液滴碰撞亲—疏水性组合壁面过程的数值模拟

发布时间：2018-10-16 21:34

【摘要】：浸润性是影响液滴碰撞动态特性的重要因素之一:亲水性壁面有利于液滴快速铺展,而疏水性壁面有利于液滴发生反弹。利用该特性,将不同的浸润性组合在同一个壁面上,可以产生很多新的应用。比如:在超疏水性壁面上设置亲水点将壁面分割成多个微小的单元,可以形成“液滴微阵列”,实现细胞筛选等实验的高度并行化和小型化。这个过程中,为了保证实验的准确性,需要避免相邻区域的液滴发生相互干涉。这就要求在掌握液滴碰撞特性的前提下,对壁面上亲水点的大小及亲水点间的距离进行设计。针对以上问题,本文采用VOF方法对液滴碰撞亲-疏水性组合壁面的过程进行数值模拟。为了得到一般性结论,将液滴性质、壁面亲水点尺寸等影响参数合并成三个主要无量纲参数圫、Oh、和β_i。其中,We数代表惯性力与表面张力的比值,Oh数代表粘性力与表面张力的比值,β_i代表亲水点与液滴初始直径的比值。通过无量纲计算,分别改变以上参数,重点研究三个问题:(1)液滴发生破碎的条件;(2)液滴最大铺展直径的变化规律;(3)液滴最终稳定直径的变化规律。在We=1～100,Oh=0.001～01β1和β_i=0.5～10的工况范围内进行计算,结果表明,随着We、Oh、和β_i的变化液滴表现出三种碰撞结果,分别为不破碎、单液滴破碎、多液滴破碎。在Oh-We图中,液滴不发生破碎发生在右边及下边区域;单液滴破碎发生在中间区域;多液滴破碎发生在左上角区域;增大β_i可以使不破碎的区域增大。其中单液滴破碎是指液滴只发生一次破碎,主要受表面张力和壁面粘附力的影响;多液滴破碎是指液滴发生多次破碎,除上述原因外还受到毛细波的影响。在We数较小时,如果β_i也较小,则亲水点的存在不会影响最大铺展直径;如果β_i较大,则会将液滴限制在亲水点内,最大铺展直径≤亲水点直径。在We数较大时,惯性力处于主导地位,最大铺展直径则不受亲水点大小影响。液滴稳定状态下,动能全部耗尽,稳定直径与初始状态无关,由壁面性质决定。因此稳定铺展因子β_s(液滴稳定直径与亲水点直径之比)与Re、We无关,而与β_i呈一一对应的关系。当β_i=0.5时,液滴边缘超出了亲水性区域,β_s≈1.26;当β_i=(?)、1.5时,液滴边缘被限制在亲-疏水性分界线上,β_s≈1.0;当β=2.0,液滴稳定在亲水性区域内部,β_s≈0.81。
[Abstract]:Wettability is one of the important factors affecting the dynamic characteristics of droplet collision: hydrophilic wall is conducive to rapid spread of droplets, while hydrophobic wall is conducive to droplet rebound. Using this property, different wettability can be combined on the same wall, and many new applications can be generated. For example, by setting hydrophilic points on the super-hydrophobic wall, the wall can be divided into several tiny elements, which can form a "droplet microarray", and achieve high parallelization and miniaturization of experiments such as cell screening. In this process, in order to ensure the accuracy of the experiment, it is necessary to avoid the interference of droplets in adjacent regions. This requires the design of the size of hydrophilic points and the distance between hydrophilic points on the wall under the premise of grasping the characteristics of droplet collision. In order to solve the above problems, the VOF method is used to simulate the hydrophilic and hydrophobic composite wall process of droplet collision. In order to obtain the general conclusion, the influence parameters such as droplet properties and the size of hydrophilic point on the wall are combined into three main dimensionless parameters, Oh, and 尾 _ I. The We number represents the ratio of inertial force to surface tension, the Oh number represents the ratio of viscous force to surface tension, and 尾 _ I represents the ratio of hydrophilic point to the initial diameter of the droplet. By dimensionless calculation, the above parameters are changed, and three problems are studied: (1) the condition of droplet breakage; (2) the variation law of the maximum spreading diameter of droplet; (3) the variation law of the final stable diameter of droplet. Under the operating conditions of We=1~100,Oh=0.001~01 尾 _ 1 and 尾 _ I _ (0.5) ~ (10), the results show that with the change of We,Oh, and 尾 _ I, there are three kinds of collision results: no breakage, single drop breakage and multiple droplet breakage. In the Oh-We diagram, droplets do not break in the right and lower regions; single droplets break in the middle region; multiple droplets break in the upper left corner; and 尾 _ I increases the unbroken regions. Single droplet breakup means that droplet breakage occurs only once, which is mainly affected by surface tension and wall adhesion force, and multi-droplet breakup refers to multiple breakup of droplets, which is influenced by capillary wave in addition to the above reasons. When the We number is small, if 尾 _ I is small, the existence of hydrophilic point will not affect the maximum spreading diameter, and if 尾 _ I is larger, the liquid droplet will be confined to the hydrophilic point, and the maximum spreading diameter will be 鈮，

本文编号：2275688