Inspired by a few phenomena in nature such as the lotus leaf, red rose petal, gecko’s feet and Nepenthes Alata plant, much attention has been paid to use simple and feasible means to achieve remarkable wetting behaviour for many applications in various areas including self-cleaning for building exteriors and windshields, oil/water separation, anti-icing, liquid collecting, anti-fogging and anti-corrosion. Based on the established theoretical models, wetting behaviour of a liquid droplet obtained by molecular dynamics simulation method is generally in good agreement with the experimental results. In macro and micro scale, the previous theories can explain and predict the wetting behaviors well. However, these theories are invalid for nanoscale. It is essential to reveal the underlying physical mechanism of the wetting behaviors of the droplet on solid surface with nanoroughness.
Extensive studies on nanosale wettability focus on the effect of nano structures on wettability state. Desired wetting behavior of rough material surface achieved by nanosize reentrant geometry like “T” or mushroom shape and other variant geometry with solid overhangs has been widely used in self-cleaning surfaces, heat exchange and many applications. For example, “T” shape groove with different depths and widths under nanoscale has been considered to confer superhydrophobicity to hydrophilic surfaces gradually.
In this paper, wettability transition of a liquid droplet on geometrically heterogeneous solid substrate with nanoscale structures of inverted triangular grooves is investigated by using molecular dynamics simulation method under the parameter space spanned by structure geometry and solid-liquid molecular interaction potential strength. Three wettability states, namely Cassie nonwetting state, Cassie-to-Wenzel transition state and Wenzel wetting state, are identified with various geometries and potential strength. For Cassie nonwetting state, increasing height of the triangles has less effect on wettability transition with weak solid-liquid molecular interaction. Besides, the Cassie nonwetting state is less sensitive to different interval between the triangles as solid-liquid molecular interaction is weak. For Cassie-to-Wenzel transition state, increasing height of the triangles and decreasing interval between the triangles decrease wettability. For Wenzel wetting state, increasing interval between the triangles with low height increases wettability. With strong solid-liquid molecular interaction, different interval between the triangles results in wetting state transition from Wenzel to transition state. What’s more, liquid droplet changes its state from Wenzel wetting state to Cassie-to-Wenzel transition state with increasing height of the triangles or decreasing interval between the triangles. Three wettability transition regions are identified in the parameter space.