Validation of Capillarity Theory at the Nanometer Scale. II: Stability and Rupture of Water Capillary Bridges in Contact with Hydrophobic and Hydrophilic Surfaces

AB Almeida and N Giovambattista and SV Buldyrev and AM Alencar, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 1556-1569 (2018).

DOI: 10.1021/acs.jpcc.7b08577

We perform molecular dynamics (MD) simulations of water capillary bridges formed between parallel walls. The underlying structure of the walls corresponds to hydroxilated (crystalline),beta-cristobalite, modified to cover a wide range of hydrophobicity/hydrophilicity. The capillary bridges are stretched during the MD simulations, from wall- wall separation h = 5 nm up to h approximate to 7.5 nm, until they become unstable and break. During the stretching process, we calculate the profiles of capillary bridges as well as the force and pressure induced on the walls, among other properties. We find that, for all walls separations and surface hydrophobicity/hydrophilicity considered, the results from MD simulations are in excellent agreement with the predictions from capillarity theory (CT). In addition, we find that CT is able to predict very closely the limit of stability of the capillary bridges, i.e., the value of h at which the bridges break. We also confirm that CT predicts correctly the relationship between the surface hydrophobicity/hydrophilicity and the resulting droplets of the capillary bridge rupture. Depending on the contact angle of water with the corresponding surface, the rupture of the capillary bridges results in (i) a single droplet attached to one of the walls, (ii) two identical, or (iii) two different droplets, one attached to each wall. This work expands upon a previous study of nanoscale droplets and (stable) capillary bridges where CT was validated at the nanoscale using MD simulations. The validation of CT at such small scales is remarkable, since CT is a macroscopic theory that is expected to fail at <10 nm scales, where molecular details may become relevant. In particular, we find that CT works for capillary bridges that are approximate to 2-nm thick, comparable to the thickness of the water-vapor interface.

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