ABSTRACT
When a supercritical solution that contains a dissolved solute is expanded across a microorifice, the solvent density decreases dramatically and the solute is rejected from solution. Petersen et al. (1) were the first to call this process the rapid expansion of supercritical solutions, or RESS for short. Because the characteristic speed of the expansion is the speed of sound, the process is quite rapid, with residence times in the orifice on the order of 1 µs. The rapid pressure reduction across the expansion nozzle leads to both uniform conditions and very high supersaturation ratios in the postexpansion environment. These two characteristics are a key feature of RESS and favor the formation of small particles with narrow size distributions (2). When materials such as polymers are used, other product morphologies are possible. For example, RESS solutions can be sprayed to form thin films (3). In other cases, the very high extensional rates in the expansion nozzle can be used to make microfibers (4-6).
