纳米结构表面上冷凝液滴的生长模式及部分润湿液滴的形成机制

Growth Modes of Condensates on Nanotextured Surfaces and Mechanism of Partially Wetted Droplet Formation

分析并计算了纳米结构表面上冷凝液滴按照不同途径长大的过程中液滴能量的增加速率,并以能量增加最小为判据来确定液滴的生长途径.结果表明,纳米结构内形成的冷凝液斑在初期按接触角(CA)增加的模式生长时,其能量增加速率远低于其它模式,于是,初始液斑先按增大接触角、并保持底面积不变的模式生长,直至液滴达到前进角状态.此后,沿接触角增加的模式长大所导致的能量增加速率开始远高于其它生长模式,于是液滴三相线开始移动,底面积开始增加,但接触角保持不变.液滴所增加的底面积可以呈润湿或复合两种状态,分别形成Wenzel液滴及部分润湿液滴,前者的表观接触角一般小于160°,而后者则明显大于160°.液滴的生长模式及其润湿状态均与纳米结构参数密切相关,仅当纳米柱具有一定高度、且间距较小时,冷凝液滴才能呈现部分润湿状态.最后,本模型对纳米结构表面上冷凝液滴润湿状态的计算结果与绝大部分实测结果相一致,准确率达到91.9%,明显高于已有公式的计算准确率.

The energy increasing rate （EIR） of a condensed droplet was analyzed during its growth in three different modes. The lowest EIR corresponding to one of the three ways was used as the criterion to determine the mode in which a condensed drop will increase its volume. The results show that the EIR according to the mode of increasing contact angle （CA） is much smaller than that according to the two other modes during the first period of growth of a condensate spot formed within a nanostructure. This means that the drop will grow, with CA increasing but the base area remaining constant, until a certain CA. After this, the EIR according to the mode of CA increasing becomes much higher than that according to the two other modes. The three-phase contact line of the drop starts to shift and the base area begins to increase while the CA remains constant. During this second period, the state of increased base area can be wetted; i.e., a Wenzel-state droplet forms with an apparent CA less than 160°. In contrast, the expanded base area can be in a composite state; i.e., a partially wetted droplet forms with a CA greater than 160°. The growth mode and its wetted state of a condensed droplet are strongly related to nanostructure. Partially wetted condensed drops can appear only on surfaces with nanopillars of a certain height and small pitch. The calculated results were consistent with experimental observations reported in the literature for the wetting states of condensed drops on nanotextured surfaces, with an accuracy of 91.9%, which is obviously higher than those calculated with reported formulas.