Cutting-edge chemistry
See it
work�
Watch beads of solution
roll around and observe
how it resists attack from
acid and base:
http://bit.ly/15p7z5S
© american chemical society
The super
material
consists of a
stainless steel
mesh coated
with a layer of
polymer beads
8 | The Mole | March 2013
A material that is equally good at repelling water, oil,
concentrated acid and alkali solutions, and non-Newtonian
fluids like polymer solutions has been created by chemists
in the US. This chemical resistance combined with the
simple, scalable production process makes it promising for
protective and self-cleaning surface applications.
© american chemical society
See droplets of liquid and
jets of fluid bounce off the
superomniphobic surface
in this YouTube video.
Super surface repels non-Newtonian
fluids
Anish Tuteja from the University of Michigan in Ann Arbor
explains that while a lot of effort has been directed towards
creating ‘self-cleaning’ superomniphobic surfaces that
repel both oily and water-based liquids, less attention has
been paid to non-Newtonian fluids.
Honey, custard and polymers
Viscous substances like custard, honey and solutions
containing polymers change the way they flow depending
on the forces applied to them. They can also absorb a lot
more energy by deforming when they hit a surface. The
deformation and flow of matter is known as ‘rheology’.
Adding 0.2 wt% of a polymer to water can make a droplet
stick to a surface where pure water droplets would bounce
off, Tuteja explains. ‘But in this case [...] we can still get
droplets or jets of these solutions to bounce off’.
‘Normally, when people talk about superhydrophobic or
superomniphobic surfaces, they talk about wetting, which
is a measure of the shape that droplets make on the
surface and their contact angles,’ says Sergiy Minko, who
researches smart polymer materials at Clarkson University
in Potsdam, US.
less impact,’ he adds, which is why the surface can repel
Newtonian and non-Newtonian fluids equally well.
The surfaces are also highly resistant to chemical attack.
Tuteja and his group covered one side of aluminium
plates with their material and dunked them into baths of
The crucial aspect of Tuteja’s surface, Minko says, is that it
concentrated hydrochloric acid and sodium hydroxide. The
has a very low wetting hysteresis, which means that as a
droplet rolls over the surface, the contact angles at the front uncoated sides of the aluminium were quickly attacked, but
and rear of the droplet are almost the same, so the droplet the coated surfaces were completely protected.
does not deform very much. ‘This means the rheology has
Pockets of air
All of these properties stem from two aspects of the
material’s construction, Tuteja explains. The material is
based on a fine stainless steel wire mesh. This is coated
with a layer of polymer beads, made from a mixture of
polydimethylsiloxane (PDMS) and fluorodecyl polyhedral
oligomeric silsesquioxane (POSS). The roughly spherical
shape of the beads gives the surface the geometry required
to make it superomniphobic and also means it traps a layer
of tiny pockets of air, which prevents the acid or base from
coming into contact with the surface, so it can’t react. The
fluorinated POSS molecules also migrate to the surface of
the beads, lowering the surface energy and enhancing the
chemical resistance. Phillip Broadwith
www.rsc.org/TheMole
Platinum plating at the flick of
a switch
Atom thick catalytic layers of platinum can be
deposited on surfaces from solution rapidly and
cheaply thanks to a new technique developed by
scientists in the US.
atoms are deposited first at steps or defects on the
surface. ‘But platinum is happier growing on platinum
than it is on gold,’ says Moffat, so you end up with islands
that eventually join up into relatively thick, lumpy layers.
Platinum films are used as catalysts in devices such
as fuel cells, as well as in microelectronics and various
other applications. Because of the rising price of
platinum and the interesting properties of very thin
films, it is desirable to make these films as thin as
possible, explains Thomas Moffat from the National
Institute of Standards and Technology in Gaithersburg,
who led the project.
Reversing the polarity
Abandoning tradition
Atomically thin films of platinum can already be made,
Moffat acknowledges, but these techniques involve
expensive high vacuum chambers and each layer forms
quite slowly. ‘We’re essentially using beaker chemistry
and can lay down a monolayer in under a second,’
Moffat says, ‘so from an engineering perspective it’s
much simpler.’
As part of their investigations, Moffat’s team tried
cranking up the voltage. ‘We went right to the threshold
of when you start reducing the protons in solution into
hydrogen gas, which is not normally what you’d want to
do,’ he says. Instead of getting a very thick film of metal
laid down, they got a single atomic layer of platinum,
capped with a layer of hydrogen atoms. Switching briefly
to a positive potential oxidises the hydrogen atoms off
the surface, leaving the platinum ready to add another
layer if needed.
This, says Jay Switzer from Missouri University of
Science and Technology in Rolla, US, is the biggest
advantage of the technique. ‘In one beaker, just by
pulsing the potential back and forth, you can put down
one monolayer at a time. So if you wanted to look at
properties of a material as a function of thickness, you
But to get a smooth, thin layer of platinum atoms on
can grow anywhere between one and several layers
their surface, the team had to abandon traditional
electrodeposition techniques. Normally, Moffat explains, fairly easily.’ This kind of investigation will be particularly
deposition is done very slowly. The item is immersed in a useful for people working on catalysts and magnetic
bath containing a platinum salt and a very small potential materials, where the properties of thin film materials can
vary considerably. Phillip Broadwith
is applied. The platinum complex is reduced and metal
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© science/aaas
Scientists can
build a platinum
coating one
layer at a time
by pulsing
the electrode
potentials
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March 2013 | The Mole | 9