Formation of argon clusters by homogeneous nucleation in supersonic shock tube flow

The formation of argon clusters by homogeneous nucleation of pure argon diluted in helium was investigated in the supersonic outflow of a shock tube, where the clusters were detected by laser light scattering from the particles in supersaturated states. The thermodynamic states for the onset of homogeneous nucleation varied between 48 and 85 K with corresponding argon partial pressures in the range 2–850 Torr. Values of adiabatic supercooling with respect to the argon phase equilibrium ranged between 3 and 10 K. The experiments are in agreement with previous investigations in subsonic shock tube and supersonic Ludwieg tube expansions but disagree with previous investigations in steady supersonic nozzle expansions. It is possible to explain qualitatively the observed differences in argon nucleation behavior on the basis of a thermodynamic and kinetic concept independent of the basic assumptions of existing nucleation theories.