Layer-by-layer water filling in molecular-scale capillaries

TL;DR

This study investigates how water fills nanocapillaries at molecular scales, revealing layer-by-layer or abrupt filling depending on wall flexibility, and highlights the impact of molecular discreteness on capillary phenomena.

Contribution

It demonstrates the influence of wall flexibility on water filling regimes in nanocapillaries and elucidates the role of molecular discreteness in capillary condensation.

Findings

Flexible capillaries fill in ~3 Å steps matching water layers.

Rigid capillaries exhibit abrupt water filling.

Wall deformation energy influences filling behavior.

Abstract

Under ambient humidity, water spontaneously condenses in pores only a few nanometers in size, making nanoscale capillarity central to numerous natural phenomena and technological applications. At these dimensions, water may no longer be treated as a continuous fluid, yet the consequences of molecular discreteness for capillary condensation and filling remain poorly understood. Here we study nanocapillaries fabricated by van der Waals assembly and, using atomic force microscopy, monitor their wall deformations during humidity-driven water uptake. We observe two distinct regimes: layer-by-layer filling of flexible capillaries and abrupt filling of rigid ones. Flexible walls deform in steps of ~3 {\AA}, corresponding to the sequential entry of individual water molecular layers. The different filling regimes are explained by the competition between deformation energy and oscillatory wall-water interactions. Our findings show that the molecular discreteness of water can profoundly affect ubiquitous capillary phenomena, with wall compliance selecting between discrete and abrupt filling.