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Chemical & Petroleum, Industrial, MEMS
PITTSBURGH (Nov. 12, 2019) — Carbon
nanotubes (CNTs) are valuable for a wide variety of applications. Made of
graphene sheets rolled into tubes 10,000 times smaller than a human hair, CNTs
have an exceptional strength-to-mass ratio and excellent thermal and electrical
properties. These features make them ideal for a range of applications,
including supercapacitors, interconnects, adhesives, particle trapping and
structural color. New research reveals even more
potential for CNTs: as a coating, they can both repel and hold water in place,
a useful property for applications like printing, spectroscopy, water
transport, or harvesting surfaces. When water is dropped on a CNT forest, the
CNTs repel the water, and it forms a sphere. However, when flipped over, the
drop does not fall to the ground but rather clings to the surface. “In contrast to superhydrophobic surfaces where droplets
roll off easily when tilted, CNTs forests are parahydrophobic, where the droplet
is both repelled and attracted to the CNT surface,” explains Ziyu Zhou, lead
author of the paper and graduate student in the LAMP Lab. “It is a love-hate relationship.” The key to this wetting behavior is the use of CNT forests
that are densely, vertically packed on the surface and the inherently hydrophilic
CNT surface. The forests are about 100 microns in height and so dense that
there are over 100 billion (1011) CNTs in 1 cm2 area. Some amount of water sinks below the carbon
nanotubes and clings to the hydrophilic material, while the rest is repelled
into a sphere. This research represents the first observation of
parahydrophobicity of CNT forests, where the droplet can roll along the surface
but does not fall off when turned upside down. Other surfaces in nature such as
peach fuzz or rose petals also exhibit this wetting behavior, which may be used
to for liquid transportation, fabrics coating design, membrane selectivity and
even wall-climbing robotics. This wetting behavior could also be used to as a way to construct
CNTs into various arrangements. “Previous research showed CNT forests to be unstable under
the application of water, but we show that water droplets are, in fact, stable
on these dense CNT forests,” explains Paul Leu, PhD, associate professor of
industrial engineering at the University of Pittsburgh’s Swanson School of
Engineering and author on the paper. “This wetting behavior may be used to
assemble CNTs into dense vertical arrays, surface stripes, and other unique shapes
that could be used for supercapacitors, interconnects, and other applications.” Leu also has appointments in chemical engineering and mechanical
engineering and material science. His lab, the Laboratory
for Advanced Materials at Pittsburgh (LAMP), conducted the research. The paper, “Parahydrophobicity and stick-slip wetting
dynamics of vertically aligned carbon nanotube forests,” (DOI: 10.1016/j.carbon.2019.06.012)
was published in the journal Carbon
and was coauthored by Ziyu Zhou, Tongchuan Gao, Sean McCarthy, Andrew Kozbial,
Susheng Tan, David Pekker, Lei Li, and Paul W. Leu.
Maggie Pavlick