Mechanism governing microparticle morphology during precipitation by a compressed antisolvent: Atomization vs nucleation and growth

This study provides insight into the mechanisms that govern morphology in m icroparticles processed using precipitation by a compressed antisolvent. We explore the time scale of surface tension evolution in jets of miscible fl uids injected into critical and near-critical solvents to determine whether the jets atomize into droplets or simply evolve as gaseous plumes. Classic al jet breakup length equations, modified with time-dependent surface tensi on, accurately predict observed breakup lengths over a range of liquid misc ibilities. Linear jet breakup theory can be applied successfully to near cr itical conditions. The aerodynamic reduction factor remains constant over a wide range of pressures. However, under miscible conditions, calculations show that surface tension in a 10-cm/s round jet of methylene chloride in c arbon dioxide at 8.5 MPa and 35 degrees C approaches 0.01 mN/m in I mu m. B ecause this distance is shorter than characteristic breakup lengths, distin ct droplets never form. Rather, the jets spread in a fashion characteristic of gaseous jets, whose mixing is well described by the gaseous fluid mixin g theory. Presumably, microparticle formation results from gas phase nuclea tion and growth within the expanding plume, rather than nucleation within d iscrete Liquid droplets.