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 $1$ or $2$ 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: $\cos(\theta)\to -1$. We speculate that the existence of such a transition might account for earlier simulation observations of strongly enhanced density fluctuations.