Tip-induced flipping of droplets on Janus pillars: From local reconfiguration to global transport

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Science Advances  08 Jul 2020:
Vol. 6, no. 28, eabb4540
DOI: 10.1126/sciadv.abb4540
  • Fig. 1 Geometry of pine needles and of our PNAS arrays.

    (A) Optical images of a pine needle (S. chinensis), exhibiting a gradient height α ≈ 30° and a tilt angle β ≈ 50°. As shown in the close-up, its two sides are respectively flat and curved, and we denote r as the local radius of curvature of the curved side. (B) Scanning electron microscopy (SEM) images of our pine needle–inspired asymmetric surface (PNAS), made of tilted (β ≈ 70°) Janus (flat-curved) pillars with a gradient of height α ≈ 20° and an interpillar distance s = 300 μm. (C) SEM images of tilted Janus pillars without gradient of height (β ≈ 70°, α ≈ 0). (D) SEM images of tilted conical pillars with a height gradient (β ≈ 70°, α ≈ 20°). (E) SEM overview of PNAS arrays, showing parallel textured stripes (periodicity w ≈ 1.5 mm). Other PNAS surfaces are shown in fig. S3, and SEM overviews of Janus and conical surfaces are shown in fig. S4. The total size L of each sample is 10 mm. Photo credit: Shile Feng, City University of Hong Kong.

  • Fig. 2 Droplet transport on the different samples.

    (A) On PNAS, water harvested from a fog self-propels after successive coalescences. Transport takes place along positive x, as highlighted by dashes (see also movie S1). (B) On equally high Janus pillars, transport starts the same way: Droplets sit on the right side of the Janus pillars and get ejected in the +x direction after coalescence. However, interpillar coalescence later launches droplets in the opposite direction (movie S2). (C and D) On conical needles, coalescence can eject droplets in both +x and −x directions, due to the absence of orientation of droplets at the needle tips (movies S3 and S4). Photo credit: Shile Feng, City University of Hong Kong.

  • Fig. 3 TIF effect.

    (A) TIF (TIF effect) on Janus pillars: Droplets tend to accumulate at the pillar tip (movie S5), either directly on the curved side of the pillar (first picture) or after flipping from flat to curved (second picture). In the latter case, large droplets can be attracted by small ones, showing that the TIF effect overcomes the difference of Laplace pressure between droplets (movie S6). (B) On conical pillars, droplets also gather at the pillar tips, but they sit indifferently on the right or left side of the pillar (movies S7 and S8). Photo credit: Shile Feng, City University of Hong Kong. (C) Droplet surface energy (compared to that in air) as a function of φ, the flip angle around the Janus tip. ΔE is normalized by Ea, the adhesion energy of the droplet on a flat solid. (D) Probability p to find a droplet on the curved side of a Janus pillar as a function of z/R, the normalized distance to the tip (green data). p is deduced from statistics performed on 200 droplets and compared to the probability of finding droplets on the right side of conical pillars (blue data). (E) Sketch and top view of the coalescence of two water drops with R ≈ 2.5 mm sitting on a superhydrophobic material and placed respectively on the flat and curved sides of a vertical hemicylinder (with r = 0.5 mm), whose profile is highlighted with dots. Time starts at the onset of coalescence, after which the merged drop rotates around the solid until it pins on its curved side, confirming the generality of the TIF effect (movie S9). Photo credit: Antoine Malod, ESPCI Paris.

  • Fig. 4 Large-scale droplet rectification.

    (A) Top view of centimetric samples subject to fog (released 3 cm above the central part, as marked with dashes), as a function of time. PNAS not only rectifies the flux of water droplets but also ejects it out of the rectifying unit. (B) Same experiment on inclined conical fibers. Rectification is less efficient and takes place in the opposite direction. Photo credit: Shile Feng, City University of Hong Kong. (C) Water repartition e = m/mtot defined as the proportion of the injected water found on the different sections of the sample, that is, x < 0, 0 < x < L, and x > L. Repartition is compared on PNAS (green columns) and inclined cones (blue columns) at different times.

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