平衡接触角对受热液滴在水平壁面上铺展特性的影响

壁面温度是影响壁面润湿性的重要外部条件. 为解决液滴铺展中三相接触线处应力集中问题, 已有研究多采用预置液膜假设, 但无法探究壁面温度对润湿性的影响. 本文针对受热液滴在固体壁面上的铺展过程, 基于润滑理论建立了演化模型, 通过数值模拟, 从平衡接触角角度分析了温度影响壁面润湿性及铺展过程的内部机理. 研究表明: 随温度梯度增大, 液滴所受Marangoni效应增强, 致使液滴向低温区的铺展速率加快; 铺展过程中, 位于高温区的接触线与液滴主体部分间形成一层薄液膜, 重力与热毛细力先后主导该区域的铺展; 当液-固或气-液界面张力对温度的敏感度高于另两个界面时, 低温区方向的平衡接触角不断增大, 使壁面润湿性恶化, 导致液滴铺展减慢; 而当气-固界面张力对温度的敏感度高于其他两个界面时, 低温区方向上的平衡接触角将减小, 由此改善壁面润湿性, 加快液滴铺展; 在温度影响壁面润湿性和液滴铺展过程中, 平衡接触角起关键作用.     

微细通道内蒸发薄液膜区壁面滑移及微流动特性

基于扩展Young-Laplace方程和动力学理论研究微通道中蒸发薄液膜区固液界面附近流动和传热特性,考虑压力特征、壁面滑移和温度跳跃建立物理模型.利用边界层近似,提出一种计算固液界面吸附微液层热阻的方法,导得固液界面的热阻和温度.数值模拟结果表明,壁面微流动会降低毛细压力梯度,增加壁面热阻,降低液相过热度,恶化液膜铺展和传热性能,在薄液膜区不可忽略.阐明壁面微流动含义,指出滑移系数与吸附流动微液层厚度的关系.     

自润湿流体液滴的热毛细迁移特性

采用数值模拟方法研究了自润湿流体液滴的热毛细迁移特性.基于润滑理论和滑移边界条件建立了二维液滴运动的演化模型,分析了液气界面张力极小值对应温度在壁面上的位置(临界点)与液滴位置间的相对关系对液滴运动特性的影响.结果表明,对于壁面润湿性不随温度变化的情形,随液滴初始位置相对临界点的向左移动,液滴的迁移方向发生改变,但液滴受热毛细力驱动总是向界面张力高的方向移动.对于壁面润湿性随温度变化的情形,无论液滴初始放置于临界点何处,受高温侧壁面润湿性恶化的影响,液滴均向低温区迁移;随液滴初始位置相对临界点的向左移动,液滴受方向向左的热毛细力增大,提高了其向低温区的迁移速率.控制自润湿流体液滴运动可通过调控临界点与液滴位置间的关系来实现,欲抑制液滴向低温区的迁移,则应将液滴放置于临界点右侧.     

微液滴在不同能量表面上润湿状态的分子动力学模拟

微小液滴在不同能量表面上的润湿状态对于准确预测非均相核化速率和揭示界面效应影响液滴增长微观机理具有重要意义. 通过分子动力学模拟, 研究了纳米级液滴在不同能量表面上的铺展过程和润湿形态. 结果表明, 固液界面自由能随固液作用强度增加而增加, 并呈现不同液滴铺展速率和润湿特性. 固液作用强度小于1.6的低能表面呈现疏水特征, 继续增强固液作用强度时表面变为亲水, 而固液作用强度大于3.5的高能表面上液体呈完全润湿特征. 受微尺度条件下非连续、非对称作用力影响, 微液滴气液界面存在明显波动, 呈现与宏观液滴不同的界面特征. 统计意义下, 微小液滴在不同能量表面上铺展后仍可以形成特定接触角, 该接触角随固液作用强度增加而线性减小, 模拟结果与经典润湿理论计算获得的结果呈现相似变化趋势. 模拟结果从分子尺度为核化理论中的毛细假设提供了理论支持, 揭示了液滴气液界面和接触角的波动现象, 为核化速率理论预测结果和实验测定结果之间的差异提供了定性解释.     

