Research ArticleMATERIALS SCIENCE

When and how self-cleaning of superhydrophobic surfaces works

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Science Advances  17 Jan 2020:
Vol. 6, no. 3, eaaw9727
DOI: 10.1126/sciadv.aaw9727
  • Fig. 1 Self-cleaning of superhydrophobic surfaces.

    (A) The surfaces are contaminated with particles of different sizes (80 nm to 50 μm) and polarities (hydrophobic/hydrophilic). (B) Water drops roll over the contaminated surface. (C) Can the water drops remove the contamination and how is superhydrophobicity affected? How does the self-cleaning evolve on the micrometer scale, and which forces are involved in the self-cleaning process?

  • Fig. 2 Effect of differently sized hydrophobic particle powder contamination on nanoporous superhydrophobic surfaces.

    (A) Schematic illustration of the self-cleaning process of hydrophobic particle powders (purple) by a water drop (gray) on a superhydrophobic surface (blue). Colors and texture were chosen to match the LSCM images. (B) Photograph of a 10-μl water drop cleaning a nanoporous superhydrophobic surface contaminated with Oil Red dye particles (appear black). (C to E) LSCM images after contaminating the nanoporous superhydrophobic surface (left) with powders of hydrophobic particles with diameters of 10 to 50 μm, 200 nm, and 80 nm (see Materials and Methods for details of the image processing). Efficient cleaning of all hydrophobic powders was verified by LSCM (center) and SEM (right). Scale bars, 200 nm (SEM). (F and G) Contact and roll-off angles using 6-μl water drops after self-cleaning of a nanoporous surface consecutively contaminated with hydrophobic particle powders.

  • Fig. 3 Effect of hydrophilic particle contamination having various particle sizes deposited from ethanol dispersion on nanoporous superhydrophobic surfaces.

    (A) Schematic illustration of the self-cleaning process of hydrophilic particles (purple; 2R > p) deposited from ethanol dispersion by a water drop (gray). (B) Particles of smaller diameter than the pore diameter (2R < p) can penetrate the coating (blue), affecting wetting properties. (C to E) LSCM images (left) after contamination of the superhydrophobic surfaces with hydrophilic particles with diameters of 10 to 50 μm, 600 nm, and 80 nm (see Materials and Methods and fig. S9 for details of image processing). LSCM (center) and SEM images (right) show the surfaces after rinsing. Scale bars, 200 nm (SEM). (F and G) Contact and roll-off angles using 6-μl water drops after self-cleaning of nanoporous surfaces contaminated with various hydrophilic particles (dried from ethanol dispersion).

  • Fig. 4 Contamination and self-cleaning of superhydrophobic microstructured SU-8 pillars (rectangular, 10-μm height with 5 × 5–μm2 top areas; center-center distance of pillars, 20 μm).

    (A) LSCM (top) and SEM (bottom) images showing a surface contaminated with 1.5-μm particles. On the left side of the SEM image, the micropillar array is only partially filled with particles (hc < hP), whereas on the right, the particles completely covered the microstructure (hc > hP). Scale bar, 10 μm (SEM). (B) Roll-off angles of 6-μl water drops after contamination of the micropillar array with hydrophilic and hydrophobic particles of various sizes and subsequent self-cleaning.

  • Fig. 5 Illustration of the self-cleaning of a contaminated superhydrophobic surface using confocal microscopy and friction force measurements.

    (A) A 10-μl water drop (dyed with ATTO 488; navy blue) is dragged over a nanoporous superhydrophobic surface contaminated with 10- to 50-μm hydrophilic particles (purple). The interface between the drop and the surface is monitored by LSCM. Particle contamination is completely taken along by the water drop (see Materials and Methods and fig. S9 for details of the image processing). (B and C) High-magnification LSCM images showing the contact angle θ of the hydrophilic and hydrophobic particles in contact with water. Smaller particles lost contact with the solid surface. (D) Sketch of the pickup process of particles. The deformed meniscus pulls on the particle. (E) Macroscopic observation of a 10-μl drop being dragged over a surface heavily contaminated with hydrophilic 10- to 50-μm particles. (F) Force required to clean a surface contaminated with hydrophilic 1.5-μm and 600-nm particles. The drop is moved at a velocity of v = 250 μm s−1. (G) Effect of the thickness of the contamination layer (<0.1 mm, 0.1 to 0.2 mm, and 0.2 to 0.4 mm) for hydrophilic 10- to 50-μm particles on the force required to clean the surface. For strongly contaminated surfaces (0.2- to 0.4-mm contamination layer), a continuous increase in the force during the self-cleaning can be observed (1 and 2). Upon complete coverage of the drop’s surface with particles (between 3 and 4), a sudden increase in force can be observed. Drop velocity, v = 250 μm s−1.

