The spreading of a liquid droplet on flat surfaces is a well-understood phenomenon, but little is known about how liquids spread on a rough surface. When the surface roughness is of the nanoscopic length scale, the capillary forces dominate and the liquid droplet spreads by wetting the nanoscale textures that act as capillaries. Here, using a combination of advanced nanofabrication and liquid-phase transmission electron microscopy, we image the wetting of a surface patterned with a dense array of nanopillars of varying heights. Our real-time, high-speed observations reveal that water wets the surface in two stages: 1) an ultrathin precursor water film forms on the surface, and then 2) the capillary action by nanopillars pulls the water, increasing the overall thickness of water film. These direct nanoscale observations capture the previously elusive precursor film, which is a critical intermediate step in wetting of rough surfaces.

液滴在平坦表面上的扩散是一种众所周知的现象，但人们对液体如何在粗糙表面上扩散知之甚少。当表面粗糙度为纳米级长度尺度时，毛细管力占主导地位，液滴通过润湿充当毛细管的纳米级纹理而扩散​​。在这里，我们结合先进的纳米加工和液相透射电子显微镜，对具有不同高度的密集纳米柱阵列图案的表面的润湿进行成像。我们的实时高速观察表明，水分两个阶段润湿表面：1）在表面形成超薄的前体水膜，然后 2）纳米柱的毛细管作用拉动水，增加水的整体厚度电影。这些直接的纳米级观察捕获了以前难以捉摸的前体薄膜，这是润湿粗糙表面的关键中间步骤。