Directed droplet migration along thin fibers

Small droplets can spontaneously move on conical fibers to minimize the surface energy of the system. This is a feature used by cacti, and other plants, to harvest water from the air. Similar ideas are being developed to carry out fog harvesting in arid regions of the globe. Here we investigate a similar concept: propelling droplets with two fibers. We deposit micrometric droplets between pairs of thin fibers. By controlling the angle between the fibers, we can direct the motion of the droplets. We find that droplets spontaneously move towards the apex of the angled fibers. The speed of the droplet motion increases both with the angle between the fibers and the distance the droplet spans across the fibers.