Nanoscale heterogeneous phase separation kinetics in binary mixtures: Multistage dynamics

In order to find a measure of the dynamical features of phase separation kinetics during spinodal decomposition of a liquid binary mixture (like water and cyclohexane , water and 2,6 lutidiene or methanol and cyclohexane), we study both the initial fast exponential-like growth (the Cahn-Hilliard regime) and the subsequent cross-over to a much slower, non-exponential long time growth (the so-called scaling regime), by atomistic molecular dynamics (MD) simulation of a structure breaking binary liquid mixture. In particular, we combine our MD simulations with a coarse grained multi scale modelling (CGMSM) capable of capturing both length and time scales of phase separation kinetics within simulation box. The system is quenched from a higher temperature to two lower temperatures well below the coexistence temperature of the phase diagram. We observe a multiscale phase separation dynamics. Initially the growth is exponential up to a regime of 80-100ps having strong dependence over quench depth. Subsequently a cross-over regime appears where the dynamics slows down considerably due to coarsening through a power law phase. For deeper quench power law growth dominates over the initial exponential and the cross-over becomes transient. We find that for the present parameter values for the binary mixture, the initial rapid growth of structure formation is practically over within 200 ps which is followed by slow structural coarsening. When scaled by the respective viscosities, this time translates to 50-200 ns for water-lutidine binary mixture. The last part of dynamics may extend into ms. The dynamics of phase separation is slowest in regions that have equitable distribution of the two species, and one can observe the signatures of "up-hill diffusion" that is a trade mark of spinodal decomposition.

Milan K. Hazra, Sarmistha Sarkar, Biman Bagchi · 2016