Phase Diagram of Nanoscale Water on Solid Surfaces with Various Wettabilities

Understanding structural and dynamic properties of water in contact with solid surfaces is essential for diverse fields, including environmental sciences, nanofluidics, lubrication, and electrochemistry. Despite tremendous efforts, how interfacial water phase behaviors correlate with a surface's wettability remains elusive. Here, we investigate the structure and dynamics of nanoscale water droplets or adlayers on solid surfaces with wettabilities spanning from strongly hydrophobic to strongly hydrophilic using extensive molecular dynamics simulations. It is shown that liquid water drops on solid surfaces with contact angles greater than 42.6° transform into drops of ordinary hexagonal ice (I_h) upon cooling. In contrast, water forms a liquid disc on a completely wetted surface with a zero contact angle, which freezes into a hexagonal bilayer ice disc at low temperatures. Unexpectedly, on surfaces with a mild contact angle in the range of 21.9°-29.2°, the originally stable liquid drop at room temperature further wets the surface upon cooling and eventually transforms into a bilayer ice disc. These results establish a phase diagram of nanoscale water at the wettability versus temperature plane, which may expand our knowledge of water-surface interactions as well as enrich the complexity of water behaviors at interfaces.