Geckos are able to maintain a grip on wet surfaces not because their toe pads repel water, but because they attract it. A new polymer, which was inspired by this phenomenon, could find use in shoe soles that keep people from slipping on ice.
The fundamental reason that ice is slippery is because even in sub-freezing temperatures, it has a very thin film of liquid water on its surface.
In a nutshell, this film is present due the fact that the water molecules at the surface of a block of ice aren't held in place by any molecules above them. By contrast, molecules within that solid block are firmly secured by other molecules that are packed above, below, and to all sides of them.
Additionally, the pressure that our feet exert on ice lowers its melting temperature, plus the friction that we produce when shuffling our feet across ice produces heat that may ever-so-slightly melt it.
Most anti-slip shoe sole materials attempt to address this problem by repelling liquid water. Such is not the case with the experimental new polymer, however, which draws inspiration from gecko toes.
Each of a gecko's toe pads is made up of rows of tiny hair-like structures known as setae. The narrow grooves between these rows naturally draw in liquid water via capillary action. This process produces suction. Therefore, when the reptile walks across a solid surface covered by a film of water, its toes are actually sucked down against that underlying surface.
An international team of scientists set out to replicate that phenomenon in a human-made polymer, by combining silicone rubber with nanoparticles of a hydrophilic (water-attracting) ceramic known as zirconia. This polymer was rolled into a thin sheet, which was then laser-etched with a grooved pattern.
When that sheet was pressed against sections of wet ice, the nanoparticles drew the liquid water up into the grooves, creating suction that pulled the material down into contact with the underlying solid ice within just 1.5 milliseconds.
After experimenting with a number of formulations, it was found that the polymer was most slip-resistant when it contained just 3% to 5% zirconia nanoparticles by weight. There's currently no word on when it might reach commercial production.
The study was led by Dr. Vipin Richhariya from Portugal's University of Minho; Assoc. Prof. Ashis Tripathy from India's Vellore Institute of Technology; and Dr. Md Julker Nine from The University of Adelaide in Australia. It is described in a paper that was recently published in the journal ACS Applied Materials & Interfaces.
Source: American Chemical Society