Validation of Capillarity Theory at the Nanometer Scale by Atomistic Computer Simulations of Water Droplets and Bridges in Contact with Hydrophobic and Hydrophilic Surfaces

Abstract

Capillarity is the study of interfaces between two immiscible liquids or between a liquid and a vapor. The theory of capillarity was created in the early 1800s, and it is applicable to mesoscopic and macroscopic (>1 μm) systems. In general, macroscopic theories are expected to fail at the <10 nm scales where molecular details may become relevant. In this work, we show that, surprisingly, capillarity theory (CT) provides satisfactory predictions at 2–10 nm scales. Specifically, we perform atomistic molecular dynamics (MD) simulations of water droplets and capillary bridges of different symmetry in contact with various surfaces. The underlying structure of the surfaces corresponds to hydroxilated (crystalline) silica which is modified to cover a wide range of hydrophobicity/hydrophilicity. In agreement with CT, it is found that water contact angle is independent of the droplet/bridge geometry and depends only on the hydrophobicity/hydrophilicity of the surface employed. In addition, CT provides the correct ...