Imagine a dry compound that can conductively bind
components to a printed-circuit board without the
high heat associated with various soldering processes.
Or, how about an adhesive that never dries out in a
vacuum—a common problem in aerospace applications? For
the adventurous, imagine wearing a suit that would allow you
to easily scale walls and hang from ceilings like a superhero.
These fantasies may not be too far from reality as scientists
look to the gecko for answers. For some time now, researchers
have observed the ability of geckos to cling to surfaces, be they
rocks, trees, walls, and ceilings, vertically and horizontally, and
even upside down, with their toes. This ability is attributable to
microscopic hairs with an elastic quality in the gecko’s toes that
exert attractive forces on an atomic level (Fig. 1).
GECKOS AND NANOTUBES
In their attempts to replicate these adhesive hairs, scientists
have been experimenting with certain polymers and carbon
nanotubes. In October, researchers at the University of Dayton,
Georgia Institute of Technology, Air Force Research Laboratory,
and University of Akron unveiled the first carbon nanotube
material that not only apes the gecko’s anisotropic adhesive
properties, but does so with an adhesive ability about three
times better than previous models and with 10 times greater
resistance to perpendicular shear forces than the gecko.
Using the unique carbon nanotube array, these researchers
expect to develop artificial gecko feet that can grip vertical
surfaces while being easy to remove. According to Georgia
Tech Regents Professor Zhong Lin Wang, the unique material
relies on rationally designed, multiwalled carbon nanotubes that form arrays with entangled tops (Fig. 2). These
tops mimic the architecture of gecko feet and hairs of varying
diameters.
Pressing these nanotubes upon a surface, their tangled tops
align with the surface, and contact between them significantly
increases. This increase in contact of the tops with each other
causes an increase in van der Waals forces occurring on the
atomic level, which promotes adhesion.
“The contact surface area matters a lot. When you have line
contact along, you have van der Waals forces acting along the
entire length of the nanotubes. But when you have a point
contact, the van der Waals forces act only at the tip of the
nanotubes. That allows us to truly mimic what the gecko does
naturally,” Wang says. “When lifted off the surface in a direction
parallel to the main body of the nanotubes, only the tips
remain in contact, minimizing the attraction forces.”
Liangti Qu, a research assistant at the University of Dayton,
fabricated the vertically aligned, multiwalled nanotube arrays
via a low-pressure, chemical-vapor deposition process on a
silicon wafer. The first segments grew in random directions,
creating the coiled and entangled tops.
During lab tests, performed with a number of surface types
including glass, Teflon, polymer sheets, and sandpaper, the
researchers measured adhesive forces in the shear direction of
about 100 Newtons per square centimeter. Measurement in the
normal direction reveals an adhesive force approximately the
same as a gecko at 10 Newtons per square centimeter (Fig. 3).
In essence, resistance to shear force increases with the length
of the nanotubes while resistance to normal force appears to be
independent of nanotube length.
APPLICATIONS AND FUTURE RESEARCH
Since carbon nanotubes conduct both heat and electrical current,
a dry adhesive formed with the gecko-like nanotubes could
provide an efficient method for connecting electrical components.
“Thermal management is a real problem, and if you could use a
nanotube dry adhesive, you could simply apply the devices and
allow van der Waals forces to hold them together. That would
eliminate the heat required for soldering,” Wang says.
With eyes on the skies, the researchers also foresee adhesives
that will hold up for long periods in outer space. “In space, there
is a vacuum and traditional adhesives dry out. But nanotube dry
adhesives would not be bothered by space environments,” says
University of Dayton researcher Liming Dai.
A bit more work is necessary before we see these dry adhesives
on the market. The researchers need to analyze various surface
interactions to boost adhesive force as well as determine longterm
reliability.
“As surfaces may not be uniform, the adhesive force produced
by a larger patch may not increase linearly with the size. There is
much we need to learn about the contact between nanotubes and
different surfaces,” Dai says.