Most solid materials are crystals; the atoms are arranged
in regular patterns, for example, stacked like cannonballs.
The structure of crystals has been carefully studied for quite a
long time, and is related to properties such as the strength of
materials, their conductivity, how they break, and how they form.|
Glass has no underlying regular structure. Instead,
the atoms or molecules making up a glass are jumbled together,
like the picture at left. They may be packed in so tightly
they cannot move, but they are not packed in a regular way.
Some materials naturally form glasses when they are cooled,
such as silica (SiO2) (the primary chemical component of
normal glass). What's weird is that structurally, glasses
are the same as liquids -- if you just look at a microscopic
snapshot of the position of the atoms, you can't tell the
difference (whereas it's really obvious that a crystal is
something different from a liquid). So why is a glass a glass,
and not a liquid? Or is it a liquid...?|
When a glass-forming liquid is cooled, its viscosity increases -- it
flows slower and slower. A simple definition of a glass is a liquid
with a viscosity that is 10,000,000,000,000 times larger than
the viscosity of water. This is somewhat arbitrary. In fact, a big
question is, do glasses actually flow, or are they completely solid
My best answer is that I believe glasses do not flow any more than any
other solid object flows, although perhaps even rocks and glasses
and crystals will flow in some fashion if we wait eons.|
A colloid is simply a fluid filled with lots of very small
solid particles; this includes black ink, blood (filled with
blood cells), and paint (filled with particles which stick to
surfaces when they dry). Typically these particles are very small,
between 1 nm and 1000 nm (one-millionth to one-thousandth
of a millimeter). We use particles that are 0.002 mm in diameter,
made from PMMA (the same material which is Plexiglas, if you have a
much larger hunk of it). These particles basically act like
marbles, that is, they ignore each other unless they bump into each
other -- they don't have electric charges on them, for example.|
Sometimes, our colloidal particles form crystalline arrays,
like the picture at left, or the hexagons shown above.
These are similar in many ways to regular atomic
crystals. In other cases, the colloidal particles pack close
together in a random way, and form a colloidal glass; this
is what is shown in some of the other pictures on this page.
While normal atomic solids are formed by cooling, colloidal
crystals and glasses are formed by cramming the particles
together, usually by centrifuging them.|
We use a
confocal microscope to take 3D pictures of our
samples, to see what the individual
particles are doing as they move around in a colloidal glass.
We can follow several thousand particles simultaneously, and
watch their motion for several hours (sometimes several days).
We look at how their motion changes when they are packed
closer together, as the sample becomes
a glass. Hopefully by understanding what occurs in a colloidal
glass system, we can learn general properties of all glasses.
The larger size of colloidal particles (as compared to atoms
in regular glasses) make them possible to see, and it also means
they move slower -- thus we don't need really fast electronics
or techniques to see what they're doing, unlike people who study
molecular glasses, who have to be extremely clever. (We make no such
We find that particles have to cooperate to move:
if one particle can move a little ways, then one of its
neighboring particles can move into the space left behind
by the first particle; and then perhaps a third particle can
follow the second particle, and so on. The more glass-like
the sample is, the more particles cooperate at the same time.
However, it takes longer and longer times before we see one of
these cooperative events. Thus, it is possible that when all
of the particles have to cooperate in order for any of them to
move, you have a glass. Perhaps the time between cooperative
rearrangements diverges, as well as the number of particles
needed to cooperate, and it is the divergence of these two
dyanamical quantities that distinguishes a glass from a liquid.
Our data isn't inconsistent with this hypothesis -- which is a
weasly scientist way of saying we have no idea if this really
happens, but it's an intriguing idea that could be possible,
so I mention it here on this web page. At least, the possibility
is one reason why I find this interesting.|
This is from the experimental data, showing cooperative
clusters of particles (the largest in this image is highlighted).
All of the particles are actually the same size (the size of
the large ones), I have just made the slower particles smaller
so that the fast ones stand out.|
This work was conducted at Harvard University, before I came to Emory University.
I also have two movies you can download, below. Each of them
is 10 MB in size. They are animated GIFs, so they should
display in any graphical browser without any additional
work or plugins.
The movies are similar to the picture above: fast particles are drawn big (to scale) and slow particles are drawn small (not to scale). The scale bar on the left indicates the time scale, as noted below. The scale bar on the right indicates the fraction of the sample that is "fast" according to our somewhat arbitrary definition that over time, on average 5% of the sample will be fast (thus this scale bar will fluctuate around 5%).
volume fraction = 56%
dt* = 1000 s
For a supercooled fluid, we have very large clusters of cooperative fast particles. This movie is from the sample that is closest to the glass transition, without actually being a glass.
volume fraction = 60%
dt* = 3000 s
For glasses, there are no large cooperative clusters. Since we're defining the fastest 5% of the particles as "fast", there is always some activity going on. However, it does not appear to involve many particles moving in a cooperative fashion as occurs for the liquid-like samples. Instead, there are small groups of particles which move slowly side-to-side, in somewhat of a cooperative fashion. This motion never results in large-scale rearrangements, unlike the supercooled fluids.
If you have questions or comments send me email:
An article in Science by C. A. Angell (see the references below) lists a few other intriguing facts about glasses. While glasses occur naturally (such as volcanic glass), it has recently been discovered that another natural glass is in comets. Water in comets exists in a glass state (unlike water here on Earth!). It has also been found that the glass transition is related to the suspension of insect life in a desert during a drought.