Chemical Technology March 2015

Self-cleaning surfaces have widespread applications

by Gavin Chait A surface that is hydrophobic has an

astonishing number of properties. Besides the obvious one of water pouring off, bacteria, fungi, algae and other pathogens cannot get a grip either. The difficulty in creating synthetic surfaces with similar properties is that they are damaged over time.

W hen I was a child, I discovered the simple de- light of the way water behaves in a nasturtium leaf. Gathering into round blobs and refusing to disperse. The trick was to see how big a blob of water you could create and how long you could control it for before the leaf itself collapsed and the water bounced off. As a kid, I imagined it had something to do with the fine ‘hairiness’ of the leaf surface. Turns out that this was some- what advanced thinking as a huge amount of research went into specific chemicals which could mimic such superhydro- phobic properties. The development of the scanning electron microscope permitted researchers to see the physical structure of the leaf. What has become called the Lotus Effect is widespread in na- ture. Besides nasturtium ( Tropaeolum ) and lotus ( Nelumbo ), it is also seen in insects such as the morpho butterfly. These surfaces are able to repel water to such an extent that dirt is shed too, resulting is self-cleaning. Sincewater can- not stick, they are immune to ice-formation. And, it turns out, these properties are a result of physical-chemical properties at the nanoscopic scale. The hydrophobicity of a surface is a property of the contact angle of water to that surface. If less than90 º , then the surface is hydrophilic. Beyond about 140 º , the surface is defined as superhydrophobic. The three stars listed above can reach a contact angle of 170 º meaning that the contact area is only 0,6 % of the overall surface area of the droplet. A surface that is hydrophobic has an astonishing number

of properties. Besides the obvious one of water pouring off, bacteria, fungi, algae and other pathogens cannot get a grip ei- ther. Dust doesn’t settle. This is not to say that purely chemical approaches to creating ‘non-stick’ surfaces don’t work. Polytet- rafluoroethylene (PTFE, aka Teflon) is a synthetic fluoropolymer which is highly hydrophobic, just not superhydrophobic. Even with a more limited repertoire, PTFE is used through- out the aerospace and computer industry as an insulator, as well as in bearings. Gore-Tex incorporates PTFE for waterproof- ing. Most non-stick frying pans are coated in PTFE. And the medical industry uses PTFE in everything frombypass stenotic arteries, todental fillings, and inwound-care toprevent chafing. The surfaces of superhydrophobic materials exhibit pat- terns and structures at multiple scales – not simply a regular and constant set of ridges or grooves. Such hierarchical structures canmanipulate thehydrophobic responseandeven reverse it, rendering the surface hydrophilic. Morpho wings or lotus leaves are extremely fragile but, be- ing organic, can also heal. The difficulty in creating synthetic surfaces with similar properties is that they are damaged over time. Abrasion caused simply by water-runoff can be sufficient to destroy surface efficacy. There are a number of different ways in which superhy- drophobic properties can be imparted on surfaces. The first is straight-out chemical bonding to create a new coating over the original surface. P2i, based in the UK, has developed a fluoropolymer coating systemwhich is now used by Samsung, amongst others, to waterproof mobile phones.

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Chemical Technology • March 2015