Vapour-mediated sensing and motility in two-component droplets

Controlling the wetting behaviour of liquids on surfaces is important for a variety of industrial applications such as water-repellent coatings(1) and lubrication(2). Liquid behaviour on a surface can range from complete spreading, as in the 'tears of wine' effect(3,4), tominimal wetting as observed on a superhydrophobic lotus leaf(5). Controlling droplet movement is important in microfluidic liquid handling(6), on self-cleaning surfaces(7) and in heat transfer(8). Droplet motion can be achieved by gradients of surface energy(9-13). However, existing techniques require either a large gradient or a carefully prepared surface(9) to overcome the effects of contact line pinning, which usually limit dropletmotion(14). Here we show that two-component droplets of well-chosen miscible liquids such as propylene glycol and water depositedon clean glass are not subject to pinning and cause the motion of neighbouring droplets over a distance. Unlike the canonical predictions for these liquids on a high-energy surface, these droplets do not spread completely but exhibit an apparent contact angle. We demonstrate experimentally and analytically that these droplets are stabilized by evaporation-induced surface tension gradients and that they move in response to the vapour emitted by neighbouring droplets. Our fundamental understanding of this robust system enabled us to construct a wide variety of autonomous fluidic machines out of everyday materials.