液滴在气固交界面变形移动问题的光滑粒子流体动力学模拟

为准确模拟液滴在气固交界面变形移动问题,对基于连续表面张力模型的表面张力光滑粒子流体动力学方法进行了改进.改进方法采用新的边界处理方式和界面法向修正方法,即将固体边界虚粒子色函数值根据液面的位置进行相应设定以保证气-液-固三相交界处流体粒子的界面法向沿接触线法线方向,引入Brackbill提出的壁面附着力边界条件处理方法,对在气-液-固三相交界处的流体粒子及部分固体边界虚粒子的界面法向进行修正,修正前后保持法向模值不变,得到了含壁面附着力边界条件的表面张力算法.模拟了受壁面附着力影响的水槽中液面的变化过程、液滴润湿壁面过程和剪切气流驱动液滴在固体表面变形脱落过程,并与流体体积函数方法进行了对比.结果表明,该方法在处理壁面附着力问题时精度较高,稳定性较好,适合处理工程中液滴在气固交界面变形移动问题.     

液滴撞击热壁面的相变行为研究

本文基于格子Boltzmann方法建立了液滴撞击热壁面的数学模型,研究了撞击过程液滴相变传热机制,重点分析了壁面温度与We数的影响规律.结果表明,随着壁面温度升高,液滴撞击壁面出现接触沸腾、膜态沸腾相变模式.接触沸腾过程中液滴内部与壁面接触的三相线附近出现最大局部热流.当壁面温度超过Leidenfrost点后,撞击液滴因界面蒸发产生的气膜托举作用而与壁面无接触地铺展、回缩直至完全反弹形成膜态沸腾,这一阶段液滴所能达到的最大铺展因子与液滴We数呈幂次关系(指数为0.313).     

固着液滴蒸发薄液膜区传质传热特性数值研究

本文基于扩展Young-Laplace方程和动力学理论,以水为工质,建立固着液滴蒸发薄液膜区、宏观弯月面区、固体基底内部及气液、液固、气固界面的耦合传热模型,并探究固体基底厚度和热物性对蒸发薄液膜区传热特性的影响。结果表明,由于液滴蒸发过程中的冷却效应,三相接触线附近的基底温度低于周围固体温度;蒸发速度越快,温度差越大。蒸发薄液膜的长度、最大质量流量和传热量与基底厚度与液滴半径比值h_r有关;当h_r=0.4时,薄液膜区传热对总体传热贡献最大;基底的热导率越小,蒸发液滴具有更长的蒸发薄液膜长度及更强的界面传热,薄液膜区传热对总体传热影响越显著.     

液滴振荡模型及与数值模拟的对比

由于碰撞壁面后液滴内部流动的复杂性, 以及气-液-固三相间的相互作用, 对液滴碰撞壁面形态变化的数学理论研究有较大的难度, 因此所见者多为实验和数值模拟. 本文通过对液滴受力状态的分析, 得到了惯性力、黏性力和表面张力带经验系数的表达式, 并进一步建立了液滴碰撞壁面振荡模型, 得到了液滴铺展半径的振荡表达式, 以及表面张力、黏性系数等参数对液滴铺展的影响. 最后通过与液滴衰减振荡数值模拟结果的对比, 确定了液滴振荡模型中的修正系数, 验证了模型的可行性. 关键词： 液滴碰撞 振荡 铺展半径/高度 数值模拟     

蒸发相变与界面流动耦合机理研究

本文提出了一种新模型来研究液层在其纯蒸气中的蒸发热动力学特征,尤其是当蒸发界面张力驱动流占主导作用时(如微重力环境中)液层热毛细对流和界面蒸发始终耦合在一起。气-液界面的传热传质规律有待深入研究。本文数值模拟研究了蒸发相变界面热毛细对流与蒸发效应的耦合机质,得到了不同蒸发模式和不同强度热毛细对流蒸发液层的温度分布、蒸发速率以及对流流场分布的数值解。论述了蒸发Biot数和Marangoni数对界面传热传质的影响,发现并解释了蒸发和热毛细耦合的三种模式。     

