Vapour-mediated sensing and motility in two-component droplets

Abstract

Controlling the wetting behaviour of liquids on surfaces is important 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.

Vapor-Induced Motion of Liquid Droplets on an Inert Substrate

“…Accordingly, they always approach each other. This explains the experimentally observed motions of pair W or PG droplets [7] which have not been explained so far.…”

“…Equally complex motion of droplets (attraction, repulsion, chasing etc) has been reported for volatile droplets slowly evaporating at room temperature [2][3][4][5][6][7], but, apart from the conjecture that the phenomena is caused by the Marangoni effect (flow induced by the gradient of surface tension), no quantitative theory has been given. Cira et al [7] reported that evaporating droplets show complex motion even if they are placed on an inert substrate where the wettability is uniform and unchanged. Figure 1 schematically shows their results for the case of pure liquid droplets.…”

“…Such motion of evaporating droplets has conventionally been explained by the vapormediated Marangoni effect [3][4][5][6][7]. Liquid vapor evaporating from one droplet condenses in other droplets and changes the local composition of the droplet (see Fig.…”

“…If the vapor density is not uniform, the evaporation rate of the droplet becomes non-uniform [7][8][9][10].…”

Magnetic Actuation of Drops and Liquid Marbles Using a Deformable Paramagnetic Liquid Substrate

“…In the large majority of cases, the drop motion is induced by an interfacial energy gradient at a solid/liquid interface (wettability gradient) and/or at a free interface (Marangoni stress) 3. This has resulted in a plethora of studies to elucidate the fundamental mechanisms that convert such gradients into drop motion4 as well as to develop strategies to manipulate drops under the control of various external signals, such as thermal,5 electrical2, 6, 7 and optical8, 9, 10, 11, 12, 13 stimuli. Most of these approaches necessitate the implementation of rather complex components, such as electrodes or optical elements, or the development of systems that are intrinsically responsive to the desired stimulus, such as photo‐ or thermosensitive substrates 8, 9.…”