"Imagine you're a water molecule in a glass of ice water, and you're
floating right on the boundary of the ice and the water," proposes Emory
University physicist Eric Weeks. "So how do you know if you're a solid
or a liquid?"
 | | "Imagine you're a water molecule in a glass of ice water, and you're floating right on the boundary of the ice and the water," proposes Emory University physicist Eric Weeks. "So how do you know if you're a solid or a liquid?" Weeks' lab recently captured the first images of what's actually happening in this fuzzy area of the crystal/liquid interface. Credit: Eric Weeks Lab/Emory University |
Weeks' lab recently captured the first images of what's actually happening
in this fuzzy area of the crystal/liquid interface. The lab's data, published
this week in the Proceedings of the National Academy of Sciences (PNAS), make
the waves between the two states of matter visible for the first time.
"The theory that surface waves move along the crystal/liquid boundary
– the intrinsic interface – dates back to 1965 and is well established,"
says Weeks, associate professor of physics. "What we've done is found a
way to take a picture of the intrinsic interface, measure it, and show how it
fluctuates over time."
The visual evidence shows that the fuzzy region between the two states is extremely
narrow, Weeks says. "The transition from completely organized to completely
disorganized goes very quickly, spatially." To see the transition, and
hear Weeks explain the process, visit: http://esciencecommons.blogspot.com/2009/08/crystal-liquid-interface-visible-for.html
Modeling states of matter
Weeks' lab uses tiny plastic balls, each about the size of a cell nucleus,
to model states of matter. Samples of these colloids can be fine-tuned into
liquid or crystal states by changing the concentrations of the particles suspended
in a solution.
"Water molecules are too small too study while they are fluctuating,"
Weeks explains. "We used the plastic spheres to resize an experiment to
a scale that we could observe. You lose some of the detail when you do this,
but you hope it's not the critical detail."
The experiment took a great deal of trial and error, says Jessica Hernández-Guzmán,
a graduate student in physics and the lead author of the PNAS article. "I
was looking for that transition," she says. "I knew what the colloids
looked like in a crystal state, and I knew what they looked like as a liquid,
but I didn't know what they looked like in-between. When I finally saw (the
transition), I felt like I had won the lottery."
The samples of plastic spheres were confined in wedge-shaped glass slides and
loaded onto a confocal microscope turned sideways, so that gravity gradually
changed the concentration gradient. Rapid, three-dimensional digital scans were
made to record the Brownian motion of the particles over one hour. Algorithms
were applied to the images to classify the degree of organization of each of
the particles. The particles were then digitally colored: from dark blue for
the most crystalline, to dark red for the most liquid. The series of images
were stitched together and speeded up, becoming microscopy movies that reveal
the action along the crystal/liquid interface.
'The zone of confusion'
"You can watch as the boundary fluctuates," Weeks says. "The
yellow area along the bumpy line is liquid, but almost crystal. The light blue
area is crystal, but almost liquid. The zone of confusion is less than two particles
thick. By looking at the tiniest scale possible, we can see that the fuzzy region
between the two areas is much smaller than we previously thought."
The research was funded by the National Science Foundation Faculty Early Career
Development (CAREER) Program. Better understanding of the crystal/liquid interface
could have industrial applications, such as investigating the use of colloidal
crystals as optical switches, Weeks says.
Weeks is used to working in fuzzy territory. He has devoted most of his career
to probing the mysteries of substances that cannot be pinned down as a solid,
liquid or gas. Referred to as "soft condensed materials," they include
everyday substances such as toothpaste, peanut butter, shaving cream, plastic
and glass.
Emory University is known for its demanding academics, outstanding undergraduate
experience, highly ranked professional schools and state-of-the-art research
facilities. Perennially ranked as one of the country's top 20 national universities
by U.S. News & World Report, Emory encompasses nine academic divisions as
well as the Carlos Museum, The Carter Center, the Yerkes National Primate Research
Center and Emory Healthcare, Georgia's largest and most comprehensive health
care system.
Posted August 11th, 2009
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