Theoretical analysis of droplet transition from Cassie to Wenzel state
Theoretical analysis of droplet transition from Cassie to Wenzel state
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DOI：
10.1088/1674-1056/24/11/116801
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发表时间：
2015-10
影响因子：
1.7
通讯作者：
Li Tian-qing;Yanjie Li;Xiangqin Li;Wei Sun
中科院分区：
文献类型：
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作者：
Li Tian-qing;Yanjie Li;Xiangqin Li;Wei Sun
微滴在织构化表面上是否从Cassie态转变为Wenzel态（C-W）是衡量织构化表面超疏水性能否保持的试金石。然而，C-W转变的机制，特别是小液滴的自发转变，至今仍不十分清楚。在能量和梯度分析的基础上，首次提出并推导了小液滴界面自由能梯度作为其C-W演化的驱动力。在此基础上，建立了C-W转变的物理和数学模型，得到了C-W转变的驱动力或转变压力、阻力和液滴下方弯月面的参数。结果表明，微纳结构参数显著影响C-W驱动力和阻力。柱直径和柱间距越小，C-W转变压力越小，阻力越大。因此，纳米织构表面上的液滴很难完成C-W转变。同时，如果柱太短，即使三相接触线（TPCL）没有脱钉，液滴下方的弯曲的液体-空气界面的前部也将容易接触结构基底。当柱足够高时，液滴下方的TPCL必须在弯月面能够到达衬底之前首先移动。结果表明，在短柱织构表面上的液滴更容易完成C-W演化。另一方面，液滴越小，C-W转变越容易，因为转变压力变大，这很好地解释了为什么蒸发液滴会自发地从复合态坍缩到Wenzel态。此外，本征接触角和前进接触角也会影响C-W转变。两个角度越大，C-W转变越难。最后计算了C-W跃迁参数和临界条件，并与文献中的测量值进行了比较，计算值与实验值符合雅阁。
Whether droplets transit from the Cassie to the Wenzel state (C–W) on a textured surface is the touchstone that the superhydrophobicity of the surface is still maintained. However, the C–W transition mechanism, especially the spontaneous transition of small droplets, is still not very clear to date. The interface free energy gradient of a small droplet is firstly proposed and derived as the driving force for its C–W evolution in this study based on the energy and gradient analysis. Then the physical and mathematical model of the C–W transition is found after the C–W driving force or transition pressure, the resistance, and the parameters of the meniscus beneath the droplet are formulated. The results show that the micro/nano structural parameters significantly affect the C–W driving force and resistance. The smaller the pillar diameter and pitch, the minor the C–W transition pressure, and the larger the resistance. Consequently, the C–W transition is difficult to be completed for the droplets on nano-textured surfaces. Meanwhile if the posts are too short, the front of the curved liquid–air interface below the droplet will touch the structural substrate easily even though the three phase contact line (TPCL) has not depinned. When the posts are high enough, the TPCL beneath the drop must move firstly before the meniscus can reach the substrate. As a result, the droplet on a textured surface with short pillars is easy to complete its C–W evolution. On the other hand, the smaller the droplet, the easier the C–W shift, since the transition pressure becomes larger, which well explains why an evaporating drop will collapse spontaneously from composite to Wenzel state. Besides, both intrinsic and advancing contact angles affect the C–W transition as well. The greater the two angles, the harder the C–W transition. In the end, the C–W transition parameters and the critical conditions measured in literatures are calculated and compared, and the calculations accord well with the experimental results.