New material could lead to water-proof surfaces

Scientists have created a new low-cost, non-toxic material that mimics the lotus leaf to repel water droplets, an advance that could help create environment-friendly water-proof surfaces.
The superhydrophobic nanomaterial can be applied to a variety of surfaces via spray- or spin-coating.
The hydrocarbon-based material may be a green replacement for costly, hazardous fluorocarbons commonly used for superhydrophobic applications, said Andrew Barron of Rice University in US, who led the research.
"Nature knows how to make these materials and stay environmentally friendly. Our job has been to figure out how and why, and to emulate that," Barron said.
The lotus leaf was very much on their minds as the researchers tried to mimic one of the most hydrophobic or water-repelling surfaces on the planet.

Barron said the leaf's abilities spring from its hierarchy of microscopic and nanoscale double structures.

"In the lotus leaf, these are due to papillae within the epidermis and epicuticular waxes on top," Barron said.
"In our material, there is a microstructure created by the agglomeration of alumina nanoparticles mimicking the papillae and the hyperbranched organic moieties simulating the effect of the epicuticular waxes," he said.
Fabrication and testing of what the researchers call a branched hydrocarbon low-surface energy material (LSEM) were carried out by lead author Shirin Alexander, from University of Swansea in UK.
Alexander coated easily synthesised aluminium oxide nanoparticles with modified carboxylic acids that feature highly branched hydrocarbon chains.

These spiky chains are the first line of defence against water, making the surface rough.
This roughness, a characteristic of hydrophobic materials, traps a layer of air and minimises contact between the surface and water droplets, which allows them to slide off.
To be superhydrophobic, a material has to have a water contact angle larger than 150 degrees. Contact angle is the angle at which the surface of the water meets the surface of the material. The greater the beading, the higher the angle.
An angle of 0 degrees is basically a puddle, while a maximum angle of 180 degrees defines a sphere just touching the surface.

LSEM, with an observed angle of about 155 degrees, is essentially equivalent to the best fluorocarbon-based superhydrophobic coatings, Barron said.
Even with varied coating techniques and curing temperatures, the material retained its qualities, the researchers said.
Potential applications include friction-reducing coatings for marine applications where there is international agreement in trying to keep water safe from such potentially dangerous additives as fluorocarbons, Barron said.
The study was published in the journal ACS Applied Materials and Interfaces.