Microscopic structure and elasticity of particle gels
Many materials consist of random aggregates of microscopic
particles, known as particle gels. How does the microscopic
arrangement of particles dictate the elastic properties of the gel as
a whole? With the confocal microscope, we can completely characterize
the architecture of the disordered gel and, in the same experiment,
measure its local elastic properties. We want to understand in detail
how these are related. The microscope is ideally suited to these
experiments because it reveals heterogeneities and topological
Gels that differ microscopically share many macroscopic properties,
such as fractal dimension, fractal correlation length and a remarkable scaling
of the frequency-dependent shear modulus. These experiments,
however, show that there are some intriguing differences at the
microscopic scale. In gels with a stronger interaction potential, the
'chains' that make up the gel become thinner and more tenuous.
Gel Structure in 3D
A 3D view of gels reveals that the gel is made of chains, not clumps.
We measure the topology, shape, and elasticity of these chains using
confocal microscopy. The figure at the top of this page is a
false-color image of a 20x18x10-micron section of the gel,
reconstructed using the measured particle positions. The color code
indicates the number of bonds per particle: red particles have 0 or 1
bond, green have 2, blue have 3, yellow have 4 and purple have 5 or
more bonds. All of the particles in this image are connected in a
single large cluster.
Dynamics of the Gel
The particles in the gel are not stationary; they move due to thermal
fluctuations (small kicks from the solvent molecules). By watching
these small fluctuations, we can measure the mechanical properties
(viscosity, elasticity) of the gel. We are especially interested in seeing how the elasticity scales with length, which will allow us to predict the bulk rheology from microscopic information.
I thank Eric Weeks for teaching me
how to make these movies using POV-Ray.