CRYSTALLIZATION OF HARD-SPHERE COLLOIDS IN MICROGRAVITY

The structure of, and transitions between, liquids, crystals and glass es have commonly been studied with the hard-sphere model(1-5), in whic h the atoms are modelled as spheres that interact only through an infi nite repulsion on contact. Suspensions of uniform colloidal polymer pa rticles are good approximations to hard spheres(6-11), and so provide an experimental model system for investigating hard-sphere phases. The y display a crystallization transition driven by entropy alone. Becaus e the particles are much larger than atoms, and the crystals are weakl y bound, gravity plays a significant role in the formation and structu re of these colloidal crystals. Here we report the results of microgra vity experiments performed on the Space Shuttle Columbia to elucidate the effects of gravity on colloidal crystallization. Whereas in normal gravity colloidal crystals grown just above the volume fraction at me lting show a mixture of random stacking of hexagonally close-packed pl anes (r.h.c.p.) and face-centred cubic (f.c.c.) packing if allowed tim e to settle(7,8), those in microgravity exhibit the r.h.c.p. structure alone, suggesting that the f.c.c. component may be induced by gravity -induced stresses. We also see dendritic growth instabilities that are not evident in normal gravity, presumably because they are disrupted by shear-induced stresses as the crystals settle under gravity. Finall y, glassy samples at high volume fraction which fail to crystallize af ter more than a year on Earth crystallize fully in less than two weeks in microgravity. Clearly gravity masks or alters some of the intrinsi c aspects of colloidal crystallization.