Concentrated suspensions of particles with polymer present in solution behave like pastes. The polymer makes the colloidal particles weakly attractive, and the colloid-polymer mixtures form nonequilibrium jammed states as gels and glasses: soft, fragile solids that exhibit slow dynamics. We are investigating a system consisting of micron-sized colloidal particles in a viscoelastic background fluid. If the particles are of a different density than the suspending fluid, the disordered gel structure they form may later collapse under its own weight after some characteristic time. Gravity acts as an unbalanced stress on the system. We are interested in the underlying cause of the collapse, how the collapse is driven by gravity, and in the structure, dynamics, and mechanical properties of these fragile solids. To investigate these questions, we use a variety of experimental techniques.

Structure of colloidal pastes/gels Dynamics via sedimentation Rheology Structure from microscopy images The polymer makes the micron-sized colloidal particles weakly attractive via a depletion attraction, and the particles form a fragile, sample-spanning gel. Under gravity, these gels can undergo a dramatic collapse at some characteristic later time. The structure we observe by confocal microscopy is fairly generic across different colloid-polymer systems. Surprisingly, the height profiles of the particles over time as the delayed collapse occurs, for different particle and polymer concentrations, all scale together in time. Why should we expect this intriguing universal behavior in the dynamics of the delayed collapse? We are working hard to answer this question. The shear modulus of the paste increases when the colloidal silica particles are added, even at low volume fractions, suggesting that the particles form a disordered network. We are investigating how the elastic properties of the gels change when the suspending fluid is itself viscoelastic. From the 2D FFT of the images, we calculate the radially averaged intensity. For wavevectors corresponding to length scales inside the clusters, we can extract the fractal dimension. We find that the fractal dimension increases as the gels age, prior to the collapse.

Aging	Size matters
Not everything ages gracefully, and these gels exhibit structural aging. Above is a low resolution (10X) image of side length 0.5 mm, showing the structure of an aged gel that has developed local large, visible density variations. We are investigating how the structural aging is related to the delayed collapse.	We vary the size of the silica particles in the colloid-polymer mixtures to investigate the effects of size and polydispersity on the mechanical properties of the gel formed. For half micron particles the final height and volume fraction following the collapse depend sensitively on polymer concentration, while for 1 micron silica it does not. Click here for a simple explanation for why, for the behavior, particle size matters.	We see even more striking behavior when we make the silica gels using 100 nm particles. Above, a time series of images of a single sample shows a high-scattering region moving up through one of these colloid-polymer suspensions against the direction of sedimentation. This looks like a shock wave! We think it is actually related to phase behavior. Click on the image for a movie of these "shock waves".

	Gelation under gravity			Late stage gel under gravity
We ask: what is the role of gravity in determining the onset of gelation and the aging in the gels that form in the presence of gravitational stress? This system comprises PMMA latex particles in PS solution. We work hard to match refractive index of the solution to that of the colloidal particles, and control its density.		Here, an initially homogeneous mixture of weakly attractive particles sediments to the low density, rigid, disordered network that can support its own weight under gravity. Because we can locate the centers of all the individual particles, we can reconstruct the particle positions, and plot their density profiles as they evolve with time. Click on the images for movies of the density profiles as the gelation proceeds.			The same gel at later times. The structure is arrested due to the interparticle attractions and the gravitational stress on the system. Interestingly, the density seems to be constant through the gel as it "bootstraps" its structure from the bottom up.

Scan through a low density PMMA/PS gel at lower magnification, taken on the confocal mi croscope using a high resolution 60X lens. Click on the image for a real space stroll from 12 to 87 microns deep (BIG! 50Mb).