倾斜粗糙壁面上含不溶性活性剂溶液的动力学特性

针对倾斜随机粗糙壁面上含不溶性活性剂溶液的流动过程, 采用润滑理论建立了液膜厚度和浓度的时空演化模型, 通过PDECOL程序数值求解得到了液膜流/液滴铺展的动力学特性及壁面结构参数的影响. 研究表明: 在重力分量和Marangoni效应共同作用下, 液膜流/液滴铺展速度加快, 液膜边缘和液滴中心出现毛细隆起, 液膜/液滴底部出现凹陷, 同时受粗糙壁面影响, 液膜表面变形更显著. 增加壁面倾角θ具有使重力分量和Marangoni效应增强, 导致隆起和凹陷程度均有所增加的作用. 增大壁面高度D可使液膜流/液滴铺展速度加快, 表面变形放大. 而壁面波数k₀则使液膜流/液滴铺展过程减缓, 抑制隆起和凹陷产生. 与液膜流相比, D和k₀对液滴铺展速度的影响相对较小. 关键词： 随机粗糙壁面 液膜 Marangoni效应 倾斜流动     

Sonoprocessing of wetting of SiC by liquid Al: A thermodynamic and kinetic study

The sonoprocessing of droplet spreading during the wetting process of molten aluminum droplets on SiC ceramic substrates at 700 °C is investigated in this paper. When wetting is assisted by a 20 kHz frequency ultrasonic field, the wettability of liquid metal gets enhanced, which has been determined by the variations in thermodynamic energy and wetting kinetics. Wetting kinetic characteristics are divided into two stages according to pinning and depinning states of substrate/droplet contact lines. The droplet is static when the contact line is pinning, while it is forced to move when the contact line is depinning. When analyzing the pinning stage, high-speed photography reveals the evidence of oxide films being rapidly crushed outside the aluminum droplet. In this work, atomic models of spherical Al core being wrapped by alumina shell are tentatively built, whose dioxide microstructures are being transformed from face-centered cubic into liquid at the atomic scale. At the same time, the wetting experiment reveals that the oxide films show changes in the period of sonoprocessing from 3rd to 5th second.During the ultrasonic spreading behavior in the late stage, there is a trend of evident expansion of the base contact area. The entire ultrasonic process lasts for no longer than 10 s. With the aid of ultrasonic sinusoidal waves, the wettability of metal Al gets a rapid improvement. Both molecular dynamic (MD) investigations and the experiments results reveal that the precursor film phenomenon is never found unless wetting is assisted by ultrasonic treatments. However, the precursor film appears near the triple line after using ultrasonics in the droplet wetting process, whose formation is driven by ultrasonic oscillations. Due to the precursor film, the ultrasonic wetting contact angle is lower than the non-ultrasonic contact angle. In addition, the time-variant effective ultrasonic energy has been quantitatively evaluated. The numerical expressions of thermodynamic variables are well verified by former ultrasonic spreading test results, which altogether provide an intrinsic explanation of the fast-decreasing contact angle of Al/SiC.     

Experimental investigation of liquid drop evaporation on a heated solid surface

Evaporation of a water drop was studied experimentally at a temperature difference between the solid surface and surrounding atmosphere from 30 to 60 °C. The studies were performed on the substrates with micro- and nanocoatings with different wettability. The features of evaporation were studied for the pinned, partially pinned, and depinned three-phase contact line (solid-liquid?gas interface). It is shown that with a decrease in the water drop volume, the specific evaporation rate (mass flow per unit of surface area) increases, particularly at the last stage of evaporation.     

Evaporation of liquid droplets from a surface of anodized aluminum

The results of study of evaporation of water droplets and NaCl salt solution from a solid substrate made of anodized aluminum are presented in this paper. The experiment provides the parameters describing the droplet profile: contact spot diameter, contact angle, and droplet height. The specific rate of evaporation was calculated from the experimental data. The water droplets or brine droplets with concentration up to 9.1 % demonstrate evaporation with the pinning mode for the contact line. When the salt concentration in the brine is taken up to 16.7 %, the droplet spreading mode was observed. Two stages of droplet evaporation are distinguished as a function of phase transition rate.     

