【作者】 吕勇军；

【导师】 魏炳波；

【作者基本信息】 西北工业大学 ， 材料物理与化学， 2006， 博士

【摘要】 本文从实验和理论两方面深入地探讨了声悬浮液滴的运动规律；研究了声悬浮条件下水滴的过冷与形核机制；采用分子动力学方法模拟了过冷态水的表面性质；借助声悬浮技术和分子模拟相结合的方法研究了不同浓度下NaCl和KCl水溶液的过冷度分布规律。主要取得以下几方面的研究结果： 实验研究了声悬浮条件下液滴的平衡形状演变规律。深入分析了两瓣、三瓣和四瓣液滴形状的演化过程。证实声压导致的液滴初始形状的变形是造成各个分瓣形状簇临界角速度漂移的主要原因。实验中首次观察到一种新的平衡形状分支：五瓣形状，认为它的出现与液滴初始形状的变形程度有关。 采用有限元方法模拟了声悬浮液滴各个形状簇的演化过程，预测了两瓣、三瓣和四瓣形状的临界角速度与自身演变规律。同实验结果比较，两者符合较好。 实验研究了声悬浮水滴的深过冷规律，获得的最大过冷度为32K。比较声压幅值SPL=160.6dB和SPL=164.4dB下液滴的过冷度分布，两种条件下的平均过冷度分别为13.7K和9.7K，均远小于均质形核过冷度，而且增大声压幅值会显著地降低过冷度。 根据液滴的过冷度分布，采用统计方法分析了160.6dB和164.4dB两个声压水平下水滴的形核规律。结果表明声悬浮条件下水滴的形核主要发生在表面附近。提高声压幅值会加剧水滴的变形，导致表面积扩大，形核数目增多，过冷度水平降低。造成表面形核过程的主要因素有两方面：一是声压与液滴的相互作用主要发生在表面附近，这里气泡的运动及空化效应对形核的促进作用比较明显。二是表面附近的声流加速了液滴表层与周围介质的热交换，使得表面温度较内部低，有利于形核。对比两个声压幅值下的形核率，发现高声压幅值能够在一定程度上促进形核，其主要原因是高声压能够在更大程度上促进气泡生长，加剧液滴周围介质的流动，增大形核率。 选用SPC／E和TIP4P两种势函数对过冷态水的表面性质进行了分子动力学模拟。结果表明随着过冷度的增大，气／液界面厚度减小，表面张力增大，表面势增强。SPC／E势的表面张力模拟结果与实验值符合很好，而TIP4P势的模拟值更多还原

【Abstract】 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更多还原