SLIPS: A new material concept ?

In the realms of philosophy as well as science, lotus leaf has a unique position.  In philosophy it is an excellent allegory for being  pristine,  incorruptible, by the surroundings. In material science, it is the supreme example of  water repelling (hydrophobic) surface. Material scientists have gone with their  typical tongs and tools of   Electron Microscopy and Atomic Force Microscopy to unravel the mystery of this special feature(1).  The lotus leaf looks, polished, smooth  and shiny, but has actually myriads of micro sized  (10-20 microns in size) pimples.   The pimples are crowned with a veil of ( nano layer)   a water resistant waxy material. Armed with this knowledge scientists are in the process of recreating this effect and  there have been a series of patents too. 

What about a  surface that is slippery and  repels both oil and water?    Now  Wong and his team from the Wyss Institute for for Biologically Inspired Engineering,  (School of Engineering,  Harvard University)  are attempting to achieve  just that.   Their model again comes from the plant kingdom, the Pitcher plants(2). Pitcher plants are the nonvegetarians of the  vegetarian kingdom. They   roll up their leaves  to make contraptions which look like pitchers and hence the name. Into these  colorful   traps   they  ensnare small insects and make a nice meal of them.   So what is special about the pitcher leaf surface?  Wong and his team found that these do have a textured surface just like the lotus leaf, but then the cavities in between the pimples  are filled with a self spreading watery slurry.  And it is this combination that does the trick. 

 With this model in mind Wang's team   went to work.   Their efforts are   reported   in the 22nd September issue of  Nature (3).    They call the technique  SLIPS short for Slippery Liquid Infused Porous Surface.  So how did they achieve this ?  They focused mainly on the miscibility criteria. This was essentially  a three component system:  a surface, a slippery  liquid  which wets the surface, and  the   third the immiscible  impinging liquid   Wang and his team  reduced the complexity of the system to one of  surface energy and surface tension calculations.  

This simple approach   allowed them to design  several sets of surface/slippery liquid combinations which could repel liquids of a wide range of surface tensions.  In the Nature paper 

the  demonstrates the design of a surface   from commercially available materials such as teflon and epoxy membranes  and perfluorinated  fluids which could repel, water, ice, blood, crude oil and above all inability of a carpenter ant to hold onto the surface. 

The two component SLIPS could have very interesting variations. Imagine it to be composed of not a solid surface and a fluid but two  nonNewtonian fluids : thixotropic ( liquids which gel on standing and flow when stirred).  and rheopectic ( liquids which flow  on standing and gel  when stirred).  This could spell novel ideas for the materials of Sports world indeed.

1.  http://en.wikipedia.org/wiki/Pitcher_plant; Planta 202, 1-8(1997). 

2. Proceedings of the National Academy of Sciences, (U.S.A.) 101, 14138-14143 (2004)

3.  Nature 477 pages 443-447 (2011)