溶液液滴蒸发变干的环状沉积

液体蒸发驱动的颗粒自组装现象在许多的工业技术中有重要应用. 本文利用显微镜观测含有颗粒物质的液滴变干后留在固体表面的颗粒形成的环状沉积图案. 采用微米粒径的SiO₂小球水溶液液滴蒸发变干模拟咖啡环的形成过程, 结果发现液滴蒸发过程中接触线的钉扎是环状沉积的必要条件. 在液滴蒸发过程中颗粒随着补偿流不断的向液滴边缘移动, 聚集在接触线处形成环. 液滴蒸发变干后残留在液滴内部的颗粒数随颗粒质量分数的增加而增加, 可以达到单层的颗粒排列. 而玻璃衬底上的颗粒环在颗粒质量分数很小时, 形成单层排列, 且一排一排地生长. 蒸发过程中颗粒环由于液滴边缘的尺寸限制向液滴中心缓慢移动. 这会导致液滴中不同大小颗粒的分离. 关键词： 液滴 接触线 蒸发 颗粒     

Investigations into wetting and spreading behaviors of impacting metal droplet under ultrasonic vibration control

Ultrasonic-assisted metal droplet deposition (UAMDD) is currently considered a promising technology in droplet-based 3D printing due to its capability to change the wetting and spreading behaviors at the droplet-substrate interface. However, the involved contact dynamics during impacting droplet deposition, particularly the complex physical interaction and metallurgical reaction of induced wetting-spreading-solidification by the external energy, remain unclear to date, which hinders the quantitative prediction and regulation of the microstructures and bonding property of the UAMDD bumps. Here, the wettability of the impacting metal droplet ejected by a piezoelectric micro-jet device (PMJD) on non-wetting and wetting ultrasonic vibration substrates is studied, and the corresponding spreading diameter, contact angle, and bonding strength are also discussed. For the non-wetting substrate, the wettability of the droplet can be significantly increased due to the extrusion of the vibration substrate and the momentum transfer layer at the droplet-substrate interface. And the wettability of the droplet on a wetting substrate is increased at a lower vibration amplitude, which is driven by the momentum transfer layer and the capillary waves at the liquid–vapor interface. Moreover, the effects of the ultrasonic amplitude on the droplet spreading are studied under the resonant frequency of 18.2–18.4 kHz. Compared to deposit droplets on a static substrate, such UAMDD has 31% and 2.1% increments in the spreading diameters for the non-wetting and wetting systems, and the corresponding adhesion tangential forces are increased by 3.85 and 5.59 times.     

Spreading of gallium on the surface of a contact layer of variable composition from the Pb–Bi system

An approach for obtaining substrates of variable composition of metal systems is proposed. The spreading of drops of gallium of uniform size on the surfaces of areas of variable composition, obtained via contact melting of the Bi-Pb system is studied. A change in the diameter of a spot is observed, depending on the position of the drop on the surface of the substrate. A change in the shape of the spot is observed, depending on the composition along the interface region. The need to use force to separate the crystallized drops from the surface of the substrate, depending on the positions of the gallium drops (i.e., the substrate’s composition) confirms the wettability upon spreading.     

Pattern formation during the drying of a colloidal suspension

Receding contact lines of colloidal suspensions are studied in the presence of drying, inside Hele-Shaw cells. At high velocity the contact line movement is continuous and the particle deposition is uniform. At small velocity, a periodic pinning-unpinning of the contact line is observed leading to a patterning of the substrate. We focused on the correlation between the deposition pattern and the pinning force that grows during the pinning. Our results strongly indicate that this pinning force is proportional to the macroscopic slope of the deposit and accounted by a simple capillary balance.     

The influence of the drop formation rate at spreading over a microstructured surface on the contact angle

The article presents the experimental dependences of a macro-contact angle and the diameter of a distilled water drop spreading over solid microstructured surface on surface average roughness (Ra) and fluid flow rate (G). It has been found that at changing G from 0.005 to 0.02 ml/s, the contact angle decreases, and at a liquid flow rate over 0.02 ml/s, it increases. With small values of G (0.005?0.01 ml/s), the drop diameter grows throughout the spreading process. In the range of G from 0.02 to 0.16 ml/s at the final stage of spreading, the contact line pinning, i.e., the diam-eter constancy, has been detected. The hypothesis about the mechanism of the pinning process has been formulated: it is due to the zero sum of all forces acting on the drop (inertia, viscosity, friction, gravity, and surface tension).