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Chemical Technology • March 2015
22
Self-cleaning surfaces have
widespread applications
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.
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.