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Scientists develop 'green' superhydrophobic coating

10 December 2015

An international group of scientists have developed a new class of superhydrophobic nanomaterials that match the best water repellent.

A scanning electron microscope image of the new superhydrophobic material shows the rough surface of functionalised alumina nanoparticles (courtesy of the University of Swansea)

The material, made by scientists at Rice University, the University of Swansea, the University of Bristol and the University of Nice Sophia Antipolis, is inexpensive, non-toxic and 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.

The research was led by Rice chemist, Andrew Barron, whose attention was focused on the hydrophobic properties of the lotus leaf, which 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," he 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."

Fabrication and testing of what the researchers call a branched hydrocarbon low-surface energy material (LSEM) were carried out by Shirin Alexander, a research officer at the Energy Safety Research Institute at the Swansea University Bay Campus.

There, 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.

An environmentally friendly superhydrophobic coating repels water as effectively as commercial coatings that employ hazardous materials (diagram: Shirin Alexander/University of Swansea)

To be superhydrophobic, a material has to have a water contact angle (the angle at which the surface of the water meets the surface of the material) larger than 150 degrees. 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.

Barron's team's LSEM, with an observed angle of about 155 degrees, is essentially equivalent to the best fluorocarbon-based superhydrophobic coatings. Even with varied coating techniques and curing temperatures, the material retained its qualities, the researchers reported.

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. "The textured surfaces of other superhydrophobic coatings are often damaged and thus reduce the hydrophobic nature," he said. "Our material has a more random hierarchical structure that can sustain damage and maintain its effects."

The team is now working to improve the material's adhesion to various substrates, as well as looking at large-scale application to surfaces.

An article describing this work is published in the American Chemical Society journal ACS Applied Materials and Interfaces.

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