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New "Can't Touch This" Coating

Scientists at the University of Michigan have teamed up with the Air Force to create a superomniphobic material coating that repels, well, pretty much everything. The research was published in the Journal of the American Chemical Society, on January 16, 2013.

SEM image of the coating on a stainless steel wire mesh
coated with cross-linked molecules.
Image adapted with permission from the
 J. Am. Chem. Soc.2013135 (2), pp 578–581.
Copyright 2012 American Chemical Society. 
Spill a glass of wine, a spatter of oil, a slosh of hydrochloric acid and emerge stain- and chemical-free. "Virtually any liquid you throw on [the surface] bounces right off without wetting it," said lead researcher Anish Tuteja to the University of Michigan press. 

The key to the resistant coating is its hierarchical structure which effectively lifts the spilled liquid away from the surface, repelling liquid aimed from any angle. 

A hierarchical structure is one in which there are multiple layers of structural organization that span several length scales. For example, a net of strings in which each strand is itself made of a net of smaller strings is a hierarchical structure.

The coating is made of tiny spherical plastic particles called PDMS mixed with other nanoscale cubes created by the Air Force of carbon, fluorine, silicon, and oxygen. In a process called electrospinning, the scientists applied a voltage to the liquid coating as they spun it evenly across a metal surface. Through the slight charge separation caused by the voltage potential difference, the coating forms microscopic cross-links like a mesh web. Over 95 percent of the coating is actually air pockets, Tuteja explained. This hierarchical structure-within-a-structure drastically reduces the intermolecular Van der Waals forces that usually attract liquids to surfaces. 

"Normally, when two materials get close, they imbue a small positive or negative charge on each other, and as soon as the liquid comes in contact with the solid surface it will start to spread," he said

Jets of different fluids maintain their droplet shape (E),
and bounce off at different angles (F). Image adapted with permission from the 
 J. Am. Chem. Soc.2013135 (2), pp 578–581.
Copyright 2012 American Chemical Society. 
With the coating, the intermolecular forces between the liquid ends up being stronger than the interaction between the liquid and the coating: stain avoided. 

To test their coating, the team studied over 100 organic, inorganic, acids, bases and solvent liquids including coffee, vegetable oil, sulfuric acid, hydrochloric acid, gasoline, and several alcohols. Only two kinds of liquids, both chlorofluorocarbons, stuck. The coating even worked for non-newtonian fluids– liquids that change viscosity depending on the forces applied to them. 

The researchers hope the coating can be applied to develop protective clothing for the military and scientists, and biofouling and corrosion resistant surfaces. 

All images Reprinted with permission from Superomniphobic Surfaces for Effective Chemical Shielding Shuaijun Pan, Arun K. Kota, Joseph M. Mabry, and Anish Tuteja Journal of the American Chemical Society 2013 135 (2), 578-581. Copyright 2013 American Chemical Society.


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