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

Nature Sustainability volume 2, pages 1097–1105 (2019)Cite this article

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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.

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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.

References

  1. Eliasson, J. The rising pressure of global water shortages. Nature 517, 6–7 (2015).

    Article  CAS  Google Scholar 

  2. Mekonnen, M. M. & Hoekstra, A. Y. Four billion people facing severe water scarcity. Sci. Adv. 2, e1500323 (2016).

    Article  Google Scholar 

  3. Attari, S. Z. Perceptions of water use. Proc. Natl Acad. Sci. USA 111, 5129–5134 (2014).

    Article  CAS  Google Scholar 

  4. Shannon, M. A. et al. Science and technology for water purification in the coming decades. Nature 452, 301–310 (2008).

    Article  CAS  Google Scholar 

  5. Surwade, S. P. et al. Water desalination using nanoporous single-layer graphene. Nat. Nanotechnol. 10, 459–464 (2015).

    Article  CAS  Google Scholar 

  6. Chiavazzo, E., Morciano, M., Viglino, F., Fasano, M. & Asinari, P. Passive solar high-yield seawater desalination by modular and low-cost distillation. Nat. Sustain. 1, 763–772 (2018).

    Article  Google Scholar 

  7. Parker, A. R. & Lawrence, C. R. Water capture by a desert beetle. Nature 414, 33–34 (2001).

    Article  CAS  Google Scholar 

  8. Ju, J. et al. A multi-structural and multi-functional integrated fog collection system in cactus. Nat. Commun. 3, 1247 (2012).

    Article  CAS  Google Scholar 

  9. Park, K.-C. et al. Condensation on slippery asymmetric bumps. Nature 531, 78–82 (2016).

    Article  CAS  Google Scholar 

  10. Kim, H. et al. Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science 356, 430–434 (2017).

    Article  CAS  Google Scholar 

  11. DeOreo, W. B., Mayer, P. W., Dziegielewski, B. & Kiefer, J. Residential End Uses of Water, Version 2 (Water Research Foundation, 2016).

  12. Human Development Report (United Nations Development Programme, 2006).

  13. Progress on Sanitation and Drinking Water—2015 Update and MDG Assessment (World Health Organization and UNICEF, 2015).

  14. Ghisi, E. & Ferreira, D. F. Potential for potable water savings by using rainwater and greywater in a multi-storey residential building in southern Brazil. Build. Environ. 42, 2512–2522 (2007).

    Article  Google Scholar 

  15. The United Nations World Water Development Report 2016: Water and Jobs (United Nations Educational, Scientific and Cultural Organization & World Water Assessment Programme, 2016).

  16. Mahdavinejad, M., Bemanian, M., Farahani, S. F. & Tajik, A. Role of toilet type in transmission of infections. Acad. Res. Int. 1, 110–113 (2011).

    Google Scholar 

  17. Paterson, C., Mara, D. & Curtis, T. Pro-poor sanitation technologies. Geoforum 38, 901–907 (2007).

    Article  Google Scholar 

  18. Lin, J. et al. Qualitative and quantitative analysis of volatile constituents from latrines. Environ. Sci. Technol. 47, 7876–7882 (2013).

    Article  CAS  Google Scholar 

  19. Tuteja, A. et al. Designing superoleophobic surfaces. Science 318, 1618–1622 (2007).

    Article  CAS  Google Scholar 

  20. Tuteja, A., Choi, W., Mabry, J. M., McKinley, G. H. & Cohen, R. E. Robust omniphobic surfaces. Proc. Natl Acad. Sci. USA 105, 18200–18205 (2008).

    Article  Google Scholar 

  21. Liu, T. L. & Kim, C.-J. C. Turning a surface superrepellent even to completely wetting liquids. Science 346, 1096–1100 (2014).

