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Researchers
Discover No-slip Condition Does Not Hold at the Nanoscale
New Surface Benefits Microfluidic Applications and Cell Studies
Professor Ben Wu, Professor CJ Kim,
Chang-Hwan Choi, and Professor James Dunn |
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UCLA engineers working on the development of a new ultra-slippery
nano-engineered surface have challenged a long-held concept in fluid
dynamics-the no-slip condition. Mechanical and aerospace engineering
professor CJ Kim and graduate student Chang-Hwan Choi have proven
that their nano-architectured surface in effect defeats the fundamental
notion of no-slip by a considerable margin, even in practical flow
conditions.
The no-slip condition states that fluids stick to surfaces past
which they flow, and there is no movement where a fluid touches
the surface of a solid. Most challenges to this condition thus far
have come from scientific interests because the amount of measurable
slip has been too small to be useful. The advent of micro and nano
technologies, however, has refocused attention on slip flows and
the need to measure slip accurately because microfluidic applications
can be affected by even a relatively small slip.
Since the amount of drag reduction caused by the internal slip surface
of a pipe is determined by pipe size and flow conditions as well
as the surface itself, a rather complex scientific value called
slip length should be used to objectively describe the slip as a
pure surface property, according to Kim.
Until recently, most of the reported slip lengths were less than
one micrometer and prone to measurement errors. Kim and Choi expected
to measure tens of micrometers of slip length on their new surface,
and so considered a slip of less than one micrometer as no slip.
"We started with the no-slip assumption on a flat surface in testing
our slip length," said Choi, "and in most instances it remains true."
Consider, for instance, water droplets moving along a glass surface
and along a Teflon surface. Compared to the relatively sticky (i.e.,
hydrophilic) glass surface, water beads and moves more easily on
non-stick (i.e., hydrophobic) surface, such as Teflon. Droplets,
which move mostly by a rolling motion, are unaffected by surface
slip, although they move more easily along a more hydrophobic surface.
The primary question, however, is the movement of liquid in continuous
flows, where it must slip on a surface to flow more easily. To determine
if surface wettability would make a difference to continuous flow
in microchannels, Choi measured the slip length on a planar hydrophobic
surface while at Brown University, and found it to be about 20-30
nanometers, or thousands of times smaller than the width of a human
hair.
The nano-engineered material Kim and Choi have created at UCLA has
a dense forest of sharply tipped nanoposts, which greatly limits
contact between a liquid and the surface of the solid. The height
of the posts, their shape, and the large number in a small space
combine to create a thick layer of air beneath the liquid and to
keep it from filling the gaps between the posts.
"We're using surface tension to keep the liquid out of the gaps,
and in most practical flow conditions (e.g., pressurized flows)
those gaps need to be very, very small," explained Kim. "So we've
created a surface with a high density of sharp-tipped posts - submicron
density - and then treated them to be hydrophobic."
At the suggestion of their colleague, UCLA mechanical and aerospace
engineering professor Pirouz Kavehpour, Choi used a rheometer-a
commercial tool used to measure viscosity-to track slip length along
their surfaces. Although reliable and accurate, the rheometer lacks
the precision to measure conventional miniscule slip lengths. But
it may work for the very large slip Kim and Choi have on the nano-architectured
surface.
"The rheometer gave us repeatable results, very quickly," said Kim.
"And it showed that the nano-engineered surface had a 20-30 micrometer
slip length, a thousand times larger than on a conventional hydrophobic
surface. We were expecting the results in this range based on our
analysis and others', but were still surprised and very pleased
to see it validated in testing."
When the UCLA Henry Samueli School of Engineering and Applied Science
researchers published their results in Physical Review Letters
earlier this year, they received considerable response from the
physics community.
"Fluid dynamics is a classical field, and while our results do not
change a long-held belief about the behavior of moving liquids where
they touch solids, we have worked around the assumption by creating
a surface with a minimal liquid-solid contact," noted Kim. "The
slip length along the new surface is far more than what was previously
assumed possible for flows under pressure. This degree of slip is
now large enough to be useful for engineering applications and not
just limited to the microscale."
In addition to developing a low-friction surface for use in fluidic
applications such as underwater vehicles and tools for DNA analysis
and real-time, on-site testing and monitoring for early detection
of hazardous materials, UCLA researchers are exploring new uses
for the innovative surface.
Kim and Choi are working with bioengineering professors Ben Wu and
James Dunn on the fabrication of new surfaces for cell growth.
"We know cells grow well under certain conditions, but at the nanoscale
most of the changes to date have been in the chemical conditions;
little attention has been paid to the physical conditions," said
Kim. "We're approaching it from a new direction and fabricating
different surfaces. We're able to make the surface as elaborate
as needed, which is basically a new capability at the nanoscale."
In addition to addressing basic scientific questions about the physical
manipulation of cell growth at the nanoscale, Dunn and Wu hope to
use the process for advances in medicine.
"There are many potential applications for this work," explained
Dunn, "one is tissue engineering. If we're able to change the cells'
orientation using the nano-textured surface, we can make the cells
line up in a particular way to form the shape and structure of the
tissues that we need."
Added Wu, "We are currently investigating the molecular basis of
the cells' interactions on different nanostructures. If we are successful,
we can use this knowledge to control the surfaces to regulate cell
behavior. Our research in this area is really just the tip of the
iceberg."
To create the well-regulated nano-engineered surfaces, Kim and Choi
use interference lithography to etch the pattern on a silicon substrate,
followed by deep reactive ion etching. To make sharp tips on the
posts, they heat the silicon, creating silicon oxide, which is then
removed.
The current method of manufacturing is practical for small area
applications, but the UCLA researchers are exploring polymer as
an alternative material to decrease costs for large volume area
applications, as on the surface of a torpedo. They also are exploring
applications for the silicon material in field emission displays
and tips for atomic force microscopes.
For more information on research in Kim's lab, please visit http://cjmems.seas.ucla.edu/.
- Marlys Amundson
Photos: Don Liebig, UCLA Photography
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