Vapour-mediated sensing and motility in two-component droplets

Nate J. Cira (1) , Adrien Benusiglio (1) , Manu Prakash (1)

1 Stanford University

Controlling the wetting behaviour of liquids on surfaces is import- ant for a variety of industrial applications such as water-repellent coatings and lubrication. Liquid behaviour on a surface can range from complete spreading, as in the ‘tears of wine’ effect, to minimal wetting as observed on a superhydrophobic lotus leaf. Controlling droplet movement is important in microfluidic liquid handling, on self-cleaning surfaces and in heat transfer. Droplet motion can be achieved by gradients of surface energy.However,existing techniques require either a large gradient or a carefully prepared surface to overcome the effects of contact line pinning, which usually limit droplet motion. Here we show that two-component droplets of well- chosen miscible liquids such as propylene glycol and water deposited on 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.

Domaines: Mécanique des fluides [physics.class-ph]

Nate J. Cira, Adrien Benusiglio, Manu Prakash. Vapour-mediated sensing and motility in two-component droplets. Nature, 2015, ⟨10.1038/nature14272⟩. ⟨hal-01338286⟩