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.
Subscribe to Journal
Get full journal access for 1 year
only $8.67 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
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.
Eliasson, J. The rising pressure of global water shortages. Nature 517, 6–7 (2015).
Mekonnen, M. M. & Hoekstra, A. Y. Four billion people facing severe water scarcity. Sci. Adv. 2, e1500323 (2016).
Attari, S. Z. Perceptions of water use. Proc. Natl Acad. Sci. USA 111, 5129–5134 (2014).
Shannon, M. A. et al. Science and technology for water purification in the coming decades. Nature 452, 301–310 (2008).
Surwade, S. P. et al. Water desalination using nanoporous single-layer graphene. Nat. Nanotechnol. 10, 459–464 (2015).
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).
Parker, A. R. & Lawrence, C. R. Water capture by a desert beetle. Nature 414, 33–34 (2001).
Ju, J. et al. A multi-structural and multi-functional integrated fog collection system in cactus. Nat. Commun. 3, 1247 (2012).
Park, K.-C. et al. Condensation on slippery asymmetric bumps. Nature 531, 78–82 (2016).
Kim, H. et al. Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science 356, 430–434 (2017).
DeOreo, W. B., Mayer, P. W., Dziegielewski, B. & Kiefer, J. Residential End Uses of Water, Version 2 (Water Research Foundation, 2016).
Human Development Report (United Nations Development Programme, 2006).
Progress on Sanitation and Drinking Water—2015 Update and MDG Assessment (World Health Organization and UNICEF, 2015).
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).
The United Nations World Water Development Report 2016: Water and Jobs (United Nations Educational, Scientific and Cultural Organization & World Water Assessment Programme, 2016).
Mahdavinejad, M., Bemanian, M., Farahani, S. F. & Tajik, A. Role of toilet type in transmission of infections. Acad. Res. Int. 1, 110–113 (2011).
Paterson, C., Mara, D. & Curtis, T. Pro-poor sanitation technologies. Geoforum 38, 901–907 (2007).
Lin, J. et al. Qualitative and quantitative analysis of volatile constituents from latrines. Environ. Sci. Technol. 47, 7876–7882 (2013).
Tuteja, A. et al. Designing superoleophobic surfaces. Science 318, 1618–1622 (2007).
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).
Liu, T. L. & Kim, C.-J. C. Turning a surface superrepellent even to completely wetting liquids. Science 346, 1096–1100 (2014).
Wong, T.-S. et al. Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity. Nature 477, 443–447 (2011).
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).
Leslie, D. C. et al. A bioinspired omniphobic surface coating on medical devices prevents thrombosis and biofouling. Nat. Biotechnol. 32, 1134–1140 (2014).
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).
Wang, L. & McCarthy, T. J. Covalently attached liquids: instant omniphobic surfaces with unprecedented repellency. Angew. Chem. 128, 252–256 (2016).
Israelachvili, J. N. Intermolecular and Surface Forces (Academic Press, 2011).
Dahlquist, C. A. in Treatise on Adhesion and Adhesives Vol. 2 (ed. Patrick, R. L.) 219–260 (Marcel Dekker, 1969).
Gent, A. & Schultz, J. Effect of wetting liquids on the strength of adhesion of viscoelastic material. J. Adhes. 3, 281–294 (1972).
Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 28, 988–994 (1936).
Drelich, J. & Chibowski, E. Superhydrophilic and superwetting surfaces: definition and mechanisms of control. Langmuir 26, 18621–18623 (2010).
Tenjimbayashi, M. et al. Liquid-infused smooth coating with transparency, super-durability, and extraordinary hydrophobicity. Adv. Funct. Mater. 26, 6693–6702 (2016).
Daniel, D., Timonen, J. V., Li, R., Velling, S. J. & Aizenberg, J. Oleoplaning droplets on lubricated surfaces. Nat. Phys. 13, 1020–1025 (2017).
De Gennes, P.-G., Brochard-Wyart, F. & Quéré, D. Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves (Springer Science & Business Media, 2013).
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).
Zhuravlev, L. Concentration of hydroxyl groups on the surface of amorphous silicas. Langmuir 3, 316–318 (1987).
Crisp, A., de Juan, E. & Tiedeman, J. Effect of silicone oil viscosity on emulsification. Arch. Ophthalmol. 105, 546–550 (1987).
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).
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).
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).
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).
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).
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).
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).
Woolley, S., Buckley, C., Pocock, J. & Foutch, G. Rheological modelling of fresh human faeces. J. Water Sanit. Hyg. Dev. 4, 484–489 (2014).
Yunus, A. C. & Cimbala, J. M. Fluid Mechanics Fundamentals and Applications International Edition (McGraw Hill Publication, 2006).
Vickers, A. Water-use efficiency standards for plumbing fixtures: benefits of national legislation. J. Am. Water Works Assoc. 82, 51–54 (1990).
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).
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).
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).
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).
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).
Hennigs, J. et al. Field testing of a prototype mechanical dry toilet flush. Sci. Total Environ. 668, 419–431 (2019).
Lewis, S. & Heaton, K. Stool form scale as a useful guide to intestinal transit time. Scand. J. Gastroenterol. 32, 920–924 (1997).
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.
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.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Captions for Supplementary Videos 1–8, Notes 1–11, Figs. 1–17, Tables 1–13 and refs. 1–22.
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 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 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 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 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 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 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 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.
About this article
Cite this article
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