoid effectively the heterogeneous nucleation caused by container walls and enhance the supercooling capability, which is beneficial for studying the solidification process of some organic, inorganic and low-melting point metallic materials. In the present dissertation, the equilibrium shapes and shape evolutions of acoustically levitated drops are investigated experimentally and theoretically. The solidification process of water drops under acoustic levitation is analyzed systematically, and a classical molecular dynamics simulation is performed to study the surface properties of supercooled water. The supercooling distributions of NaCl and KC1 aqueous solutions with different concentrations are also studied jointly by means of acoustic levitation technique and molecular dynamics.Equilibrium shape and shape bifurcation of rotating liquid drops under acoustic levitation are studied in experiments. The shape evolution of two-, three- and four- lobed shape families as a function of angular velocity are analyzed systematically. The experimental results show that the axisymmetric shape bifurcates into the two-, three-, and four-lobed shapes at lower critical angular velocities, which is regarded as a result of initial flattening of water drops with the effect of acoustic pressure. A new shape family, five-lobed drops are observed. That may be relevant to the initial flattening of drops.A numerical simulation with finite element method is carried out to account for the influence of acoustic field on the shape bifurcation. The results reveal that the initial flattening induced by the acoustic pressure makes the bifurcation point shift towards lower rotating rates. The simulated results agree well with the experiments.The supercooling of bulk water is measured under acoustic levitation and the maximum supercooling is up to 32 K. The comparisons of supercooling at the two different sound pressure levels (SPL): 160.6 and 164.4 dB, corresponding to the lowest and highest level of drop deformations, are carried out, and the mean supercoolings are about 13.7 and 9.7 K respectively. The supercooling level of water under acoustic levitation is lower than that of the homogeneous nucleation, and moreover the increase of SPL decreases the supercooling.According to the supercooling distributions, the nucleation characteristics of water...