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Slippery Surfaces

Harvard Professor Joanna Aizenberg shows David Pogue the Nepenthes Pitcher Plant, a carnivorous plant whose slippery surface inspired a non-stick material invented by her lab.

Projects

Unwanted surface interactions are a limiting factor nearly everywhere liquid is handled or encountered: they create drag in transport systems, trigger blood clotting, nucleate aircraft icing, and promote biofouling. Despite intensive study, state-of-the-art repellent surfaces have remained poorly suited for many substances, environments, and budgets. Inspired by the carnivorous pitcher plant, we have invented a fundamentally different conceptual approach to surface design that avoids the inherent limits of current strategies. Based on this approach, we have created surfaces that show almost perfect slipperiness toward polar and organic liquids, complex substances such as blood, oil, and ketchup, a genetically diverse range of bacteria and algae, and solid materials such as ice, dust, and insects. The surfaces can be optimized to function under extreme conditions, adapt to stimuli, and self-heal, and can be constructed from a variety of low-cost materials and applied to surfaces as diverse as medical tubing, planes, and refrigerators. 

Our approach comes from the same concept the pitcher plants use to make insects slide down their leaves into their digestive juices. We infuse a porous substrate with a lubricating fluid such that the overlying film, rather than air or solid, serves as the slippery interface. Based on extensive theoretical and experimental characterization, we have defined materials requirements for the lubricant to form a locked-in, stable, inert surface. These surfaces outperform state-of-the-art surfaces in scope (water, hydrocarbons, crude oil, blood, ice, bacteria), liquid mobility (contact angle hysteresis <2.5o), resilience (recovery within 0.1-1 s); and pressure tolerance (up to ~676 atm). Since low-surface-energy porous solids are abundant from small to large sacles, highly omniphobic surfaces can be easily generated without specialized fabrication facilities. 

Currently our group is developing this technology to meet emerging needs in biomedical fluid handling, fuel transport, antifouling, anti-icing, optical imaging, harsh environments, and other areas. 


See SLIPS in action by following the links below: 

Interviews: 

"The Makers" of The Mind of the Universe features Professor Aizenberg, International science documentary series, VPRO, May 21, 2017.

Nova episode on Making Stuff Wilder, October 23, 2013. 

Deadly pitcher-plant inspires super slippery nano-surface, Reuters, February 6, 2012. 

“Extreme Biomimetics” talk by Professor Aizenberg, Disruptive Ideas, TEDxBigApple, New York, February 4, 2012. 

“Slippery when wet (or anytime)”, CBC interview with Professor Aizenberg, October 8, 2011. 

Material World, BBC interview with Professor Aizenberg, September 21, 2011. 
 


In the news: 

Slippery liquid surfaces confuse mussels to stop them from sticking to underwater structures, Harvard press release, August 17, 2017. 

ARPA-E Making Progress Toward Achieving Mission, Report by the National Academies of Sciences, Engineering, and Medicine, June 13, 2017.

Creating a slippery slope on the surface of medical implants, Harvard press release, November 1, 2016. 

An unobstructed view into the human body, Harvard press release, September 26, 2016

Harnessing engineered slippery surfaces for tissue repair, Harvard press release, May 18, 2016.

Mid-Atlantic Seabed Drilling Expedition, BBC Science in Action podcast (starting at 14:08 minutes), October 23, 2015. 

Super-slick material makes steel better, stronger, cleaner, Harvard press release, October 20, 2015. 

The secrets of secretion, Harvard press release, June 22, 2015. 

What Is ARPA-E Up to Now?, Innovation, May 2015. 

Artificial membrane system uses fluid-filled pores for smooth moving, Materials 360, April 9, 2015. 

Literally Nothing Will Stick To This New Slippery Surface, Fast Company, March 27, 2015. 

Fabrics of Life, Nature, Outlook, March 25, 2015. 

Pores for thought over separation issues, Materials Today, March 10, 2015. 

Gating mechanism under pressure, Nature, News and VIews, March 5, 2015. 

Fluid–filled pores separate materials with fine precision, Harvard press release, March 5, 2015. 

