arXiv subjects

Rajesh K. Murarka

Publications and source records attributed to Rajesh K. Murarka.

3 recordsLinked to original sources

arXiv2003

Diffusion of small light particles in a solvent of large massive molecules

We study diffusion of small light particles in a solvent which consists of large heavy particles. The intermolecular interactions are chosen to approximately mimic a water-sucrose (or water-polysaccharide) mixture. Both computer simulation and mode coupling theoretical (MCT) calculations have been performed for a solvent-to-solute size ratio five and for a large variation of the mass ratio, keeping the mass of the solute fixed. Even in the limit of large mass ratio the solute motion is found to remain surprisingly coupled to the solvent dynamics. Interestingly, at intermediate values of the mass ratio, the self-intermediate scattering function of the solute, F_{s}(k,t) (where k is the wavenumber and t the time), develops a stretching at long time which could be fitted to a stretched exponential function with a k-dependent exponent, β. For very large mass ratio, we find the existence of two stretched exponentials separated by a power law type plateau. The analysis of the trajectory shows the coexistence of both hopping and continuous motions for both the solute and the solvent particles. It is found that for mass ratio five, the MCT calculations of the self-diffusion underestimates the simulated value by about 20 %, which appears to be reasonable because the conventional form of MCT does not include the hopping mode. However, for larger mass ratio, MCT appears to breakdown more severely. The breakdown of the MCT for large mass ratio can be connected to a similar breakdown near the glass transition.

cond-mat.soft↗

arXiv2002

Diffusion and viscosity in a supercooled polydisperse system

We have carried out extensive molecular dynamics simulations of a supercooled polydisperse Lennard-Jones liquid with large variations in temperature at a fixed pressure. The particles in the system are considered to be polydisperse both in size and mass. The temperature dependence of the dynamical properties such as the viscosity ($η$) and the self-diffusion coefficients ($D_i$) of different size particles is studied. Both viscosity and diffusion coefficients show super-Arrhenius temperature dependence and fit well to the well-known Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range investigated, the value of the Angell's fragility parameter (D $\approx 1.4$) classifies the present system into a strongly fragile liquid. The critical temperature for diffusion ($T_o^{D_i}$) increases with the size of the particles. The critical temperature for viscosity ($T_o^η$) is larger than that for the diffusion and a sizeable deviations appear for the smaller size particles implying a decoupling of translational diffusion from viscosity in deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian behavior at low temperatures where a highly nonlinear dependence on size is observed. An inspection of the trajectories of the particles shows that at low temperatures the motions of both the smallest and largest size particles are discontinuous (jump-type). However, the crossover from continuous Brownian to large length hopping motion takes place at shorter time scales for the smaller size particles.

cond-mat.soft↗

arXiv2002

Pair dynamics in a glass forming binary mixture: Simulations and theory

We have carried out molecular dynamics simulations to understand the dynamics of a tagged pair of atoms in a strongly non-ideal glass-forming binary Lennard-Jones mixture (commonly known as Kob-Andersen model). The generalized time-dependent pair distribution function is calculated separately for the three pairs (AA, BB and AB). The three pairs are found to behave differently. The relative diffusion constants are found to vary in the order $D_R^{BB} > D_R^{AB} > D_R^{AA}$, with $D_R^{BB} \simeq 2D_R^{AA}$, showing the importance of the hopping process (B hops much more than A). We introduce a non-Gaussian parameter ($α_2^P (t)$) to monitor the relative motion of a pair of atoms, and evaluate it for all the three pairs, with initial separations chosen to be at the first peak of the corresponding partial radial distribution functions. At intermediate times, significant deviation from the Gaussian behavior of the pair distribution functions is observed, with different degree for the three pairs. A simple mean-field (MF) model, proposed originally by Haan [Phys. Rev. A ${\bf 20}$, 2516 (1979)] for one component liquid, is applied to the case of binary mixture, and compared with the simulation results. While the MF model successfully describe the dynamics of the AA and AB pair, the agreement for the BB pair is less satisfactory. This is attributed to the large scale anharmonic motions of the B particles in a weak effective potential. Dynamics of next nearest neighbor pairs are also investigated.

physics.chem-ph↗