Images taken with a 3D microscope show wrinkled surfaces produced using a method developed by the MIT team. The size, spacing and angles of the wrinkles vary depending on how much the original underlying surface was stretched, and how the stretching was released. Images by Jorge Luis Yague and Felice Frankel |
The
wrinkles on a raisin result from a simple effect: As the pulp inside
dries, the skin grows stiff and buckles to accommodate its shrinking
size. Now, a team of researchers at MIT has discovered a way to harness
that same principle in a controlled and orderly way, creating wrinkled
surfaces with precise sizes and patterns.
This
basic method, they say, could be harnessed for a wide variety of useful
structures: microfluidic systems for biological research, sensing and
diagnostics; new photonic devices that can control light waves;
controllable adhesive surfaces; antireflective coatings; and antifouling
surfaces that prevent microbial buildup.
A
paper describing this new process, co-authored by MIT postdocs Jie Yin
and Jose Luis Yagüe, former student Damien Eggenspieler SM ’10, and
professors Mary Boyce and Karen Gleason, is being published in the
journal Advanced Materials.
The
process uses two layers of material. The bottom layer, or substrate, is
a silicon-based polymer that can be stretched, like canvas mounted on a
stretcher frame. Then, a second layer of polymeric material is
deposited through an initiated chemical vapor deposition (iCVD) process
in which the material is heated in a vacuum so that it vaporizes, and
then lands on the stretched surface and bonds tightly to it. Then—and
this is the key to the new process—the stretching is released first in
one direction, and then in the other, rather than all at once.
When
the tension is released all at once, the result is a jumbled, chaotic
pattern of wrinkles, like the surface of a raisin. But the controlled,
stepwise release system developed by the MIT team creates a perfectly
orderly herringbone pattern.
The
size and spacing of the herringbone ribs, it turns out, is determined
by exactly how much the underlying material was originally stretched in
each direction, the coating’s thickness, and in which order the two
directions are released. The MIT team has shown the ability to control
the exact size, periodic spacing and angles in both directions for the
first time.
The
system is unusual in its ability to produce precisely controlled
patterns without the need for masks or complex printing, molding or
scanning processes, Gleason says.
Controlling the patterns
Fundamentally,
“it’s the same process that gives you your fingerprints,” says Gleason,
the Alexander and I. Michael Kasser Professor of Chemical Engineering.
But in this case, precise control over the resulting patterns requires
the iCVD process, which Gleason and her colleagues have been developing
for years. This gives a high degree of control over the thickness of
layers deposited, and also enables control of the surface chemistry of
the coatings.
Additionally,
the iCVD method provides the high degree of adhesion that is needed to
form buckled patterns. Without sufficient adhesion, the surface layer
would simply separate from the substrate.
“One
distinguishing feature of what we’re showing is the ability to create
deterministic two-dimensional patterns of wrinkles,” such as a zigzag
herringbone pattern, says Boyce, the Ford Professor of Engineering and
head of MIT’s Department of Mechanical Engineering. “The deterministic
nature of these patterns is very powerful and yields principles for
designing desired surface topologies.”
“One
important application is the measurement of ultra-thin-film material
properties without knowing the thin-film thickness,” Yagüe says. The
film’s material stiffness and thickness could be measured by analyzing
the pattern, he says.
Many potential uses
Another
possible application, the researchers say, is microfluidic devices such
as those used to test for molecules in a biological sample, where tiny
channels of precise dimensions need to be produced on a surface. These
could potentially be used as sensors for contaminants, or as medical
diagnostic devices. Another possible use is in the control of
reflections or the wettability of a surface—making it attract or repel
water, properties that depend both on the surface shape and the chemical
composition of the material.
Such
patterns can also be used to make surfaces adhere to each other — and
in this case, the degree of adhesion can also be controlled. “You can
dynamically tune the patterning — direct stretching or other actuation
can be used to tune the pattern and corresponding properties actively
during use,” Boyce says, even letting surfaces return to perfectly flat.
This could, for example, be used to provide secure bonding with
quick-release capability or to actively alter reflectivity or
wettability.
Many
techniques have been used to create surfaces with such tiny patterns,
whose dimensions can range from nanometers (billionths of a meter) to
tens of micrometers (millionths of a meter). But most such methods
require complex fabrication processes, or can only be used for very tiny
areas.
The
new method is both very simple (consisting of just two or three steps)
and can be used to make patterned surfaces of larger sizes, the team
says. “You don’t need an external template” to create the pattern, says
Yin, the paper’s lead author.
The
predictability of the resulting patterns was a big surprise, members of
the team say. “One of the amazing things is to note how beautifully the
experiments and the simulation match,” Gleason says.
John
Hutchinson, a professor of engineering and of applied mechanics at
Harvard University who was not involved in this research, says,
“Wrinkling phenomena are highly nonlinear and answers to questions
concerning pattern formation have been slow to emerge.” He says the MIT
team’s work “is an important step forward in this active area of
research that bridges the chemical and mechanical engineering
communities. The advance rests on theoretical insights combined with
experimental demonstration and numerical simulation — it covers all the
bases.”
The work was funded by the King Fahd University of Petroleum and Minerals in Saudi Arabia.