New silicone infused with silicone oil = super-slippery, and proven to keep bacteria from growing on medical tubing, Reddit, February 20, 2015. 

SLIPS team heads to Capitol Hill, Harvard press release, February 12, 2015. 

Novel non-stick material joins portfolio of slippery surface technologies, Harvard press release, February 10, 2015. 

Wyss Institute launches SLIPS company, Harvard press release, October 29, 2014. Read more at SLIPS Technologies. 

Carnivorous Plant Inspires Anticlotting Medical Devices, Scientific American podcast, October 15, 2014. 

Slippery When Coated: Helping Medical Devices Prevent Blood Clots, NPR, October 12, 2014. 

Bioinspired coating for medical devices repels blood and bacteria, Harvard press release, October 12, 2014. 

Super-Slick Material Keeps Ice From Forming, Technology Review, July 2, 2014. 

Scientists Modify Cotton and Polyester to Display Repellent Properties, The Crimson, February 26, 2014. 

Stain-free, self-cleaning clothing on the horizon, Harvard press release, January 13, 2014. 

Pulling On The Shade, American Scientist, September 1, 2013. 

New coating turns ordinary glass into super-slippery glass, Harvard press release, August 2, 2013. 

Fluid-Infused Porous FIlms Dynamically Adjust Transparency and Wettability, Materials 360, April 22, 2013. 

Scientists Design New Adaptive Material Inspired by Tears, Popular Mechanics, April 17, 2013. 

Morphing 'fabric' shifts shape to repel or grip water, New Scientist, April 9, 2013. 

New material can halt runny liquids on demand, BBC, April 9, 2013. 

Cry me a river of possibility: Scientists design new adpative material inspired by tears, Harvard press release, April 8, 2013. 

Ice-Phobic Surfaces that are Wet, ACS Nano, August 9, 2012. 

SLIPS Blitz Biofilms, Nature, August 9, 2012. 

Harvard scientists' breakthrough could stop biofilm formation, Food Production Daily, August 9, 2012. 

SLIPS liquid repeller is inspired by carnivorous plants, enemy to insects and graffiti artists alike, Engadget, August 3, 2012. 

Super slippery surface prevents biofilms, PNAS, July 31, 2012. 

New coating evicts biofilms for good, Harvard press release, July 30, 2012. 

Ice Curbs, National Science Foundation's Discovery Files (podcast), June 27, 2012. 

'Ice-Phobic' Airplane Wings, Wall Street Journal, June 22, 2012. 

SLIPS receives a 2012 R&D 100 Award from R&D Magazine, honoring it as one of the 100 most technologically significant products of the previous year. June 20, 2012. 

No Scraper Required: Ice Rolls Off Metal, Discovery, June 16, 2012. 

Slippery Coating Keeps Metals Frost-Free, Chemical and Engineering News, June 15, 2012. 

Keeping Metal Surfaces Ice, Frost Free, The Hindu, June 14, 2012. 

Ultra-Antifreeze Keeps Ice From Even Forming, Smithsonian, June 12, 2012. 

A new spin on anti-freeze, Harvard press release, June 11, 2012. 

Super-slippery material could mean end to having to wait for ketchup, The Telegraph, November 13, 2011. 

Slippery Slope: researchers take advice from carnivorous plant, Harvard press release, September 21, 2011. 

Slippery when wetted, Nature News and Views, September 21, 2011. 

Carnivorous plant inspires super slippery material, New Scientist, September 21, 2011. 

Pitcher plant inspires super slippery surface, Chemical and Engineering News, September 21, 2011. 

Ketchup bottle problems solved, The Telegraph, November 13, 2011. 

Carnivorous plant shows its slippery side, Financial Times, September 30, 2011. 

A great non-friction story: Researchers create ‘world’s most slippery surface’, Daily Mail, September 22, 2011. 

Scientists create wonder material, Metro Herald, p. 10, September 22, 2011. 

Copied from pitcher plants; destined for ketchup bottles, Popular Science, November 14, 2011. 

Pitcher plant perfect, Nature Chemistry, vol. 3, pp. 834, October 24, 2011. 

Pitcher plant inspires ultimate non-stick surface, Chemistry World, September 22, 2011. 

