Precursor Film Spreading during Liquid Imbibition in Nanoporous Photonic Crystals

TL;DR

This study investigates how precursor films spread in nanoporous photonic crystals during liquid imbibition, revealing different dynamics for simple and polymer liquids and highlighting the importance of nanoscale transport phenomena.

Contribution

It provides the first detailed experimental analysis of precursor film spreading in nanostructured geometries using light interferometry.

Findings

Glycerol exhibits Lucas-Washburn capillary-rise dynamics.

Polymer PDMS shows a precursor film advancing ahead of the main menisci.

All imbibition fronts follow a square-root-of-time broadening behavior.

Abstract

When a macroscopic droplet spreads, a thin precursor film of liquid moves ahead of the advancing liquid-solid-vapor contact line. Whereas this phenomenon has been explored extensively for planar solid substrates, its presence in nanostructured geometries has barely been studied so far, despite its importance for many natural and technological fluid transport processes. Here we use porous photonic crystals in silicon to resolve by light interferometry capillarity-driven spreading of liquid fronts in pores of few nanometers in radius. Upon spatiotemporal rescaling the fluid profiles collapse on master curves indicating that all imbibition fronts follow a square-root-of-time broadening dynamics. For the simple liquid (glycerol) a sharp front with a widening typical of Lucas-Washburn capillary-rise dynamics in a medium with pore-size distribution occurs. By contrast, for a polymer (PDMS) a precursor film moving ahead of the main menisci entirely alters the nature of the nanoscale transport, in agreement with predictions of computer simulations.