Dynamic wetting and spreading and the role of topography
Dynamic wetting and spreading and the role of topography
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DOI：
10.1088/0953-8984/21/46/464122
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发表时间：
2009-11-18
影响因子：
2.7
通讯作者：
Shirtcliffe, Neil J.
中科院分区：
文献类型：
--
作者：
McHale, Glen;Newton, Michael I.;Shirtcliffe, Neil J.
液滴在光滑固体表面上的铺展通常由霍夫曼-德热纳定律描述，该定律将边缘速度v（e）与动态和平衡接触角θ和θ（e）联系起来，直到v（e）与θ成比例（θ（2）-θ（2）（e））。当液体完全润湿表面并且平衡接触角为零时，边缘速度与动态接触角的立方成比例。当液滴是非挥发性的，该定律在毛细管和重力主导的状态下产生接触角和其他参数随时间的简单幂律。在纹理化表面上，液滴的平衡状态由于由形貌引起的表面化学倾向的放大而被强烈地修改。最常见的例子是疏水性转化为超疏水性。然而，当表面化学有利于部分润湿时，形貌可以导致液滴完全扩散。另外一个经常被忽视的地形后果是，一个不平衡的液滴扩散的速度也应该被修改。在这份报告中，我们回顾了有关的想法地形诱导润湿的想法，并考虑这可能与动态润湿和液滴扩散的速度。我们考虑的效果的Wenzel和Cassie-Baxter方程的驱动力，并讨论如何这些可能会修改幂律的传播。我们的想法都涉及到流体动力学粘性耗散模型和分子动力学理论的传播。这表明粗糙度和固体表面分数修改霍夫曼-德热内定律的边缘速度的动态和平衡接触角。我们还考虑了毛细制度和重力制度的大液滴的非挥发性液体的小液滴和条纹的传播。在小的非挥发性液滴完全扩散的情况下，提出了一个粗糙度修正的坦纳定律，给出了动态接触角随时间的依赖关系。我们审查现有的数据的传播小滴的聚二甲基硅氧烷油表面装饰微职位。在这些表面上，初始液滴以近似恒定的体积扩散，并且边缘速度-动态接触角关系遵循与θ（p）成比例的幂律v（e）。随着表面纹理变得更强，指数从p = 3到p = 1，与Wenzel粗糙度驱动的扩展和粗糙度修改的Hoffman-de Gennes幂律一致。最后，我们建议，当一个液滴传播到一个最终的部分润湿状态在粗糙表面上，它接近其Wenzel平衡接触角的指数方式与依赖于粗糙度的时间常数。
The spreading of a droplet of a liquid on a smooth solid surface is often described by the Hoffman-de Gennes law, which relates the edge speed, v(e), to the dynamic and equilibrium contact angles theta and theta(e) through v(e) proportional to theta (theta(2)-theta(2)(e)). When the liquid wets the surface completely and the equilibrium contact angle vanishes, the edge speed is proportional to the cube of the dynamic contact angle. When the droplets are non-volatile this law gives rise to simple power laws with time for the contact angle and other parameters in both the capillary and gravity dominated regimes. On a textured surface, the equilibrium state of a droplet is strongly modified due to the amplification of the surface chemistry induced tendencies by the topography. The most common example is the conversion of hydrophobicity into superhydrophobicity. However, when the surface chemistry favors partial wetting, topography can result in a droplet spreading completely. A further, frequently overlooked consequence of topography is that the rate at which an out-of-equilibrium droplet spreads should also be modified. In this report, we review ideas related to the idea of topography induced wetting and consider how this may relate to dynamic wetting and the rate of droplet spreading. We consider the effect of the Wenzel and Cassie-Baxter equations on the driving forces and discuss how these may modify power laws for spreading. We relate the ideas to both the hydrodynamic viscous dissipation model and the molecular-kinetic theory of spreading. This suggests roughness and solid surface fraction modified Hoffman-de Gennes laws relating the edge speed to the dynamic and equilibrium contact angle. We also consider the spreading of small droplets and stripes of non-volatile liquids in the capillary regime and large droplets in the gravity regime. In the case of small non-volatile droplets spreading completely, a roughness modified Tanner's law giving the dependence of dynamic contact angle on time is presented. We review existing data for the spreading of small droplets of polydimethylsiloxane oil on surfaces decorated with micro-posts. On these surfaces, the initial droplet spreads with an approximately constant volume and the edge speed-dynamic contact angle relationship follows a power law v(e) proportional to theta(p). As the surface texture becomes stronger the exponent goes from p = 3 towards p = 1 in agreement with a Wenzel roughness driven spreading and a roughness modified Hoffman-de Gennes power law. Finally, we suggest that when a droplet spreads to a final partial wetting state on a rough surface, it approaches its Wenzel equilibrium contact angle in an exponential manner with a time constant dependent on roughness.