Plant technology used to create super-repellent material, The Engineer, September 26, 2011. 

Flesh-eating plant inspires super-slippery material that repels everything, Discover Online, September 21, 2011. 

Invention Solves Ketchup Dilemma, The Harvard Crimson, November 20, 2011. 

Source: Nature 
Michael Nosonovsky of the University of Wisconsin-Milwaukee: "The development of SLIPS typifies two themes that are likely to dominate the field of biomimetic and functional surfaces in coming years." 

Source: C&EN (ACS) 
Philippe Brunet, an expert on omniphobic materials at France’s University of Paris Diderot, says he is impressed by the innovative strategy. “In my opinion, this study does not simply represent an improvement in liquid-repellent surfaces, but rather a revolution in the field,” he says. “I am sure many researchers reading this paper would think as I do: ‘I wish I would have thought about these surfaces before.’” 

Source: Chemistry World (RCS) 
"This really is a new direction," says Steven Bell, director of innovative molecular materials at Queen's University Belfast, UK. "Many of us have been working on improving the durability of 'lotus effect' materials but this now offers us an alternative way to try to reach the same objectives." 

Source: ABC (Australia) 
Professor Robert Short, director of the Mawson Institute at the University of South Australia, says the results are highly impressive and the simplicity of the approach is particularly striking. 
"The combination of solids and liquids uses capillary action to bring lubricant to the surface of nano/microengineered structures," Short says. 
"Furthermore, the ability of the surface to 'heal' sets it apart from other cutting-edge approaches." 

Source: Discover 
Walter Federle from the University of Cambridge, who discovered the structure of the pitcher plant’s peristome, says,“It’s really exciting to see that this principle has inspired the authors and allowed them to develop something that could prove extremely useful.” 

Source: New Scientist 
"It's interesting that it combines self-lubrication, self-healing and self-cleaning, which are different 
processes," says Michael Nosonovsky of the University of Wisconsin-Milwaukee. "It's a new type of 
smart material."

Park K-C, Kim P, Grinthal A, He N, Fox D, Weaver JC, Aizenberg J. Condensation on slippery asymmetric bumps. Nature. 2016;531 (7592) :78-82.Abstract
Controlling dropwise condensation is fundamental to water-harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches based on microscale, nanoscale or molecular-scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach—based on principles derived from Namib desert beetles, cacti, and pitcher plants—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and using theoretical modelling, we show how to maximize vapour diffusion flux at the apex of convex millimetric bumps by optimizing the radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope, analogous to cactus spines, directly couples facilitated droplet growth with fast directional transport, by creating a free-energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher-plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity and overcome the effect of an unfavourable temperature gradient. We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces. We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications.
Daniel D, Mankin MN, Belisle RA, Wong T-S, Aizenberg J. Lubricant-infused micro/nano-structured surfaces with tunable dynamic omniphobicity at high temperatures. Appl. Phys. Lett. 2013;102 :231603. Full TextAbstract
Omniphobic surfaces that can repel fluids at temperatures higher than 100 °C are rare. Most state-of-the-art liquid-repellent materials are based on the lotus effect, where a thin air layer is maintained throughout micro/nanotextures leading to high mobility of liquids. However, such behavior eventually fails at elevated temperatures when the surface tension of test liquids decreases significantly. Here, we demonstrate a class of lubricant-infused structuredsurfaces that can maintain a robust omniphobic state even for low-surface-tension liquids at temperatures up to at least 200 °C. We also demonstrate how liquid mobility on such surfaces can be tuned by a factor of 1000.
Leslie DC, Waterhouse A, Berthet JB, Valentin TM, Watters AL, Jain A, Kim P, Hatton BD, Nedder A, Donovan K, et al. A bioinspired omniphobic surface coating on medical devices prevents thrombosis and biofouling. Nature Biotechnology. 2014;32 (11) :1134-1140. Full TextAbstract

Thrombosis and biofouling of extracorporeal circuits and indwelling medical devices cause significant morbidity and mortality worldwide. We apply a bioinspired, omniphobic coating to tubing and catheters and show that it completely repels blood and suppresses biofilm formation. The coating is a covalently tethered, flexible molecular layer of perfluorocarbon, which holds a thin liquid film of medical-grade perfluorocarbon on the surface. This coating prevents fibrin attachment, reduces platelet adhesion and activation, suppresses biofilm formation and is stable under blood flow in vitro. Surface-coated medical-grade tubing and catheters, assembled into arteriovenous shunts and implanted in pigs, remain patent for at least 8 h without anticoagulation. This surface-coating technology could reduce the use of anticoagulants in patients and help to prevent thrombotic occlusion and biofouling of medical devices.

