Rapid release of the applied pressure on a supercritical fluid solution allows the fluid to expand, its solvent strength to drop, and solute nucleation to occur, forming fine, airborne particles. This phenomenon was first observed more than 100 years ago by the scientists J. B. Hannay and J. Hogarth (1 ,2 ). They released the pressure on a supercritical ethanol solution of potassium iodide and observed the precipitation of solute into a “snow” or “frost” of fine particles. This remarkable phenomenon remained largely unstudied for a century after the original findings were reported (3 ). In 1984, Krukonis (4 ) described the precipitation of a wide variety of solute compounds by rapid expansion of supercritical fluid solutions. A subsequent review by Tom and Debenedetti (5 ) indicates that in recent years there has been increasing interest in supercritical fluid expansion processes for the formation of fine particles.
