Uptake of water droplets by non-wetting capillaries

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 μm to 150 μm, and water microdroplets of average radius between 100 μm and 1900 μm, we demonstrate that there is a critical droplet radius below which water droplets can be taken up by hydrophobised glass and non-wetting polytetrafluoroethylene (PTFE) capillaries. The rate of capillary uptake is shown to depend strongly on droplet size, with smaller droplets penetrating the tube 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° and 114°. Our measurements confirm recent theoretical predictions and simulations for metal nanodroplets penetrating non-wetting 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. © 2011 The Royal Society of Chemistry.