Viscoelastic solid-repellent coatings for extreme water saving and global sanitation

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Abstract

Water scarcity threatens over half of the world’s population, yet over 141 billion litres of fresh water are used globally each day for toilet flushing. This is nearly six times the daily water consumption of the population in Africa. The toilet water footprint is so large primarily because large volumes of water are necessary for the removal of human faeces; human faeces is viscoelastic and sticky in nature, causing it to adhere to conventional surfaces. Here, we designed and fabricated the liquid-entrenched smooth surface (LESS)—a sprayable non-fouling coating that can reduce cleaning water consumption by ~90% compared with untreated surfaces due to its extreme repellency towards liquids, bacteria and viscoelastic solids. Importantly, LESS-coated surfaces can repel viscoelastic solids with dynamic viscosities spanning over nine orders of magnitude (that is, three orders of magnitude higher than has previously been reported for other repellent materials). With an estimated 1 billion or more toilets and urinals worldwide, incorporating LESS coating into sanitation systems will have significant implications for global sanitation and large-scale wastewater reduction for sustainable water management.

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Fig. 1: Fabrication of LESS.
Fig. 2: Overview of state-of-the-art liquid and viscoelastic solid repellent surfaces.
Fig. 3: Comparison of liquid and synthetic faeces repellency between a state-of-the-art commercial hydrophobic glaze-coated toilet (SloanTec hydrophobic glaze) and a LESS-coated toilet.
Fig. 4: Work of adhesion and water consumption characterizations.
Fig. 5: Anti-fouling performance of LESS.
Fig. 6: Durability characterizations and lubricant replenishment of LESS-coated surfaces.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Additional data that support the findings of this study are available from the corresponding author upon request.

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Acknowledgements

We thank V. Bojan and J. Shallenberger at the Materials Research Institute of The Pennsylvania State University for help with the X-ray photoelectron spectroscopy measurements and data processing, L. Andersson for help with the longevity test, B. Boschitsch Stogin for help with manuscript preparation, and A. Turrigiano for discussion. We thank T. Laremore (director of the Huck Institutes of the Life Sciences Proteomics and Mass Spectrometry Core Facility) for assistance with the MALDI Biotyper microorganism identification. We thank J. C. Liao from Stanford University for providing the urine sample. We acknowledge funding support from the National Science Foundation (CAREER Award number 1351462; I-Corps numbers 1757165 and 1735627), Wormley Family Early Career Professorship and Humanitarian Materials Initiative Award, sponsored by Covestro and the Materials Research Institute at The Pennsylvania State University. Part of the work was conducted at the Penn State node of the National Science Foundation-funded National Nanotechnology of Infrastructure Network.

Author information

J.W. and T.-S.W. designed the overall experiments. J.W. designed the LESS coating. J.W. and L.Wang fabricated the LESS coating. J.W. and N.S. designed the adhesion tests. J.W., N.S. and M.C. performed the adhesion tests. J.W., L.Wang, H.L. and P.K.W. designed the anti-biofouling tests. J.W., L.Wang and H.L. performed the anti-biofouling tests. R.T. and L.Williams designed and performed the human faeces tests. J.W. and L.Wang designed and performed the durability tests. J.W., L.Wang, N.S., M.C., H.L., R.T. and T.-S.W. analysed and processed the data. J.W. and T.-S.W. wrote the manuscript. All authors reviewed the manuscript.

Correspondence to Tak-Sing Wong.

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Competing interests

J.W. and T.-S.W. are the inventors on a patent application (PCT/US2017/062206) submitted by the Penn State Research Foundation that describes the LESS coating technology. T.-S.W. is a co-founder of the start-up company spotLESS Materials, which commercializes the LESS coating technology. N.S. is currently employed by spotLESS Materials. All other authors have no competing interests.

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Supplementary information

Supplementary Information

Captions for Supplementary Videos 1–8, Notes 1–11, Figs. 1–17, Tables 1–13 and refs. 1–22.

Supplementary Video 1

Supplementary Video 1 Super-wetting of silicone oil (20 cSt) on a PDMS-grafted glass surface. A 10-µl droplet of silicone oil was released onto a PDMS-grafted surface, which spread and completely wet the surface. Then, 10 µl of water was put on the lubricated surface. The mobility of the water droplet indicated the formation of a stabilized silicone oil film.

Supplementary Video 2

Supplementary Video 2 Spray-coating process to form LESS coating. The substrate used in the video was glass, and was cleaned by isopropanol, ethanol and deionized water. First, we sprayed ~2 ml silane solution onto the glass surface, and let the surface dry for 3 min. Then, silicone oil (with a viscosity of 20 cSt) was sprayed onto the surface. The LESS coating was then successfully formed by testing the surface with blue dyed water and synthetic faeces at 20 wt% solid content. Both water and synthetic faeces slid off the LESS-coated surface.

Supplementary Video 3

Supplementary Video 3 Spray-coating process to form LESS coating on different substrates. These substrates included ceramic, titanium and carbon steel. Before the coating process, these substrates were all cleaned by isopropanol, ethanol and deionized water. First, we sprayed ~2 ml silane solution onto the glass surface, and let the surface dry for 3 min. Then, silicone oil (with a viscosity of 20 cSt) was sprayed onto the surface. The formation of the LESS coating was confirmed by the successful repellency of the dyed water (in blue).

Supplementary Video 4

Supplementary Video 4 Comparison between uncoated and LESS-coated ceramic substrates. Approximately 5 g of synthetic faeces (solid percentage 30%) was dropped onto the testing surfaces, then rinsed by dyed water. The synthetic faeces stuck on the uncoated surface but slid off from the LESS-coated surface.

Supplementary Video 5

Supplementary Video 5 Water repellency comparison between a commercially available hydrophobic glaze-coated toilet bowl (SloanTec) and a LESS-coated toilet bowl. The blue liquid is dyed water.

Supplementary Video 6

Supplementary Video 6 Synthetic faeces repellency comparison between a commercially available hydrophobic glaze-coated toilet bowl (SloanTec) and a LESS-coated toilet bowl. Approximately 5 g of synthetic faeces (solid percentage 30%) was dropped onto the testing surfaces from a height of ~10 cm.

Supplementary Video 7

Supplementary Video 7 Comparison between a LESS-coated surface and other control surfaces, including ceramic (a commonly used toilet material), Teflon and silicone. Approximately 10 g human faeces were dropped from a 75-mm height and landed on the testing surfaces. The faeces stuck on all three control surfaces except for the LESS-coated glass, where the faeces slid off from the surface.

Supplementary Video 8

Supplementary Video 8 Displacement wetting behaviour of silicone oil on PDMS-grafted glass. We added a number of dyed deionized water droplets, which were pinned onto the surface. Once the silicone oil was sprayed onto the surface, all water droplets started to slide off the surface due to displacement wetting of the silicone oil (that is, the silicone oil displaced the water, and adhered onto the PDMS-grafted glass surface). The lubricated surface could then repel the immiscible dyed water demonstrating the stability of the lubricating layer.

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Wang, J., Wang, L., Sun, N. et al. Viscoelastic solid-repellent coatings for extreme water saving and global sanitation. Nat Sustain 2, 1097–1105 (2019) doi:10.1038/s41893-019-0421-0

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