Remarkably, mesoscopic colloidal particles a tenth the size of a blood cell in water can spontaneously form large crystals whose regularity is immediately apparent to the senses. Their regular spacing causes light waves to interfere so that only specific colors are able to pass through. This selective passage is responsible for the iridescence seen in mother of pearl, butterfly wings, and opals. The brilliant colors give direct evidence that billions of particles have assembled themselves into a regular, repeating arrangement. This self-assembly mechanism has great potential for giving new properties to materials. It also gives a way to study how atoms sometimes form regular crystals and sometimes form disordered glasses instead.
While many theories for the behavior of glassy systems rest on assumptions about the microscopic interparticle interactions, virtually all experimental studies probe macroscopic behavior. Colloidal suspensions may help to bridge this gap by affording a direct view of the microscopic dynamics of the liquid-glass transition. Colloidal latex microspheres are commercially available in sizes large enough to observe with a microscope, yet small enough to undergo Brownian motion when suspended in water. The resulting equilibrium structures for dense monodisperse suspensions of interacting particles are all crystalline. Bidisperse suspensions of hard-sphere colloidal particles, on the other hand, have been shown qualitatively to form glasses.14 C. A. Murray (AT&T Bell Labs) similarly has demonstrated glass formation in bidisperse suspensions of charge-stabilized latex spheres. Although dynamic light scattering studies have demonstrated large-scale glassy dynamics in these suspensions, the potential to characterize directly individual particle motions has remained untapped.
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