Monotonic damping in nanoscopic hydration experiments.

We present the first accurate damping profiles acquired in atomic-resolution hydration force spectroscopy, revealing a monotonic damping profile at the nanoscopic tip apex. Atomic resolution is confirmed by the observed inhomogeneity caused by random substitution of Si by Al on the mica surface, and two distinct damping regimes are identified. Damping only appears above our detection limit of 1 nNs/m once the tip interacts with the second hydration layer, and an onset of strong damping above 100 nNs/m appears upon direct interaction with the adsorbed layer and first hydration layer. These results are compared to various simulations to interpret the damping signals and determine the tip-sample distance.