Nosonovsky Michael, Bhushan Bharat
National Institute of Standards and Technology, Gaithersburg, MD 20899-8520, USA.
Ultramicroscopy. 2007 Oct;107(10-11):969-79. doi: 10.1016/j.ultramic.2007.04.011. Epub 2007 May 1.
Superhydrophobic surfaces should have high contact angles (CA) with water and low contact angle hysteresis (CAH). High CA may be achieved by increasing surface roughness, while in order to have low CAH, superhydrophobic surfaces should be able to form a stable composite interface with air pockets between solid and liquid. Capillary waves, nanodroplets condensation, hydrophilic spots due to chemical surface inhomogeneity, and liquid pressure can destroy the composite interface. These destabilizing factors have different characteristic length scales, so a hierarchical roughness is required to resist them. It is shown that convex rather than concave profile enhances stability, so nanoscale convex bumps should be superimposed over microasperities, in order to pin the liquid-air interface. In addition, the nanoroughness is required to support nanodroplets. The ability of the interface to support high pressure requires high asperity density and size, so it is in conflict with the requirement of low fractional solid-liquid contact area for low CAH and slip length. The new parameter, spacing factor for asperities, is proposed, and requirements for optimum design, which combines conflicting conditions, are formulated and discussed. Remarkably, biological superhydrophobic surfaces satisfy these requirements.
超疏水表面应与水具有高接触角（CA）和低接触角滞后（CAH）。高接触角可通过增加表面粗糙度来实现，而要获得低接触角滞后，超疏水表面应能够在固液之间形成带有气穴的稳定复合界面。毛细波、纳米液滴凝结、由于化学表面不均匀性导致的亲水点以及液体压力都会破坏复合界面。这些不稳定因素具有不同的特征长度尺度，因此需要分级粗糙度来抵抗它们。研究表明，凸形而非凹形轮廓可增强稳定性，所以纳米级凸块应叠加在微粗糙度之上，以便固定液 - 气界面。此外，纳米粗糙度对于支撑纳米液滴是必要的。界面支撑高压的能力需要高粗糙度密度和尺寸，所以这与低接触角滞后和滑移长度所需的低固液接触面积分数的要求相冲突。提出了新的参数——粗糙度间距因子，并制定和讨论了结合相互冲突条件的优化设计要求。值得注意的是，生物超疏水表面满足这些要求。
