Although the idea that entropy alone is sufficient to produce an ordered state is an old one in colloidal science, the notion remains counter-intuitive and it is often assumed that attractive interactions are necessary to generate phases with long-range order. We study colloidal suspensions of rodlike viruses and spherical colloids under conditions approximating hard particle fluids, which are impenetrable objects with no other interaction potential. These simple, hard particle fluids exhibit unexpectedly rich phase behavior. Through experiment, simulation, and theory we explore how phase behavior is controlled solely by the shape and concentration of the particles. Because all molecules have a hard particle core, the phenomena are likely to be quite general, applying also for example to low-molecular mass liquid crystals, amphiphiles and block copolymers, to bioseparation methods and DNA partitioning in prokaryotes, and to protein crystallization, and the manufacture of composite materials.

Updated 5 September 1999.