Uptake of water droplets by nonwetting capillaries

TL;DR

This study experimentally demonstrates that water droplets can spontaneously penetrate non-wetting capillaries due to Laplace pressure, with the process depending on droplet size and confirming theoretical predictions, relevant for various technological applications.

Contribution

The paper provides direct experimental evidence of water droplet uptake in non-wetting capillaries, validating theoretical models and exploring size-dependent dynamics.

Findings

Smaller droplets are absorbed more quickly.

A critical droplet radius determines uptake capability.

Measured contact angles between 96° and 114°.

Abstract

We present direct experimental evidence that water droplets can spontaneously penetrate non-wetting capillaries, driven by the action of Laplace pressure due to high droplet curvature. Using high-speed optical imaging, microcapillaries of radius 50 to 150 micron, and water microdroplets of average radius between 100 and 1900 micron, we demonstrate that there is a critical droplet radius below which water droplets can be taken up by hydrophobised glass and polytetrafluoroethylene (PTFE) capillaries. The rate of capillary uptake is shown to depend strongly on droplet size, with smaller droplets being absorbed more quickly. Droplet size is also shown to influence meniscus motion in a pre-filled non-wetting capillary, and quantitative measurements of this effect result in a derived water-PTFE static contact angle between 96 degrees and 114 degrees. Our measurements confirm recent theoretical predictions and simulations for metal nanodroplets penetrating carbon nanotubes (CNTs). The results are relevant to a wide range of technological applications, such as microfluidic devices, ink-jet printing, and the penetration of fluids in porous materials.