University of Virginia MRSEC Irving P. Hull DMR-0080016 Nanoscopic Materials

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University of Virginia MRSEC Irving P. Hull
DMR-0080016 Nanoscopic Materials
UVa MRSEC Researchers C. Thomas Schamp and
William A. Jesser have shown that the morphology
of nanoparticles requires additional
considerations beyond the usual equilibrium
contact angle notions so useful for bulk systems.
The balance between surface energies of bulk
phases yields an equilibrium contact angle
between a liquid and a solid known as Youngs
contact angle. However, in nanoparticles of
liquid gallium contacting crystalline gallium
arsenide produced by pulsed laser deposition from
a gallium arsenide target, additional factors
determine the morphological equilibrium. It was
found that the liquid meniscus always was pinned
at the widest part of the solid thereby creating
a shape of a nano-ice cream cone. The contact
angle between the solid and liquid is no longer
given by Youngs notions but now depends on the
amount of liquid present. By comparing the
contact angles in the nanoparticle with the
Youngs contact angle the researchers have
developed equations to describe when the liquid
would spill over a corner of the solid and when
it would be pinned to a corner. These ideas,
when considering the microscopic roughness of a
surface, can explain why it is commonly observed
in bulk systems that advancing contact angles are
greater than receding contact angles, and have
broad implications for understanding liquid-solid
interfaces at the nanoscale.
Transmission electron micrograph of a GaAs nano
ice-cream cone Energy dispersive X-Ray spectra
show that the sphere is liquid Ga and the cone
is solid GaAs. Inset at upper right is a low
magnification image showing a typical field of
ice-cream-cone particles. Insets at lower right
are the enlarged region of the box highlighted on
the cone showing lattice planes and a digital
diffractogram.