Controlling the wetting behaviour of liquids on surfaces is important for a variety of industrial applications such as water-repellent coatings1 and lubrication2. Liquid behaviour on a surface can range from complete spreading, as in the ‘tears of wine’ effect3,4, to minimal wetting as observed on a superhydrophobic lotus leaf5. Controlling droplet movement is important in microfluidic liquid handling6, on self-cleaning surfaces7 and in heat transfer8. Droplet motion can be achieved by gradients of surface energy9,10,11,12,13. However, existing techniques require either a large gradient or a carefully prepared surface9 to overcome the effects of contact line pinning, which usually limit droplet motion14. 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.
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We thank all members of the Prakash Laboratory for discussions. We thank J. C. Williams for early support of this work, B. Buisson for discussions, and G. R. Dick for discussions and reagents. N.J.C. is supported by a National Science Foundation Graduate Research Fellowship Program fellowship. A.B. is supported by the Pew Foundation. M.P. is supported by the Pew Program in Biomedical Sciences, the Terman Fellowship, Keck Foundation, the Gordon and Betty Moore Foundation and a National Science Foundation Career Grant.
Extended data figures
Extended data tables
Part 1) Complex movement of droplets Highly dynamic behaviour of PG/water droplets of various concentrations and sizes when placed simultaneously on a corona treated glass slide. (Slide dimensions: 25 x 75 mm, 4x speed). Part 2) Long-range attraction, different concentrations. Two 0.5 µL droplets of 25% PG (blue) and 1% PG (orange) are placed near each other on a corona treated glass slide. First the droplets move toward each other, then the droplet of higher PG concentration ‘chases’ the droplet of lower PG concentration which ‘flees’. (1x speed). Part 3) Long-range attraction, same concentration. Two 0.5 µL droplets of 10% PG are placed near each other on a corona treated glass slide. Both droplets move toward each other, and then they merge. (1x speed).
The first clip shows flow in a droplet on clean corona treated glass as visualized in bright field by 5 µm diameter tracer beads. The beads are initially well distributed but collect into a ring at the liquid/vapour interface. Flow can be seen moving both toward the centre and toward the edge of the droplet. The second clip shows a fluorescent movie of 2 µm diameter tracer beads visualizing flow in a droplet on high energy treated glass. Like this first clip, beads move both toward the centre and the edge of the droplet, collecting in a ring at the liquid/vapour interface. The third clip shows the same droplet as in the second clip, but on an untreated, unclean glass slide (lower energy surface). The bead velocity is much slower and beads do not collect into a ring. The droplets are 10% PG. (All clips are 2x speed).
Part 1) Attraction across a break Two 10% PG droplets moving toward each other despite a break/gap in the substrate. (1x speed). Part 2) Pipette tip control. A droplet of 10% PG moves to follow a pipette tip which contains a droplet of water. (Slide dimensions: 25 x 75 mm, 4x speed).
Transfer of fluorescein arises from the back droplet (25 % PG, dyed with fluorescein) to the front droplet (1% PG, initially no fluorescein) during a short-range chasing interaction. The camera is panning to the right, following the droplet. (1x speed).
Part 1) Self-alignment device 25% PG droplets are placed in lanes and allowed to move. From initially random positions they spontaneously arrange themselves in a line. (Slide dimensions: 25 x 75 mm, 4x speed). Part 2) Circular chasing. A 25% PG droplet (blue) pursues a 1% PG droplet (red) around a 2.1 cm mean diameter circular ring several times before merging. (16x speed). Part 3) Vertical oscillator. A 1% PG droplet (red) is chased up by a 25% PG droplet (blue) which remains at the bottom of a vertical lane due to gravity. The 1% PG droplet is eventually overcome by gravity and falls back, only to oscillate again once it contacts the 25% PG droplet. (8x speed). Part 4) Movement on flexible substrates. Here we show short-range chasing on flexible strips of ITO/PET which have been treated in a plasma oven for 5 min (droplets move on the high energy ITO side). Here a 25% PG droplet chases a 1% PG droplet in two different configurations. (4x speed).
0.25 µL droplets are deposited at the top of the device and gravity acts to bring them down. As they slide down the device, they sample each well. They are chased away if the surface tension of the well is lower than their own surface tension. They merge when they have reached the well of like surface tension (same [PG]). As in all videos and figures, the colour is only present to aid in visualization and not important in the phenomena. (4x speed).
Here we demonstrate contactless remote droplet positioning. The top plate has droplets of pure PG, which act to repel the 10% PG red droplet via vapour through long-range repulsion interactions. When we arrange these PG droplets in a circle, they form a vapour trap which we move around to demonstrate positioning. (8x speed).
Part 1) Parallel plate alignmentTwo 0.5 μL 10% PG droplets (blue on top, yellow on the bottom) interact across an air gap via their vapour clouds on the adjacent side of two parallel glass slides. Here the slides are repositioned several times to show several examples of alignment. (8x speed). Part 2) Self-assembled, self-aligned 2-lens system. Here we use a similar configuration to the parallel plate aligner but use clear droplets and arrange the distance between the plates to create an image only once alignment has occurred. This system shows how lenses can be placed far apart and will self-assemble and self-align to produce images of various magnifications, depending on distances and curvatures of the lenses. (2x speed, and 4x speed). Part 3) Self-assembled, self-aligned 3-lens system with scanning. Here we show an optical system where 3 lenses with 4 optical surfaces self-assemble and self-align. The setup is similar to the 2-lens system with an additional plate inserted between the top and bottom plates. This additional plate has a hole drilled through it in which sits a pinned droplet with two optical surfaces. We then demonstrate the ability of this system to scan an area much larger than the lens itself by moving the center plate. When the center plate is moved, the other lenses follow then automatically realign (2x speed, and 8x speed).
Here we demonstrate an easy method to create the simplest version of this system and run basic experiments. For more detailed methods please refer to the methods section (Supplementary Information Section 1). (various speeds).
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