This is a microscope image of a colloidal gel. Small, 0.5 micron size, particles have aggregated together to form this fascinating structure. It acts like a solid if you press on it lightly enough, but be careful! It will easily break apart just by shaking the sample cell.

Picture provided by by Phil Segre

These are pictures of the different phases that occur when colloidal particles are made sticky. The upper left is the base case with no stickiness, so there are just single particles undergoing Brownian motion. Continuing clockwise, the stickiness is ever increasing. Those are crystals in the upper right, a fluid of mobile clusters in the lower right, and a jammed solid in the lower left.

We're still trying to figure out why all these different possible structures form, as well as how they behave. Phil Segre, Vikram Prasad, and Tony Dinsmore are hard at work on this problem, so expect to read about all this soon in prestigious physics journals, if you don't see us "on tour" in the meantime.

Aggregate

Crystal

Crystal

Yogurt

Cheese