  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.
