Ariz. – A team of researchers at Arizona State University
has demonstrated the ability to move water molecules
by light -- a phenomenon they believe could have widespread
use in analytical chemistry and possibly pharmaceutical
research. The discovery could have an important effect
on the fledgling field of microfluidics, said Tony
Garcia, an associate professor in the Harrington Department
The use of an ordinary beam of light to move water
around without the need for potentially damaging electric
fields, air bubbles (which can denature proteins),
or moving microscopic mechanical pump parts (which
are expensive to make and difficult to repair) could
significantly aid development of microfluidic devices,
which are themselves tiny, sophisticated devices that
can analyze samples.
"This discovery can speed the development of
microfluidic devices," Garcia said. "These
devices could require only one drop of blood for a
battery of 20 to 30 tests, with results provided in
the time spent waiting to consult with the physician,"
Garcia explained. "They also could help pharmaceutical
companies screen for a new drug by allowing for tests
to be run on an extremely small scale and in simultaneous
The ASU researchers discovered an amplification effect
of the surface change in water contact angles through
nanotechnology. Details of their work will appear
in a paper titled "Lotus Effect Amplifies Light-Induced
Contact Angle Switching," in the Journal of Physical
Chemistry. It is now available from the Journal's
online ASAP service (go to http://pubs.acs.org/journals/jpcbfk/
and click on articles ASAP).
In addition to Garcia, the team includes Devens Gust,
professor of chemistry and biochemistry; Tom Picraux,
professor of chemical and materials engineering; Mark
Hayes, associate professor of chemistry and biochemistry;
Rohit Rosario, a postdoctoral researcher in the Harrington
Department of Bioengineering; Jennifer Taraci, a postdoctoral
researcher in chemical and materials engineering;
and graduate students Teresa Clement and Jeff Dailey
working in Picraux's group.
In nanotechnology, devices are designed from the molecular
level up. As the overall size of these devices shrink,
the nature of the surface plays an increasingly important
role because a greater percentage of the molecules
in a nanotech device reside on the surface. The ability
to manipulate surface molecules using everyday means,
such as shining a light or connecting to a battery,
becomes very important because ordinary tools like
pumps and valves are hard to make on a nano scale.
The ASU team theorized and then proved that a change
in water wettability – the ability of the water molecules
to easily move across a surface – when induced by
light can be significantly amplified through a combination
of very high nanoscale roughness and chemically coating
the surface with molecules.
"What we found was the 'sweet spot' in surface
roughness where the amplification effect was the greatest,"
Garcia said. "Our theory showed where the sweet
spot would be, meaning the optimal roughness of the
surface, and then we proved it."
"We have been working on the problem of using
light to move microscopic amounts of water around
for drug delivery and microanalysis applications,"
said Tom Picraux. "However, we were stymied by
the vexing problem of the combined small effect created
and the high degree of attraction that water retains
on even a very waxy, or hydrophobic, flat surface.
"Our advance came when we realized that if the
surface was roughened at the nanoscale, not only would
we obtain the 'lotus leaf effect,' but we could also
magnify the small change in water repelling controlled
by light to a level that can overcome the hysteresis,
or the attraction, that causes water to stick even
when a drop is pushed along, " Picraux said.
"Rohit Rosario mathematically derived the theory
for surface change amplification and proved it in
The lotus leaf effect is a fairly well known phenomenon
that combines the microscopically rough surface of
the plant's leaves with a waxy chemical coating and
leads to high water repellency and self-cleaning of
the surface. It is already employed commercially in
stain repelling pants.
What appears to aid this effect is tiny 'nanowires'
on the surface of a material, the ASU researchers
said. Nanowires are small, high-aspect-ratio wire-like
structures composed of semiconducting and other materials.
Typical wire dimensions are tens to hundreds of nanometers
in diameter and micrometers in length.
"We have used our expertise in nanowire growth
to influence a new physical property for nanowire
surfaces, namely the behavior and motion of fluids,"
Picraux said. "While nanowires give exquisite
control over the surface for creating extremely rough
surfaces, we point out there are many practical ways
to nanostructure the surface once the basic principles
of surface amplification of switching are understood."
The ASU team now is working to design a device that
can move drugs dissolved in water, or droplets of
water and samples that need to be tested for environmental
or biochemical analyses.
Another potential application is reducing the amount
of proteins or enzymes needed for testing during drug
development. Usually, making and purifying these candidate
drugs is time-consuming and small amounts are made
at a time.
In a microfluidic device, the cells, DNA, or proteins
that are used to test the candidate drug efficacy
also are reduced so that a small amount of candidate
drug can be mixed with its target and the result recorded.
This reduces the time needed to screen all of the
drug candidates and allows as many tests as possible
to be run simultaneously.
"The payoff of this scientific collaboration
is the first demonstrated ability to use a beam of
light to move microdroplets of water around on surfaces,
in extremely small channels or place them in predetermined
positions for analysis," Garcia said. "Other
nanotechnology researchers can follow our lead and
look at ways of magnifying the triggering of surface
changes through electric fields or through solution
conditions such as temperature or acidity."
Tony Garcia, (480) 965-8798
Tom Picraux, (480) 965-5252
Contact: Skip Derra
Arizona State University