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Oil droplet self-transportation on oleophobic surfaces

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Science Advances  17 Jun 2016:
Vol. 2, no. 6, e1600148
DOI: 10.1126/sciadv.1600148
  • Fig. 1 Pattern design.

    (A) Schematic of the radially arrayed undercut microstripes for oil droplet self-transportation. (B) Radially continuously decreasing air-liquid contact fraction (f2) creates an inward surface energy gradient. (C) Undercut structure with fluoropolymer coating maximizes upward suspension force for oil droplets, preventing penetration of oil into the texture.

  • Fig. 2 The fabricated patterns composed of radially arrayed undercut microstripes.

    (A) Photographs of 12 samples with systematically varied parameters. Inset: Schematic illustration of the two neighboring stripes with the undercut structure. D is the stripe width, Φ is the intersection angle between two neighboring stripes, R is the radius of the central circle formed by intersection of the stripes, h is the undercut thickness, and L is the distance to the center. (B to E) Structure details of a typical A4 sample. (C) Scanning electron microscope (SEM) image near the center shows radially arrayed microstripes. (D) Magnified image of the radial stripes far from the center. (E) Cross-sectional view of an individual stripe reveals the undercut profile. The stripe was locally removed by focused ion beam (FIB) milling to expose the cross-sectional morphology.

  • Fig. 3 Three modes of oil droplet motion on different patterns.

    (A) Regime diagram showing three types of oil droplet behaviors depending on pattern parameters. In the green regime, oil droplets move spontaneously toward the center. In the blue regime, oil droplets remain pinned after being released. In the red regime, oil droplets exhibit outward spreading. The test liquid is hexadecane and the droplet volume is fixed at 1 μl. (B) Time-lapse images illustrating the three modes of oil droplet motion.

  • Fig. 4 Mechanistic understanding.

    (A to C) Calculated oil contact angle θ (A), wettability gradient − d(cos θ)/dL (B), and breakthrough pressure Pc/Pref (C) curves as functions of L on the 12 patterned surfaces. Different curve colors correspond to the three modes of droplet motion. The red curves in (A) are shown in dashed lines because oil droplets tend to collapse into the Wenzel state in this regime.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/6/e1600148/DC1

    fig. S1. Fabrication process flow for the radial undercut patterns with TiO2 and polymer coatings.

    fig. S2. SEM images and contact angles of TiO2 and fluoropolymer-coated surfaces and merely fluoropolymer-coated surfaces.

    fig. S3. Optical images of residual liquid on sample B3 after droplet transport.

    fig. S4. The outward spreading of an oil droplet of 1 μl on the A1 pattern.

    fig. S5. The effect of D and Φ on the driving force for droplet motion.

    fig. S6. Self-transportation of water-ethanol mixture droplets of different compositions on sample B3.

    fig. S7. Self-transportation of an ethanol droplet on sample A5.

    table S1. Geometrical parameters of different patterns and their contact angles, wettability gradients, and breakthrough pressure equations.

    table S2. Effect of pattern parameters on droplet speed.

    table S3. Effect of liquid type on droplet speed.

    movie S1. Demonstration of three modes of oil droplet motion on various sample surfaces.

    movie S2. Self-transportation of ethanol-water mixture droplets with different compositions on sample B3.

    movie S3. Self-transportation of an ethanol droplet on sample A5.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Fabrication process flow for the radial undercut patterns with TiO2 and polymer coatings.
    • fig. S2. SEM images and contact angles of TiO2 and fluoropolymer-coated surfaces and merely fluoropolymer-coated surfaces.
    • fig. S3. Optical images of residual liquid on sample B3 after droplet transport.
    • fig. S4. The outward spreading of an oil droplet of 1 μl on the A1 pattern.
    • fig. S5. The effect of D and Φ on the driving force for droplet motion.
    • fig. S6. Self-transportation of water-ethanol mixture droplets of different compositions on sample B3.
    • fig. S7. Self-transportation of an ethanol droplet on sample A5.
    • table S1. Geometrical parameters of different patterns and their contact angles, wettability gradients, and breakthrough pressure equations.
    • table S2. Effect of pattern parameters on droplet speed.
    • table S3. Effect of liquid type on droplet speed.
    • Legends for movies S1 to S3

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mov format). Demonstration of three modes of oil droplet motion on various sample surfaces.
    • movie S2 (.mov format). Self-transportation of ethanol-water mixture droplets with different compositions on sample B3.
    • movie S3 (.mov format). Self-transportation of an ethanol droplet on sample A5.

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