Quantifying Density Fluctuations in Water at a Hydrophobic Surface: Evidence for Critical Drying

Employing smart Monte Carlo sampling techniques within the grand canonical ensemble, we investigate the properties of water at a model hydrophobic substrate. By reducing the strength of substrate-water attraction, we find that fluctuations in the local number density, quantified by a rigorous definition of the local compressibility chi(z), increase rapidly for distances z within one or two molecular diameters from the substrate as the degree of hydrophobicity, measured by the macroscopic contact angle theta, increases. Our simulations provide evidence for a continuous (critical) drying transition as the substrate-water interaction becomes very weak: cos(theta) -> -1. We speculate that the existence of such a transition might account for earlier simulation observations of strongly enhanced density fluctuations.