  • Fig. 6 Real-world contamination test through outdoor exposure of superomniphobic fabrics fixed on a car for 257 days.

    (A) Photograph of 20-μl drops of water (stained with methylene blue), coffee, wine, and hexadecane on a superomniphobic fabric. (B) SEM images of a coated superomniphobic polyester fabric at different magnifications. (C) Superomniphobic fabric fixed on the side mirror of the car. The fabric remained white even after 257 days of outdoor exposure. (D and E) Receding contact angles and roll-off angles of 5-μl water drops in the course of the outdoor exposure of 257 days within a period of 426 days. Periods of outdoor exposure are marked in gray. (F and G) Receding contact angles and roll-off angles of 5-μl hexadecane drops in the course of the outdoor exposure. Periods of outdoor exposure are marked in gray. (H) SEM image of a superomniphobic fabric after 257 days of outdoor exposure. (I) Higher-magnification SEM image of a dirt particle on a superomniphobic fabric. (J) High-magnification SEM image of the nanofilaments on a superomniphobic fabric after 257 days of outdoor exposure.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaaw9727/DC1

    Supplementary Materials and Methods

    Fig. S1. SEM images of the model contamination particles.

    Fig. S2. LSCM images of nanoporous surfaces contaminated with hydrophobic particle powder.

    Fig. S3. SEM images of a nanoporous surface after contamination with hydrophobic 80-nm particles.

    Fig. S4. LSCM images of nanoporous surfaces contaminated with hydrophilic particles from ethanol dispersion.

    Fig. S5. SEM images of a nanoporous surface after contamination with a thin layer of hydrophilic 600-nm particles.

    Fig. S6. Coffee stain effect during evaporation.

    Fig. S7. SEM images of nanoporous surfaces after contamination with hydrophilic 200- and 80-nm particles.

    Fig. S8. Water droplet on a nanoporous surface contaminated with nanoparticles.

    Fig. S9. Processing of the LSCM images.

    Fig. S10. Contamination and self-cleaning of superhydrophobic microstructured SU-8 pillars.

    Fig. S11. SEM images of superhydrophobic microstructured SU-8 pillars after contamination with hydrophobic particles.

    Fig. S12. Photographs of the superomniphobic fabrics on the car after 257 days of outdoor exposure.

    Fig. S13. SEM images of abraded microfibers.

    Fig. S14. Industrial contamination test.

    Table S1. Temperatures (T) and rainfall and humidities (RH) during the outdoor exposure of the superomniphobic fabrics.

    Movie S1. Self-cleaning process of hydrophilic 10- to 50-μm particles.

    Movie S2. Self-cleaning process of hydrophobic 10- to 50-μm particles.

    Movie S3. Self-cleaning process of hydrophilic 1.5-μm particles.

    Note S1. Imaging self-cleaning.

    References (4859)

  • Supplementary Materials

    The PDFset includes:

    • Supplementary Materials and Methods
    • Fig. S1. SEM images of the model contamination particles.
    • Fig. S2. LSCM images of nanoporous surfaces contaminated with hydrophobic particle powder.
    • Fig. S3. SEM images of a nanoporous surface after contamination with hydrophobic 80-nm particles.
    • Fig. S4. LSCM images of nanoporous surfaces contaminated with hydrophilic particles from ethanol dispersion.
    • Fig. S5. SEM images of a nanoporous surface after contamination with a thin layer of hydrophilic 600-nm particles.
    • Fig. S6. Coffee stain effect during evaporation.
    • Fig. S7. SEM images of nanoporous surfaces after contamination with hydrophilic 200- and 80-nm particles.
    • Fig. S8. Water droplet on a nanoporous surface contaminated with nanoparticles.
    • Fig. S9. Processing of the LSCM images.
    • Fig. S10. Contamination and self-cleaning of superhydrophobic microstructured SU-8 pillars.
    • Fig. S11. SEM images of superhydrophobic microstructured SU-8 pillars after contamination with hydrophobic particles.
    • Fig. S12. Photographs of the superomniphobic fabrics on the car after 257 days of outdoor exposure.
    • Fig. S13. SEM images of abraded microfibers.
    • Fig. S14. Industrial contamination test.
    • Table S1. Temperatures (T) and rainfall and humidities (RH) during the outdoor exposure of the superomniphobic fabrics.
    • Legends for movies S1 to S3
    • Note S1. Imaging self-cleaning.
    • References (4859)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Self-cleaning process of hydrophilic 10- to 50-μm particles.
    • Movie S2 (.mp4 format). Self-cleaning process of hydrophobic 10- to 50-μm particles.
    • Movie S3 (.mp4 format). Self-cleaning process of hydrophilic 1.5-μm particles.

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