Left: Micrometer-scale structured polystyrene surfaces for use with micro-fluids. Right: Static wetting of this type of surface water – simulation and experiment. Image: Fraunhofer IVM |
It’s
raining cats and dogs and even the short run out to the car leaves your
vision obscured by rain on your spectacles. There might soon be no need
to reach for a cloth to wipe them off. If the surface of the lens
resembled that of a lotus leaf, the drops would all fall off by
themselves. The practicality of such self-cleaning surfaces is not
limited to eyewear. Corrosion coatings would put up a better fight
against rust without the tiny puddles of water that tend to collect on
top of them.
But
exactly what characterizes surfaces that do the best job of cleaning
themselves? Researchers at the Fraunhofer Institute for Mechanics of
Materials IWM in Freiburg have now developed simulation software that
provides the answers.
“Our
simulation shows how various liquids behave on different surfaces, no
matter if these are flat, curved or structured,” explains Dr. Adham
Hashibon, project manager at the IWM. The program simulates the form the
liquid droplets take on the surface, indicating whether the liquid
distributes itself over the surface, or contracts to form droplets in
order to minimize contact with the surface. The program is also able to
calculate the flow behavior in terms of how liquids move across
different surfaces, whereby the determinant factors at different scales
of measurement are integrated, from atomic interactions to the impact of
microscopic surface structure.
The
software analyzes what goes on within a given droplet—how the
individual water molecules interact with each other, how a droplet is
attracted by the surface and how it resists the air. Researchers refer
to a three-phase contact link between liquid, surface and air. “How
liquid behaves on a surface is influenced by a great deal of parameters,
including the surface characteristics of the material as well as its
structure, but also by substances dissolved in the liquid. We have taken
all this into account to different degrees of detail within the
simulation so that we are able to clearly reproduce our experimental
findings,” says Hashibon.
Improving microfluidic systems
The
simulation is also useful in medical examinations. When doctors have to
analyze tissue cells or parts of DNA, they often use microfluidic
systems such as constant-flow cuvettes. Liquid containing dissolved
substances is analyzed as it flows through tiny channels and minute
chambers, and it is essential that no liquid whatsoever remains after
the procedure has been completed. Any residual drops would then mix with
a new sample and distort findings. The simulation will now be used to
help optimize such microfluidic systems and to design surfaces so that
as little liquid as possible gets left behind.
“Our goal was to better understand and control the wetting behavior of liquids on structured surfaces,” says Hashibon.
But
that’s not all. This tool can also be used to implement a kind of
traffic management system within the microfluidic system. When a channel
splits into two, giving each fork a different surface structure makes
it possible to separate the various components of the liquid, sending
DNA molecules one way while other components are led along the
alternative route. This technique can be used to heighten the
concentration of certain molecules and is especially important, for
instance, in raising the detection sensitivity of analysis techniques.
