Liquid transport is a fundamental process relevant to a wide range of applications, for example, heat transfer, anti-icing, self-cleaning, drag reduction, and microfluidic systems. For these applications, a deeper understanding of the sliding behavior of water droplets on solid surfaces is of particular importance. In this study, the frictional behavior of water droplets sliding on superhydrophobic surfaces decorated with micropillar arrays was studied using a nanotribometer. Our experiments show that surfaces with a higher solid area fraction generally exhibited larger friction, although friction might drop when the solid area fraction was close to unity. More interestingly, we found that the sliding friction of droplets was enhanced when the dimension of the microstructures increased, showing a distinct size effect. The nonmonotonic dependence of friction force on solid area fraction and the apparent size effect can be qualitatively explained by the evolution of two governing factors, that is, the true length of the contact line and the coordination degree of the depinning events. The mechanisms are expected to be generally applicable for other liquid transport processes involving the dynamic motion of a three-phase contact line, which may provide a new means of tuning liquid-transfer behavior through surface microstructures.

中文翻译：

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液滴在微结构超疏水表面上滑动的摩擦

液体运输是与广泛应用相关的基本过程，例如，传热，防冰，自清洁，减阻和微流体系统。对于这些应用，深入了解水滴在固体表面上的滑动行为尤为重要。在这项研究中，使用纳米摩擦计研究了在微柱阵列装饰的超疏水表面上滑动的水滴的摩擦行为。我们的实验表明，固含量分数较高的表面通常表现出较大的摩擦力，尽管当固含量分数接近于1时摩擦力可能会下降。更有趣的是，我们发现，当微结构的尺寸增加时，液滴的滑动摩擦会增强，表现出明显的尺寸效应。摩擦力对固体面积分数和表观尺寸效应的非单调依赖性可以通过两个控制因素的演变来定性地解释，这两个因素即接触线的真实长度和脱钉事件的配合程度。该机制有望普遍应用于涉及三相接触线动态运动的其他液体传输过程，这可能提供一种通过表面微结构调整液体传输行为的新手段。