【摘要】：浸潤性是影響液滴碰撞動態(tài)特性的重要因素之一:親水性壁面有利于液滴快速鋪展,而疏水性壁面有利于液滴發(fā)生反彈。利用該特性,將不同的浸潤性組合在同一個壁面上,可以產生很多新的應用。比如:在超疏水性壁面上設置親水點將壁面分割成多個微小的單元,可以形成“液滴微陣列”,實現細胞篩選等實驗的高度并行化和小型化。這個過程中,為了保證實驗的準確性,需要避免相鄰區(qū)域的液滴發(fā)生相互干涉。這就要求在掌握液滴碰撞特性的前提下,對壁面上親水點的大小及親水點間的距離進行設計。針對以上問題,本文采用VOF方法對液滴碰撞親-疏水性組合壁面的過程進行數值模擬。為了得到一般性結論,將液滴性質、壁面親水點尺寸等影響參數合并成三個主要無量綱參數圫、Oh、和β_i。其中,We數代表慣性力與表面張力的比值,Oh數代表粘性力與表面張力的比值,β_i代表親水點與液滴初始直徑的比值。通過無量綱計算,分別改變以上參數,重點研究三個問題:(1)液滴發(fā)生破碎的條件;(2)液滴最大鋪展直徑的變化規(guī)律;(3)液滴最終穩(wěn)定直徑的變化規(guī)律。在We=1～100,Oh=0.001～01β1和β_i=0.5～10的工況范圍內進行計算,結果表明,隨著We、Oh、和β_i的變化液滴表現出三種碰撞結果,分別為不破碎、單液滴破碎、多液滴破碎。在Oh-We圖中,液滴不發(fā)生破碎發(fā)生在右邊及下邊區(qū)域;單液滴破碎發(fā)生在中間區(qū)域;多液滴破碎發(fā)生在左上角區(qū)域;增大β_i可以使不破碎的區(qū)域增大。其中單液滴破碎是指液滴只發(fā)生一次破碎,主要受表面張力和壁面粘附力的影響;多液滴破碎是指液滴發(fā)生多次破碎,除上述原因外還受到毛細波的影響。在We數較小時,如果β_i也較小,則親水點的存在不會影響最大鋪展直徑;如果β_i較大,則會將液滴限制在親水點內,最大鋪展直徑≤親水點直徑。在We數較大時,慣性力處于主導地位,最大鋪展直徑則不受親水點大小影響。液滴穩(wěn)定狀態(tài)下,動能全部耗盡,穩(wěn)定直徑與初始狀態(tài)無關,由壁面性質決定。因此穩(wěn)定鋪展因子β_s(液滴穩(wěn)定直徑與親水點直徑之比)與Re、We無關,而與β_i呈一一對應的關系。當β_i=0.5時,液滴邊緣超出了親水性區(qū)域,β_s≈1.26;當β_i=(?)、1.5時,液滴邊緣被限制在親-疏水性分界線上,β_s≈1.0;當β=2.0,液滴穩(wěn)定在親水性區(qū)域內部,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，

本文編號：2275688