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A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S |
M A R C H
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As an alternative, NIST researchers
used electron backscatter diffraction
(EBSD) to read, in the crystal structure
pattern, imprints on steel that had been
removed by polishing. The more perfect
the crystal structure, the stronger and
clearer the pattern. Software can then
calculate the pattern quality to reveal
crystal damage; areas with more dam-
age produce lower quality patterns.
Ordinary SEM imaging meth-
ods reveal very faint outlines of the X
stamps in the metal grains, whereas
pattern quality mapping shows the out-
lines more clearly. The latter technique
is significantly more sensitive to small
amounts of crystal lattice damage.
While the method is still experimental,
it holds a lot of promise, say research-
ers.
CHARACTERIZING
SUPERHYDROPHOBICITY
Scientists recently discovered that
inherently water repellent surfaces can
be made superhydrophobic by pattern-
ing themwith micro or nanoscale struc-
tures. On such surfaces, water droplets
can be either suspended across neigh-
boring protrusions or impaled between
them. The transition between these
two states as well as the effect of mi-
crostructures on vapor condensation
have been investigated, but few com-
putational studies have explored how
droplets initially form by condensation
from vapor.
Now, Weiqing Ren of the A*STAR
Institute of High Performance Com-
puting, Singapore, and Yunzhi Li of the
National University of Singapore have
systematically analyzed how micropil-
lars on a hydrophobic surface affect the
condensation of water vapor. They used
a powerful computational technique
known as the string method, which Ren
developed in a previous study.
Ren and Li used this technique to
investigate the effect of parameters such
as the height and spacing of micropillars
and the supersaturation and intrinsic
wettability of the surface on the con-
densation process. They discovered that
both the pathway and configuration of
the initial nucleus from which droplets
form—the critical nucleus ―depend on
the geometry of the surface patterns. In
particular, they found that for tall, close-
ly spaced pillars on a surface with low
supersaturation and low wettability, the
critical nucleus prefers the suspended
state, whereas for the opposite case it
prefers the impaled state. By generat-
ing a phase diagram, critical values of
the geometrical parameters at which
the configuration of the critical nucleus
changes from the suspended state to the
impaled state can be determined.
.
Superhydrophobic surfaces hold promise
for industrial applications as well as
self-cleaning, defrosting, and anti-icing
surfaces. Courtesy of Yurok Aleksan-
drovich/iStock/Thinkstock.