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slow dynamics in condensed matter (supercooled liquids and liquid crystals)

A new model of binary mixture has been introduced in order to study particularly the orientational dynamics across the supercooled regime. In a molecular dynamics simulation study, the system is found to possess the canonical characteristics of slow dynamics in supercooled liquids. The energy landscape analysis reveals correlations between the decoupling phenomena, observed almost universally in supercooled molecular liquids, and the manner of exploration of the energy landscape of the system.

Inherent structure energy analysis

Figure caption:

The temperature dependence of the ratio of the first to second-rank rotational correlation time (open squares). On a different scale (appearing on the right) shown is the temperature dependence of the average inherent structure energy per particle for the purpose of comparison (solid circles).

Analytical and numerical techniques have been applied to solve a kinetic model of glassy dynamics for various observables. The model could reproduce many of the experimentally observed features of anomalous heat capacity behavior during a temperature cycle through the glass transition. The model can also predict a frequency dependent heat capacity that reflects the two-step relaxation behavior and capture salient features of structural relaxation in glassy systems.

Kinetic model of glassy dynamics

Figure caption:

A schematic representation of the kinetic model of glassy dynamics. The horizontal lines within a well represent different excitation levels.

Heat capacity versus reduced temperature

Figure caption:

Heat capacity versus reduced temperature, when subjected to cooling-heating cycle with different rate given in reduced unit.

Recent OKE experiments have suggested the analogy in orientational dynamics between the isotropic phase of thermotropic liquid crystals near the isotropic-nematic (I-N) transition and the supercooled molecular liquids. This analogy has been investigated and explored in further details through extensive molecular dynamics simulation studies of a model thermotropic liquid crystalline system. When the orientational relaxation near the I-N transition is investigated in a variety of model systems of thermotropic liquid crystals, a universal power law decay is observed at short to intermediate times. 

Single-particle second rank orientational time correlation function

Collective second rank orientational time correlation function

Figure caption:

(top) Time evolution of the single-particle second rank orientational time correlation function in log-log plots for (a) the calamitic system, (b) the discotic system, and (c) the lattice system.

(bottom) The short-to-intermediate time decay of the OKE signal in log-log plots for same systems. The solid lines show the linear fits to the data showing the power law decay regimes.

The energy landscape view of phase transitions and slow dynamics in thermotropic liquid crystals has been studied. This study throws light on the interplay between the orientational order and the translational order in the mesophases these systems exhibit.

The potential energy landscape

Figure caption:

The potential energy landscape explored by the system of 256 Gay-Berne ellipsoids of revolution with the parameterization GB (3, 5, 2, 1) as the system makes a transit upon mesophases upon cooling. (a) The temperature dependence of the average inherent structure energy per particle along three different isochors

(b) The evolution of the average orientational order parameter with temperature both for the inherent structures (filled) and the corresponding prequenched ones (opaque).

Anisotropic translational diffusion in the nematic phase has been investigated in model systems and the observed non-monotonic temperature dependence of the translational diffusion coefficient parallel to the director D|| has been explained from an energy landscape analysis to be viewed as a dynamical signature of the coupling between orientational and translational order at the microscopic level. Study of rotational diffusion near the I-N phase boundary shows breakdown of the Debye model of rotational diffusion which is attributed to the growth of orientational pair correlation.

Coupling between the orientational order and translational order in the Gay-Berne system

Figure caption:

The coupling between the orientational order and translational order in the Gay-Berne system GB(3, 5, 2, 1) at three state points: nematic (red), smectic (blue) and nematic-smectic transition region (green) along an isochor. psi and S denote the translational and orientational prder parameter respectively.

Currently the group is interested in exploring and understanding the dynamics of polydisperse supercooled liquid and the orientational dynamics of different model systems (e.g. calamatic, discotic etc.) of thermotropic liquid crystals.