Yao X, Hu Y, Grinthal A, Wong T-S, Mahadevan L, Aizenberg J. Adaptive fluid-infused porous films with tunable transparency and wettability. Nature Materials. 2013;12 :529-534. Full TextAbstract
Materials that adapt dynamically to environmental changes are currently limited to two-state switching of single properties, and only a small number of strategies that may lead to materials with continuously adjustable characteristics have been reported1-3. Here we introduce adaptive surfaces made of a liquid film supported by a nanoporous elastic substrate. As the substrate deforms, the liquid flows within the pores causing the smooth and defect-free surface to roughen through a continuous range of topographies. We show that a graded mechanical stimulus can be directly translated into finely tuned, dynamic adjustments of optical transparency and wettability. In particular, we demonstrate simultaneous control of the film's transparency and its ability to continuously manipulate various low-surface-tension droplets from free-sliding to pinned. This strategy should make possible the rational design of tunable, multifunctional adaptive materials for a broad range of applications.
Sunny S, Vogel N, Howell C, Vu TL, Aizenberg J. Lubricant-infused Nanoparticulate Coatings Assembled by Layer-by-layer Deposition. Adv. Funct. Mater. 2014;24 (42) :6658-6667.Abstract

Omniphobic coatings are designed to repel a wide range of liquids without leaving stains on the surface. A practical coating should exhibit stable repel- lency, show no interference with color or transparency of the underlying substrate and, ideally, be deposited in a simple process on arbitrarily shaped surfaces. We use layer-by-layer (LbL) deposition of negatively charged silica nanoparticles and positively charged polyelectrolytes to create nanoscale sur- face structures that are further surface-functionalized with fluorinated silanes and infiltrated with fluorinated oil, forming a smooth, highly repellent coating on surfaces of different materials and shapes. We show that four or more
LbL cycles introduce sufficient surface roughness to effectively immobilize the lubricant into the nanoporous coating and provide a stable liquid inter- face that repels water, low-surface-tension liquids and complex fluids. The absence of hierarchical structures and the small size of the silica nanoparti- cles enables complete transparency of the coating, with light transmittance exceeding that of normal glass. The coating is mechanically robust, maintains its repellency after exposure to continuous flow for several days and prevents adsorption of streptavidin as a model protein. The LbL process is conceptu- ally simple, of low cost, environmentally benign, scalable, automatable and therefore may present an efficient synthetic route to non-fouling materials.

Cui J, Daniel D, Grinthal A, Lin K, Aizenberg J. Dynamic polymer systems with self-regulated secretion for the control of surface properties and material healing. Nat. Mater. 2015;14 (8) :790-795. Publisher's VersionAbstract
Approaches for regulated fluid secretion, which typically rely on fluid encapsulation and release from a shelled compartment, do not usually allow a fine continuous modulation of secretion, and can be difficult to adapt for monitoring or function-integration purposes. Here, we report self-regulated, self-reporting secretion systems consisting of liquid-storage compartments in a supramolecular polymer-gel matrix with a thin liquid layer on top, and demonstrate that dynamic liquid exchange between the compartments, matrix and surface layer allows repeated, responsive self-lubrication of the surface and cooperative healing of the matrix. Depletion of the surface liquid or local material damage induces secretion of the stored liquid via a dynamic feedback between polymer crosslinking, droplet shrinkage and liquid transport that can be read out through changes in the system’s optical transparency. We foresee diverse applications in fluid delivery, wetting and adhesion control, and material self-repair.

  • See SLIPS in action by viewing the videos below:

  • The full collection is found here. 
    Stay tuned: more movies and demonstrations to come!