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Vital Statistics:
In the 1800s, new golf balls sported smooth surfaces, but became dimpled
over time as impacts left permanent dents. New balls were typically used for
tournament play, but in one match, legend has it that a player ran short,
had to use a dented one, and realized that he could drive the dimpled ball
much further than a smooth one. Years of testing prove that a golf ball’s
irregular surface dramatically increases the distance it travels because it
cuts drag-induced air resistance in half.
Researchers at Massachusetts Institute of Technology, Cambridge, are
harnessing the same idea to reduce drag on other surfaces. Aerodynamics studies
show that while a ball with a dimpled surface has half the drag of a smooth one at lower
speeds, this advantage reverses at higher speeds. So, the ideal would be a surface whose
smoothness can be altered in real-time.
Success Factors:
Pedro Reis and his team created a hollow ball of soft material covered with a stiff skin and then extracted
air from the interior to make the ball shrink and its surface wrinkle. At a certain degree of shrinkage, the surface produces
a dimpled pattern similar to that of a golf ball and with the same aerodynamic properties. Balls with smooth surfacesmight be expected to sail through air more easily than ones with irregu-
lar surfaces. Yet the reason for the opposite re-
sult involves a small layer of air next to the
surface: The irregular pattern holds air-
flow close to the ball’s surface longer, de-
laying the separation of this boundary
layer. This reduces the wake size, the
primary cause of drag for blunt objects.
Because surface texture can be con-
trolled by adjusting interior pressure,
the degree of drag reduction can be
controlled at will. Due to the variabil-
ity, the team named these
smart
morphable surfaces or smorphs.
About the Innovators:
Pedro Reis is an assistant pro-
fessor of mechanical engineering
at MIT. His team includes former
MIT postdocs Denis Terwagne
and Miha Brojan.
What’s Next:
Drag reduction of textured surfaces has already moved beyond golf balls: The
soccer ball from this year’s FIFA World Cup used a similar effect, as do some
track suits worn by elite athletes. For these purposes, constant dimpling is ade-
quate. But in other cases, the ability to alter surfaces on the fly could prove use-
ful. For example, many radar antennas are housed in spherical domes, which
can collapse catastrophically in high winds. A dome that could adjust its surface to
reduce drag in strong winds might avert such failures. Another application is automobile exteriors,
where the ability to adjust panels to minimize drag at different speeds could increase fuel efficiency.
Contact Details:
Pedro Reis • Elasticity,
oratory
Department of Mechanical Engineering, Massachusetts Institute of Technology617.324.3325,
mit.edu
77 Massachusetts Ave, Cambridge, MA 02139
Comprised of a soft polymer with a hollow
center and a thin coating of a stiffer
polymer, the sphere becomes dimpled when
air is pumped out, causing it to shrink.
ADVANCED MATERIALS & PROCESSES • SEPTEMBER 2014
Smart Morphable Surfaces
Specimen
Name:
SucceSS AnAlySiS
The smorphs/Smurfs pun is intentional.
Denis Terwagne, a Belgian comics fan,
points out that one characteristic of
Smurfs is that no matter how old they get,
they never develop wrinkles. Courtesy of
Pere prlpz.
In the lab, the morphable
surface can have its sur-
face texture changed at
will by adjusting the pres-
sure inside. When interior
pressure is reduced, the
flexible material shrinks,
but the stiffer outer layer
wrinkles as it shrinks,
like a plum becoming a
prune. Unlike a prune, the
material can bounce back
to a smooth state with in-
creased pressure.