Oh yes, it’s the last bit of ketchup that always give you the biggest trouble; how to get the ketchup out of the bottle? We have all been there, swinging, shaping, slapping, tapping and finally give up to physics. Okay, we all can live with little bit of ketchup left in the jar. But undesirable interactions between solid-liquid interfaces like these, presents a much bigger problem, amounts up to losses of billions of dollars in every year. These interactions, clog up oil lines, create drag in transport systems, promote fouling in sewage tunnels, block gas lines and the list goes on.
The surface that SLIPS
The solution for this problem is a perfect slippery surface that repels most of the liquids, ideally simple and inexpensive to produce and, of course last long too. This kind of a coating would immediately have relevance in number of applications ranging from textiles to coatings in ship hulls. In search of solutions, it’s natural to look for answers with lotus effect: a surface phenomenon associated with lotus leaf surface where water just rolls off without making any physical contact. This is due to microscopic roughness on these surfaces coupled with waxy coating that force water droplets to bead up and roll off. Most of the cutting edge liquid repellent surfaces are inspired by the lotus effect. However, after decades of research and development, most of these surfaces lack crucial properties for them to be employed in day today applications. Primarily, the robustness of these surfaces are quite low for practical applications and they show limited repellency to oil and liquids with low surface tension.
In recent years scientists started looking at yet another ingenious plant that pulls off the same trick in completely different way. It is the pitcher plant which has developed- through millions of years of evolution- an extremely slippery surface that can trap insects. Pitcher plant slippery surface can repel the oily feet of the insects thanks to a thin slippery liquid infused in to tiny pore structures in the plant leaf. Insects slip and fall in to the cavity formed by the cupped leaf surface which is then digested by the carnivorous plant. These slippery liquid-infused porous surfaces (SLIPS) can pave the way to a completely novel class of slippery surfaces that can repel other liquids. SLIPS technology have lot of advantages over superhydrophobic surfaces. First, liquid surfaces are inherently smooth and due to possibility of flow, self-healing. Secondly, these surfaces can be tuned for optical transparency and much easily tuned to repel wide variety of liquids ranging from water to crude oil.
SLIPS technology is based on fabrication of nano and microscale porous structures on the substrate surface followed by an infusion with a lubricant liquid. The liquid is physically locked in by the porous structure to form a slippery surface on the interface. Since wide variety of liquids can be infused in to the porous structure, a perfect lubricant can be selected to repel specific liquid type. These surfaces are shown repel both simple and complex liquids like water, hydrocarbons, blood, tomato ketchup, bee honey etc.
Scientists at work
Two research groups at Harvard University and MIT have independently developed techniques to fabricate slippery liquid-infused porous surfaces in to everyday applications. Both these teams have formed spinoff companies with the aim of commercializing SLIPS technology to consumer and industrial applications.
The research team at Harvard University founded a company called SLIPS Technologies who boasts that their technology can be applicable on any substrate including glass, metal, plastic, fabric, etc. Their SLIPS technology is already in market in applications such as optical components to prevent fouling, on refrigeration coils to delay icing and to accelerate de-icing, on undersea structures to resist marine fouling.
The MIT team has taken up a completely different problem to solve. The company name is LiquiGlide. They focus on getting commonly used sticky liquids like tomato ketchup, mayonnaise, toothpaste and hair conditioner out of the bottle little too easy to the consumer. Their first product is to be launched in this year (2015).
- Tak-Sing Wong, Sung Hoon Kang, Sindy K. Y. Tang, Elizabeth J. Smythe, Benjamin D. Hatton, Alison Grinthal, and Joanna Aizenberg, “Bioinspired Self-Repairing Slippery Surfaces with Pressure-stable Omniphobicity”, Nature, vol. 477, pp. 443 – 447 (2011).
- Philseok Kim, Tak-Sing Wong, Jack Alvarenga, Wilmer Adorno, and Joanna Aizenberg, “Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance”, ACS Nano, vol. 6, pp. 6569 – 6577 (2012).
- J.D. Smith, R. Dhiman, S. Anand, E.R. Garduno, R.E. Cohen, G.H. McKinley, K.K. Varanasi, Droplet Mobility on Lubricant-Impregnated Surfaces, Soft Matter, 2012.