| Abstract: | 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.The wettability of surfaces plays an important role in many natural and industrial processes (1–3). Wetting of a smooth surface by a liquid is commonly described by the Young’s equation, which quantifies the wettability of a solid surface using the contact angle of a liquid droplet (4, 5). When the surfaces are rough, the spreading of a droplet is governed by contact angle and capillary effects, and the droplet spreads via hemiwicking (6, 7). The rough features of the surface act as capillaries, or wicks, that imbibe liquid from the droplet (8–10). Earlier studies show that modifying the surface roughness can be a powerful approach to tuning the surface wettability (11–18), and such surface modifications have been used for applications in biomedicine (19, 20), textile industry (21, 22), and water treatment (23, 24).Despite the recent progress in designing rough superhydrophilic surfaces (17, 24), the nanoscale details of the wicking process on rough surfaces remain unknown because these processes are extremely challenging to visualize. Current approaches to study wetting are based on optical methods (25–28), and, while providing valuable insights, these methods lack the spatial resolution needed to discern the nanoscale details of wetting. Alternative approaches based on scanning probe microscopy techniques provide high spatial resolution but lack temporal resolution and thus have been limited to study the adsorption of water (29) and wetting of smooth surfaces (30, 31). Recent advances in liquid-phase transmission electron microscopy (TEM) (32–36) and fast electron detection cameras (37, 38) enabled direct imaging of the nanoscale dynamics of liquids on surfaces with high temporal and spatial resolutions. Here, we used this liquid-phase TEM approach to study the wetting of patterned nanostructures at high speeds (200 to 300 frames per second fps]). |
