Sanjay Puri

Kinetics of Phase Transitions

We review developments in the field of phase ordering dynamics, viz., the evolution of a homogeneous multi-component mixture which has been rendered thermodynamically unstable by a rapid quench below the critical temperature. We discuss results for cases with both nonconserved and conserved order parameters. We also highlight recent research directions in this field, i.e., the incorporation of experimentally relevant effects in the studies of phase ordering systems.

Surface-directed spinodal decomposition : modelling and numerical simulations

We critically review the modelling and simulations of surface-directed spinodal decomposition, namely, the dynamics of phase separation of a critical or near-critical binary mixture in the presence of a surface with a preferential attraction for one of the components of the mixture.

Effect of noise on spinodal decomposition

The long-time behaviour of two-dimensional systems undergoing spinodal decomposition is studied numerically with the aid of a cell-dynamical approach both without and with noise. In both cases, the representative length scale of the pattern behaves as l(t) approximately tphi , where the exponent phi crosses over from approximately 0.28 to approximately 0.33. The crossover time increases with an increase in amplitude of the noise.

Surface-directed spinodal decomposition

We review analytical and numerical results for surface-directed spinodal decomposition (SDSD), namely, the interplay of wetting kinetics and phase separation in a binary (AB) mixture in contact with a surface S which prefers one of the components (say, A). Depending on the relative strengths of the A–B, A–S and B–S interactions, the surface is either partially wetted or completely wetted by A in equilibrium. We discuss the theoretical framework for modelling SDSD, and review results obtained from both microscopic and coarse-grained models. We clarify the differences between diffusion-driven SDSD in solids, and SDSD in fluids, where velocity fields play an important role. Furthermore, we discuss the dependence of wetting-layer kinetics on the composition of the mixture. Some results are also presented for phase separation in a confined geometry, e.g., thin films. Finally, we discuss the problem of surface-enrichment kinetics, namely, the kinetics of enrichment of an attracting surface when the bulk mixture is stable. These nonequilibrium processes have important applications in the preparation of nanomaterials and multi-layered structures.

Domain growth in random magnets

We study the kinetics of domain growth in ferromagnets with random exchange interactions. We present detailed Monte Carlo results for the nonconserved random-bond Ising model, which are consistent with power law growth with a variable exponent. These results are interpreted in the context of disorder barriers with a logarithmic dependence on the domain size. Further, we clarify the implications of logarithmic barriers for both nonconserved and conserved domain growth.

A new numerical scheme for the Fisher equation

In the context of the one-dimensional Fisher equation (1937) the authors study a new numerical scheme for reaction-diffusion equations (proposed by Oono and Puri (1987)). They find that this scheme enables a reasonable simulation of the Fisher equation at high values of the time increment, where conventional schemes are not applicable. For lower values of the time increment, the new scheme compares favourably with conventional schemes.

Spinodal decomposition in thin films: molecular-dynamics simulations of a binary Lennard-Jones fluid mixture

We use molecular dynamics (MD) to simulate an unstable homogeneous mixture of binary fluids (AB), confined in a slit pore of width D. The pore walls are assumed to be flat and structureless and attract one component of the mixture (A) with the same strength. The pairwise interactions between the particles are modeled by the Lennard-Jones potential, with symmetric parameters that lead to a miscibility gap in the bulk. In the thin-film geometry, an interesting interplay occurs between surface enrichment and phase separation. We study the evolution of a mixture with equal amounts of A and B, which is rendered unstable by a temperature quench. We find that A-rich surface enrichment layers form quickly during the early stages of the evolution, causing a depletion of A in the inner regions of the film. These surface-directed concentration profiles propagate from the walls towards the center of the film, resulting in a transient layered structure. This layered state breaks up into a columnar state, which is characterized by the lateral coarsening of cylindrical domains. The qualitative features of this process resemble results from previous studies of diffusive Ginzburg-Landau-type models [S. K. Das, S. Puri, J. Horbach, and K. Binder, Phys. Rev. E 72, 061603 (2005)], but quantitative aspects differ markedly. The relation to spinodal decomposition in a strictly two-dimensional geometry is also discussed.

Non-algebraic domain growth in random magnets: a cell dynamical approach

The authors develop a novel numerical approach, based on a computationally efficient cell dynamical system (CDS) model, for studying the kinetics of ordering in systems (described by a non-conserved order parameter) with quenched disorder, evolving from unstable initial states. They use this model to study the kinetics of domain growth in a coarse-grained version of the random exchange Ising model. Their numerical data strongly indicate quantitative agreement with the theoretically predicted asymptotic growth law over a limited range of disorder amplitudes. They also compare their observations with those in laboratory experiments and make important predictions regarding dynamical scaling in these systems.

Kinetics of phase separation in fluids: A molecular dynamics study

We present results from extensive three-dimensional molecular dynamics (MD) simulations of phase separation kinetics in fluids. A coarse-graining procedure is used to obtain state-of-the-art MD results. We observe an extended period of temporally linear growth in the viscous hydrodynamic regime. The morphological similarity of coarsening in fluids and solids is also quantified. The velocity field is characterized by the presence of monopolelike defects, which yield a generalized Porod tail in the corresponding structure factor.

Surface-directed spinodal decomposition in binary fluid mixtures.

We consider the phase separation of binary fluids in contact with a surface, which is preferentially wetted by one of the components of the mixture. We review the results available for this problem and present numerical results obtained using a mesoscopic level simulation technique for the three-dimensional problem.