Research ArticleMATERIALS SCIENCE

Hydrophilic directional slippery rough surfaces for water harvesting

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Science Advances  30 Mar 2018:
Vol. 4, no. 3, eaaq0919
DOI: 10.1126/sciadv.aaq0919
  • Fig. 1 Hydrophilic directional SRS inspired by pitcher plants and rice leaves.

    Side view (top left) and three-dimensional view (top right) of the hydrophilic directional SRS. Photos and schematics showing the pitcher plant–inspired slippery surface (bottom left) and rice leaf–inspired directional structured surface (bottom right). The photography of rice leaf was reprinted with permission from Bixler and Bhushan (39).

  • Fig. 2 Wetting characteristics of water droplets on lubricant-infused nanostructured surfaces.

    Schematic showing a liquid droplet sitting on a chemically homogeneous and smooth solid surface (A) and a SLIPS (B). (C) CA hysteresis of 5-μl water droplets on SLIPS with different lubricants. Note that ionic liquid used in the experiment is 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]). (D) Comparison of theoretical prediction and experimental measurements of water CAs on hydrophobic and hydrophilic SLIPS. The capital letters correspond to the lubricants used (C). Note that, in our surfaces, the surface wettability is governed by the lubricant, not the underlying silane coating. Optical images showing a macroscopic water droplet on a hydrophobic (E) and a hydrophilic (F) SLIPS without any noticeable oil meniscus at the contact line. Krytox 101 and hydroxy PDMS-25 are used as our hydrophobic and hydrophilic lubricants, respectively.

  • Fig. 3 Influence of surface chemistry on dropwise condensation performance.

    (A) Schematic showing dynamic coalescence on a hydrophobic slippery surface with small nucleation density. (B) Microscale condensate on a hydrophobic slippery surface (with Krytox 101 as the lubricant; CA = 121.5° ± 2.2°). The droplets are highly mobile, resulting in dynamic coalescence, but the hydrophobic nature of the surface limits droplet nucleation. (C) Schematic showing dynamic coalescence on a hydrophilic slippery surface. The surface has a large number of nucleation sites, resulting in efficient coalescence. The large contact area of the droplets facilitates heat conduction from the hot side to the cold side and coalescence of neighboring droplets. (D) Microscale condensate on a hydrophilic slippery surface. The surface has numerous nucleation sites, and the droplets are highly mobile (with hydroxy PDMS-25 as the lubricant; CA = 76.2° ± 1.8°). (E) Visible droplet density (that is, the maximum number of droplets per unit area with droplet size <14 μm during the nucleation process) on the hydrophilic and hydrophobic slippery surfaces. (F) Coalescence rate on the hydrophilic and hydrophobic slippery surfaces. (G) Shedding frequency of coalesced droplets on the hydrophilic and hydrophobic slippery surfaces. The tilt angle of the surface is 60°. Scale bars, 100 μm. Error bars represent SDs of measurements from three separate image analyses of a given condensation experiment.

  • Fig. 4 MD simulations of water condensed on SAMs.

    Representative snapshots of water droplets/layers for (A) –CF3 functional groups, (B) –CH3 functional groups, (C) –OH functional groups, and (D) hydroxy PDMS (80% –CH3 and 20% –OH functional groups). The CA during droplet growth is consistent with the macroscopic angle expected for each surface chemistry but with large fluctuations. (E) Water condensation timeline for a representative simulation of each surface chemistry in (A) to (D). The y axis records the number of water molecules within 4 nm (in the z direction) of the surface; this includes some vapor and thus has a nonzero baseline, as shown in the plot.

  • Fig. 5 Dropwise condensation on hierarchical SHS, hydrophilic SLIPS, and hydrophilic directional SRS.

    (A) Condensation on a hierarchical SHS. The droplets are highly pinned, and the SHS shows complete flooding after 100 s. The presence of gas holes (highlighted with white circles) acts as a thermal barrier and hinders heat transport. (B) Condensation on a hydrophilic SLIPS. Smaller droplets coalesce and are rapidly removed, but the larger droplets act as thermal barriers on the surface. (C) Condensation on a hydrophilic directional SRS. Smaller droplets move into the slippery microchannels, and larger droplets can be effectively drained away by the slippery microchannels. Hydroxy PDMS-25 was used on the SLIPS and on the SRS to ensure that they had the same surface chemistry. Note that the orientations of the SEM images in (C) were rotated 180° so that the direction of gravity in the image and in reality (for the reader) is aligned. The surface was tilted by 60° during the tests. Scale bar, 100 μm (for all images).

  • Fig. 6 Fog harvesting on hydrophobic SLIPS, hydrophilic SLIPS, hydrophilic SRS, and hierarchical SHS.

    Optical images showing water droplet nucleation and mobility comparisons for the following: (A) the influence of surface chemistry (fog harvesting on hydrophobic and hydrophilic SLIPS), (B) the influence of surface structure (fog harvesting on hydrophilic SLIPS and hydrophilic directional SRS), and (C) the influence of slippery interface (fog harvesting on hierarchical SHS and hydrophilic directional SRS). (D) Fog harvesting rates on different surfaces. The surfaces were positioned vertically during the tests. Scale bars, 5 mm. Error bars represent SDs from three individual measurements.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaaq0919/DC1

    section S1. Descriptions of movies S1 to S10

    section S2. Design principle of surface structures

    section S3. Wetting equations on liquid-infused surfaces

    section S4. Design of SRS

    section S5. Longevity of SRS

    section S6. MD simulations for droplet nucleation

    fig. S1. Design principle of the surface structures.

    fig. S2. SEM images of nanotextures and SLIPS.

    fig. S3. Wetting characteristics on liquid-infused slippery surfaces.

    fig. S4. Wetting models of a water droplet on difference surfaces.

    fig. S5. Design of directional SRS.

    fig. S6. The effect of nanotextures on lubricant retention.

    fig. S7. Nucleation on various functionalized surfaces.

    table S1. CAs and thermal conductivities of different lubricants used in this study.

    table S2. Time for most (70%) of the water vapor molecules to condense on the SAM surfaces.

    Reference (40)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Descriptions of movies S1 to S10
    • section S2. Design principle of surface structures
    • section S3. Wetting equations on liquid-infused surfaces
    • section S4. Design of SRS
    • section S5. Longevity of SRS
    • section S6. MD simulations for droplet nucleation
    • fig. S1. Design principle of the surface structures.
    • fig. S2. SEM images of nanotextures and SLIPS.
    • fig. S3. Wetting characteristics on liquid-infused slippery surfaces.
    • fig. S4. Wetting models of a water droplet on difference surfaces.
    • fig. S5. Design of directional SRS.
    • fig. S6. The effect of nanotextures on lubricant retention.
    • fig. S7. Nucleation on various functionalized surfaces.
    • table S1. CAs and thermal conductivities of different lubricants used in this study.
    • table S2. Time for most (70%) of the water vapor molecules to condense on the SAM surfaces.
    • Reference (40)

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