Physics of Attractive Colloids

Images: (click to enlarge)
Spinodal decomposition arrests to form a gel
Arrested colloidal gel in coexistence with gas
Colloidal fluid

Colloids are microscopic particles so small that they move diffusively when dispersed in a fluid, exhibiting Brownian motion, controlled by the temperature of the system, like atoms. However, unlike atoms, colloids are big enough to see with light, so they can be probed with microscopes and laser light scattering. The interactions between atoms are fixed, dictated by quantum mechanics, but those in colloids can be very finely tuned. This makes them an ideal model system for investigating the structure and properties of all sorts of materials, constructed from colloidal particle building blocks, such as crystals, glasses, gels, fluids and gases.

In the lab, we create and finely control attractions between particles by using special polymers, and thus can create a number of different phases: equilibrium fluids, kinetically-arrested gels, and intermediate combinations. We image the particles in 3D dimensions over time, using a special confocal microscope that allows us to locate each particle individually. How these particle configurations change over time gives insight into the physics of the system.

So far, we have found that colloidal gelation is always driven by liquid-gas spinodal decomposition — a thermodynamic instability leading to equilibrium liquid-gas phase separation — for short-ranged attractive particles (see paper in Nature, and accompanying movies in the player). We have also found that, under some circumstances, clusters of colloidal particles remain stable for our particles which are purely attractive. We also investigate phase separation itself near the critical point, where the long evolution of samples requires conducting experiments on orbit, aboard the International Space Station.

Peter J. Lu  |  Harvard University  |  Cambridge, MA 02138 USA |  plu_at_fas.harvard.edu