    Article  CAS  Google Scholar 

  22. Wong, T.-S. et al. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 477, 443–447 (2011).

    Article  CAS  Google Scholar 

  23. Epstein, A. K., Wong, T.-S., Belisle, R. A., Boggs, E. M. & Aizenberg, J. Liquid-infused structured surfaces with exceptional anti-biofouling performance. Proc. Natl Acad. Sci. USA 109, 13182–13187 (2012).

    Article  Google Scholar 

  24. Leslie, D. C. et al. A bioinspired omniphobic surface coating on medical devices prevents thrombosis and biofouling. Nat. Biotechnol. 32, 1134–1140 (2014).

    Article  CAS  Google Scholar 

  25. Wang, J., Kato, K., Blois, A. P. & Wong, T.-S. Bioinspired omniphobic coatings with a thermal self-repair function on industrial materials. ACS Appl. Mater. Interfaces 8, 8265–8271 (2016).

    Article  CAS  Google Scholar 

  26. Wang, L. & McCarthy, T. J. Covalently attached liquids: instant omniphobic surfaces with unprecedented repellency. Angew. Chem. 128, 252–256 (2016).

    Article  Google Scholar 

  27. Israelachvili, J. N. Intermolecular and Surface Forces (Academic Press, 2011).

  28. Dahlquist, C. A. in Treatise on Adhesion and Adhesives Vol. 2 (ed. Patrick, R. L.) 219–260 (Marcel Dekker, 1969).

  29. Gent, A. & Schultz, J. Effect of wetting liquids on the strength of adhesion of viscoelastic material. J. Adhes. 3, 281–294 (1972).

    Article  CAS  Google Scholar 

  30. Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 28, 988–994 (1936).

    Article  CAS  Google Scholar 

  31. Drelich, J. & Chibowski, E. Superhydrophilic and superwetting surfaces: definition and mechanisms of control. Langmuir 26, 18621–18623 (2010).

    Article  CAS  Google Scholar 

  32. Tenjimbayashi, M. et al. Liquid-infused smooth coating with transparency, super-durability, and extraordinary hydrophobicity. Adv. Funct. Mater. 26, 6693–6702 (2016).

    Article  CAS  Google Scholar 

  33. Daniel, D., Timonen, J. V., Li, R., Velling, S. J. & Aizenberg, J. Oleoplaning droplets on lubricated surfaces. Nat. Phys. 13, 1020–1025 (2017).

    Article  CAS  Google Scholar 

  34. De Gennes, P.-G., Brochard-Wyart, F. & Quéré, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves (Springer Science & Business Media, 2013).

  35. Preston, D. J., Song, Y., Lu, Z., Antao, D. S. & Wang, E. N. Design of lubricant infused surfaces. ACS Appl. Mater. Interfaces 9, 42383–42392 (2017).

    Article  CAS  Google Scholar 

  36. Zhuravlev, L. Concentration of hydroxyl groups on the surface of amorphous silicas. Langmuir 3, 316–318 (1987).

    Article  CAS  Google Scholar 

  37. Crisp, A., de Juan, E. & Tiedeman, J. Effect of silicone oil viscosity on emulsification. Arch. Ophthalmol. 105, 546–550 (1987).

    Article  CAS  Google Scholar 

  38. Graiver, D., Farminer, K. & Narayan, R. A review of the fate and effects of silicones in the environment. J. Polym. Environ. 11, 129–136 (2003).

    Article  CAS  Google Scholar 

  39. Seah, M. P. An accurate and simple universal curve for the energy-dependent electron inelastic mean free path. Surf. Interface Anal. 44, 497–503 (2012).

    Article  CAS  Google Scholar 

  40. Liu, H., Zhang, P., Liu, M., Wang, S. & Jiang, L. Organogel-based thin films for self-cleaning on various surfaces. Adv. Mater. 25, 4477–4481 (2013).

    Article  CAS  Google Scholar 

  41. Zhang, C., Xia, Y., Zhang, H. & Zacharia, N. S. Surface functionalization for a nontextured liquid-infused surface with enhanced lifetime. ACS Appl. Mater. Interfaces 10, 5892–5901 (2018).

