Study of critical dynamics in fluids via molecular dynamics in canonical ensemble

Hydrodynamic effects in kinetics of phase separation in binary fluids: Critical versus off-critical compositions

Via hydrodynamics-preserving molecular dynamics simulations we study growth phenomena in a phase-separating symmetric binary mixture model. We quench high-temperature homogeneous configurations to state points inside the miscibility gap, for various mixture compositions. For compositions at the symmetric or critical value we capture the rapid linear viscous hydrodynamic growth due to advective transport of material through tubelike interconnected domains. For state points very close to any of the branches of the coexistence curve, the growth in the system, following nucleation of disconnected droplets of the minority species, occurs via a coalescence mechanism. Using state-of-the-art techniques, we have identified that these droplets, between collisions, exhibit diffusive motion. The value of the exponent for the power-law growth, related to this diffusive coalescence mechanism, has been estimated. While the exponent nicely agrees with that for the growth via the well-known Lifshitz-Slyozov particle diffusion mechanism, the amplitude is stronger. For the intermediate compositions we observe initial rapid growth that matches the expectations for viscous or inertial hydrodynamic pictures. However, at later times these types of growth cross over to the exponent that is decided by the diffusive coalescence mechanism.

Accelerated inertial regime in the spinodal decomposition of magnetic fluids

Furukawa predicted that at late times, the domain growth in binary fluids scales as (t) ∼ t^2/3, and the growth is driven by fluid inertia. The inertial growth regime has been highly elusive in molecular dynamics (MD) simulations. We perform coarsening studies of the (d = 3) Stockmayer (SM) model comprising of magnetic dipoles that interact via long-range dipolar interactions as well as the usual Lennard-Jones (LJ) potential. This fascinating polar fluid exhibits a gas-liquid phase coexistence, and magnetic order even in the absence of an external field. From comprehensive MD simulations, we observe the inertial scaling [(t) ∼ t^2/3] in the SM fluid for an extended time window. Intriguingly, the fluid inertia is overwhelming from the outset - our simulations do not show the early diffusive regime [(t) ∼ t^1/3] and the intermediate viscous regime [(t) ∼ t] prevalent in LJ fluids.

Advantages of the Rayleigh-Lowe-Andersen thermostat in soft sphere molecular dynamics simulations

The Rayleigh-Lowe-Andersen thermostat is a momentum-conserving, Galilean-invariant analogue of the Andersen thermostat, like the original (Maxwellian) Lowe-Andersen thermostat. However, the Rayleigh-Lowe-Andersen thermostat remains local even if the fluid density becomes low. By using a minimized thermostat interaction radius we show with a molecular dynamics simulation that the Rayleigh-Lowe-Andersen thermostat affects the natural dynamics of a low-density Lennard-Jones fluid in a minimal fashion. We also show that it is no longer necessary to consider a separate simulation just to determine the optimal value of the thermostat interaction radius. Instead, this value is computed directly during the main simulation run. Because the Rayleigh-Lowe-Andersen thermostat can be combined with the velocity Verlet integration scheme, we expect a widespread applicability of the thermal mechanism presented here.

Fast Nosé-Hoover thermostat: molecular dynamics in quasi-thermodynamic equilibrium

An extension of the Nosé-Hoover thermostat equation for molecular dynamics (MD) simulation is introduced, which perturbs fast degrees of freedom out of canonical equilibrium, while preserving the average temperature of the system. Based on the generalised Liouville equation, it is shown that the newly introduced fast Nosé-Hoover (FNH) equations give rise to a dynamical system with a well-defined non-equilibrium probability distribution. Corresponding thermostat parameters are identified, which in principle allow to sample arbitrarily close to canonical equilibrium. Results show that the dynamic system properties (e.g. self-diffusion, shear viscosity, dielectric permittivity and rotational relaxation times) are only moderately perturbed for typical FNH setups. However, the non-equilibrium FNH equations modify the occurrence of rare events substantially and thus offer a novel approach for enhanced sampling in MD. In particular, it is shown that the FNH thermostat increased significantly the frequency of the folding and unfolding process of short β-peptides. The efficiency of the phase-space exploration solely depends on the additionally imposed quasi-equilibrium conditions, i.e. it does not rely on any modification of the potential-energy surface.

Finite-size scaling study of dynamic critical phenomena in a vapor-liquid transition

Via a combination of molecular dynamics (MD) simulations and finite-size scaling (FSS) analysis, we study dynamic critical phenomena for the vapor-liquid transition in a three dimensional Lennard-Jones system. The phase behavior of the model has been obtained via the Monte Carlo simulations. The transport properties, viz., the bulk viscosity and the thermal conductivity, are calculated via the Green-Kubo relations, by taking inputs from the MD simulations in the microcanonical ensemble. The critical singularities of these quantities are estimated via the FSS method. The results thus obtained are in nice agreement with the predictions of the dynamic renormalization group and mode-coupling theories.