Lotus leaf inspires new waterproofing material

A new class of water-repelling nanomaterials, inspired by the lotus leaf, could replace more expensive and environmentally-damaging materials used for waterproofing.

The water-repelling – or “superhydrophobic”– material has been developed by scientists at Swansea University, with colleagues at Rice University in Houston, Texas, the University of Bristol and the University of Nice Sophia Antipolis.

The researchers, led by Professor Andrew Barron of the Energy Safety Research Institute at Swansea University, reported their find in the American Chemical Society journal ACS Applied Materials and Interfaces.

Read the abstract of the paper

The hydrocarbon-based material may be a "green" replacement for costly, hazardous fluorocarbons commonly used for superhydrophobic applications.   One possible application is for waterproofing ships and other marine vessels.  

Professor Andrew Barron said:

"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."

The lotus leaf was very much on the team’s minds as they tried to mimic one of the most hydrophobic - water-repelling - surfaces on the planet.  

Watch:

A wipe and piece of cardboard treated with the new material, compared to untreated ones.

 
Professor 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.

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."

Waterproof nanomaterial

Picture:  a scanning electron microscope image of the new superhydrophobic material shows the rough surface of functionalized alumina nanoparticles.

Scientists at Swansea University and Rice University led the creation of the environmentally friendly material.

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

‌There, Alexander coated easily synthesized 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 minimizes 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.

Prof Andrew BarronThe Barron team's 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 reported.

Professor Barron (pictured) said:

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

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

Co-authors of the paper are Julian Eastoe, a professor of chemistry at the University of Bristol; Alex Lord, a researcher at the Swansea University Center for Nanohealth; and Frédéric Guittard, a professor at the University of Nice Sophia Antipolis.

Andrew Barron is the Charles W. Duncan Jr.–Welch Professor of Chemistry and a professor of materials science and nanoengineering at Rice and the Sêr Cymru Chair of Low Carbon Energy and Environment at Swansea University.

The Robert A. Welch Foundation and the Welsh Government Sêr Cymru Program supported the research.

College of Engineering, Swansea University

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