    Article  CAS  Google Scholar 

  42. Urata, C., Cheng, D. F., Masheder, B. & Hozumi, A. Smooth, transparent and nonperfluorinated surfaces exhibiting unusual contact angle behavior toward organic liquids. RSC Adv. 2, 9805–9808 (2012).

    Article  CAS  Google Scholar 

  43. Rose, C., Parker, A., Jefferson, B. & Cartmell, E. The characterization of feces and urine: a review of the literature to inform advanced treatment technology. Crit. Rev. Environ. Sci. Technol. 45, 1827–1879 (2015).

    Article  CAS  Google Scholar 

  44. Woolley, S., Cottingham, R., Pocock, J. & Buckley, C. Shear rheological properties of fresh human faeces with different moisture content. Water SA 40, 273–276 (2014).

    Article  Google Scholar 

  45. Woolley, S., Buckley, C., Pocock, J. & Foutch, G. Rheological modelling of fresh human faeces. J. Water Sanit. Hyg. Dev. 4, 484–489 (2014).

    Article  Google Scholar 

  46. Yunus, A. C. & Cimbala, J. M. Fluid Mechanics Fundamentals and Applications International Edition (McGraw Hill Publication, 2006).

  47. Vickers, A. Water-use efficiency standards for plumbing fixtures: benefits of national legislation. J. Am. Water Works Assoc. 82, 51–54 (1990).

    Article  Google Scholar 

  48. Awad, T. S., Asker, D. & Hatton, B. D. Food-safe modification of stainless steel food processing surfaces to reduce bacterial biofilms. ACS Appl. Mater. Interfaces 10, 22902–22912 (2018).

    Article  CAS  Google Scholar 

  49. Halvey Alex, K., Macdonald, B., Dhyani, A. & Tuteja, A. Design of surfaces for controlling hard and soft fouling. Phil. Trans. R. Soc. A 377, 20180266 (2019).

    Article  CAS  Google Scholar 

  50. Evans, C., Coombes, P. J. & Dunstan, R. Wind, rain and bacteria: the effect of weather on the microbial composition of roof-harvested rainwater. Water Res. 40, 37–44 (2006).

    Article  CAS  Google Scholar 

  51. Segura, C. G. Urine flow in childhood: a study of flow chart parameters based on 1,361 uroflowmetry tests. J. Urol. 157, 1426–1428 (1997).

    Article  Google Scholar 

  52. Yang, P. J., Pham, J., Choo, J. & Hu, D. L. Duration of urination does not change with body size. Proc. Natl Acad. Sci. USA 111, 11932–11937 (2014).

    Article  CAS  Google Scholar 

  53. Hennigs, J. et al. Field testing of a prototype mechanical dry toilet flush. Sci. Total Environ. 668, 419–431 (2019).

    Article  CAS  Google Scholar 

  54. Lewis, S. & Heaton, K. Stool form scale as a useful guide to intestinal transit time. Scand. J. Gastroenterol. 32, 920–924 (1997).

    Article  CAS  Google Scholar 

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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

Author notes

  1. These authors contributed equally: Lin Wang, Nan Sun.

Authors and Affiliations

  1. Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA, USA

    Jing Wang, Nan Sun, Margo Corsetti, Pak Kin Wong & Tak-Sing Wong

  2. Materials Research Institute, The Pennsylvania State University, University Park, PA, USA

    Jing Wang, Lin Wang, Nan Sun & Tak-Sing Wong

  3. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA

    Lin Wang

  4. Centre for Competitive Creative Design, Cranfield University, Cranfield, UK

    Ross Tierney & Leon Williams

  5. Department of Biomedical Engineering, The Pennsylvania State University, University Park, PA, USA

    Hui Li, Pak Kin Wong & Tak-Sing Wong

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Contributions

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.

Corresponding author

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). https://doi.org/10.1038/s41893-019-0421-0

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