Texas—Chemical engineers at The University of Texas
at Austin have discovered a new way to predict the
mobility of confined fluids at nanometer scales.
At these scales, often just a few molecules across,
fluids exhibit significantly different properties
than at the macroscopic level.
The ability to predict these changes has applications
in fields such as cell biology and geophysics, as
well as important implications for the design of
The research by graduate student Jeetain Mittal
and Dr. Thomas Truskett, assistant professor in the
Department of Chemical Engineering at The University
of Texas at Austin, along with Dr. Jeffrey Errington
of the University at Buffalo, SUNY, appears in the
May 5 issue of Physical Review Letters.
results will help engineers understand how a variety
of confined fluid systems function, from the performance
of new materials for chemical separation and environmental
remediation to transport processes in biological
membranes. The discovery also provides a way to
study the behavior of fluids in nanodevices, such
as miniaturized “lab on a chip” tools for biomedical
and analytical chemistry applications.
Confining fluids in very small, nanometer-scale
channels can affect how the molecules pack together,
how they withstand compression, and their ability
to rapidly mix or flow. Changes to the first two
properties are relatively well understood, but predicting
the third, which is connected to the mobility of
the molecules, has proven elusive.
“One of the most dramatic changes you see going
from macroscopic scales to nanometer scales is that
materials can actually change their state,” Truskett
said. “A solid may become liquid upon confinement.
If that solid material is a bonding agent and it
turns into a runny fluid, it doesn't do its job.
Likewise, a liquid can become a solid when confined
to small scales. If it is a lubricant, it fails.
So in the engineering of nanoscale devices, these
kinds of changes can have potentially catastrophic
In a bulk fluid, such as a glass of water, fluid
molecules interact primarily with other fluid molecules.
Relatively few are in contact with the surface of
the container. At nanometer scales, however, a much
higher proportion of molecules come in contact with
the confining material. This surface interaction
can significantly alter fluid properties, including
The key to successfully predicting changes to mobility
in a confined fluid, the researchers discovered,
is the relationship between mobility and excess entropy.
“One way to think about how mobility relates to
entropy is to think of entropy as measuring a sort
of randomness at the molecular level,” Truskett said. “In
a gas, where the molecules are randomly distributed,
entropy is high and the gas mixes readily. In a solid,
the molecules are aligned in a regular spatial pattern;
there is little randomness and the solid barely mixes
at all. Our discovery is that while both excess entropy
and mobility of a fluid are affected by confinement,
the relationship between the two quantities essentially
remains the same down to very small scales.”
Because scientists already have reliable methods
for predicting how confinement will affect excess
entropy, they can now use this information together
with the group's findings to predict how confinement
will affect fluid mobility.
The group performed computer simulations to study
the behavior of fluids in highly restrictive channels
with different shapes and boundary interactions.
They were able to successfully model changes to fluid
mobility and entropy in these conditions, a critical
breakthrough that will allow engineers to learn how
these changes occur while avoiding the difficult
task of gathering experimental data on such small
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