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Using the molecular dynamics method we simulate the hydration layer probed by an atomic force microscope (AFM) tip. We investigate how the AFM tip affects the phase, structure, and dynamics of the intrinsic hydration layer formed on a hydrophobic carbon plate. Without the AFM tip the molecular packing and orientation of the hydration layer are ordered up to the second molecular layer. With approaching the tip, the hydration layer is perturbed and eventually evaporates. The force−distance curve in AFM lacks an oscillation typically found for the hydration layer formed on a hydrophilic surface. The molecular diffusions parallel and perpendicular to the plate are, respectively, enhanced and restricted by the tip. The molecular reorientation is significantly slowed down by the tip and plate, which disrupt the hydrogen-bond network present in the bulk water.

Utilizing a fully atomistic molecular dynamics simulation, we uncovered the molecular structure of a hydration layer formed on a hydrophobic carbon sample. By varying the wettability of a probe tip, we investigated how the hydration layer manifests as an image (force) measured in atomic force microscopy. With approaching a tip within 1 nm, the intrinsic hydration layer was significantly perturbed. A hydrophobic tip eventually evaporated the hydration layer at a closer distance, giving an attractive force characteristic of a hydrophobic force. With a hydrophilic tip, the hydration layer persisted with some compression, giving an oscillating force with a period close to the molecular diameter of water.

Using all-atom molecular dynamics simulation, we investigated the wettability of a surface texturized with nanoscale pillars of domed, rectangular, or cylindrical shapes. The dewetted and wetted states of the gaps between the pillars were related to the Cassie−Baxter (CB) and Wenzel (WZ) states of a macroscopic water droplet resting on top of the pillars. We uncovered the structures and free energies of the intermediate states existing between the CB and WZ states. The contact line of the liquid−vapor−solid interface could not be depinned for the domed pillars due to their smooth curvatures unlike for the rectangular or cylindrical pillars. The liquid symmetrically penetrated down into the gap between the domed pillars by a liquid−vapor interface shape like a paraboloid, while the penetration for the rectangular or cylindrical pillars was often asymmetrical, giving a half-tubular liquid−vapor interface.

Water molecules are orderly when stacked on a material surface in a liquid or under ambient conditions. Such a hydration layer plays a crucial role in various chemical and biological processes at interfaces. Significant gaps exist however in our understanding of the molecular structure and dynamics of a hydration layer. Atomic force microscopy (AFM) and vibrational sum frequency generation (VSFG) are widely used to probe the molecular stacking and orientation in a hydration layer. We review the molecular features of a hydration layer extracted from AFM and VSFG and how a molecular simulation can give a clear and quantitative interpretation of these experiments.