Anomalous capillary filling and wettability reversal in nanochannels

TL;DR

This paper investigates capillary filling in nanometric channels, revealing non-monotonic behaviors and confinement-induced wettability reversal due to molecular structuring, challenging classical continuum models.

Contribution

It demonstrates that at nanoscales, capillary dynamics and wettability can reverse due to confinement effects, supported by molecular simulations and a new theoretical framework.

Findings

Filling velocity varies non-monotonically with radius.

Wettability reverses from hydrophilic to hydrophobic at certain radii.

Effects are observed in both water and liquid metal simulations.

Abstract

This work revisits capillary filling dynamics in the regime of nanometric to subnanometric channels. Using molecular dynamics simulations of water in carbon nanotubes, we show that for tube radii below one nanometer, both the filling velocity and the Jurin rise vary non-monotonically with the tube radius. Strikingly, with fixed chemical surface properties, this leads to confinement-induced reversal of the tube wettability from hydrophilic to hydrophobic for specific values of the radius. By comparing with a model liquid metal, we show that these effects are not specific to water. Using complementary data from slit channels, we then show that they can be described using the disjoin-ing pressure associated with the liquid structuring in confinement. This breakdown of the standard continuum framework is of main importance in the context of capillary effects in nanoporous media, with potential interests ranging from membrane selectivity to mechanical energy storage.