BINARY NUCLEATION KINETICS .3. TRANSIENT-BEHAVIOR AND TIME LAGS

Transient binary nucleation is more complex than unary because of the bidimensionality of the cluster formation kinetics. To investigate thi s problem qualitatively and quantitatively, we numerically solved the birth-death equations for vapor-to-liquid phase transitions. Our previ ous work [J. Chem. Phys 103, 1137 (1995)] showed that the customary sa ddle point and growth path approximations are almost always valid in s teady state gas phase nucleation and only fail if the nucleated soluti on phase is significantly nonideal. The current work demonstrates that in its early transient stages, binary nucleation rarely, if ever, occ urs via the saddle point. This affects net only the number of particle s forming but their composition and may be important for nucleation in glasses and other condensed mixtures for which time scales are very l ong. Before reaching the state of saddle point nucleation, most binary systems pass through a temporary stage in which the region of maximum flux extends over a ridge on the free energy surface. When ridge cros sing nucleation is the steady state solution, it thus arises quite nat urally as an arrested intermediate state that normally occurs in the d evelopment of saddle point nucleation. While the time dependent and st eady state distributions of the fluxes and concentrations for each bin ary system are strongly influenced by the gas composition and species impingement rates, the ratio of nonequilibrium to equilibrium concentr ations has a quasiuniversal behavior that is determined primarily by t he thermodynamic properties of the liquid mixture. To test our quantit ive understanding of the transient behavior, we directly calculated th e time lag for the saddle point flux and compared it with the availabl e analytical predictions. Although the analytical results overestimate the time lag by factors of 1.2-5, they should be adequate for purpose s of planning experiments. We also found that the behavior of the sadd le point time lag can indicate when steady state ridge crossing nuclea tion will occur. (C) 1996 American Institute of Physics.