Polydispersity is ubiquitous in nature. It is present in clays, minerals, paint pigments, metal and ceramic powders, food preservatives and in simple homogeneous liquids. It is common in synthetic colloids, which frequently exhibit considerable size polydispersity and is also found in industrially produced polymers, which contain macromolecules with a range of chain length.
We are interested in investigating how polydispersity influences the
potential energy landscape, fragility and heterogeneous dynamics of polydisperse
Lennard-Jones (LJ) systems in supercooled regime near the glass transition.
Polydispersity is found to have a significant effect on the potential energy
landscape. Increasing polydispersity at fixed volume
fraction decreases the glass transition temperature
and the fragility of glass formation
analogous to the antiplasticization seen in some polymeric melts.
Polydispersity is found to suppress the rate of
growth of dynamic heterogeneity, which can be attributed to the loss of structural correlations (as measured by the structure factor and the local bond orientational order) with polydispersity. While a critical polydispersity is required to avoid crystallization, we find that further increase in polydispersity lowers the glass forming ability.
Positive and negative deviations from the prediction of the composition dependence of ideal Raoult's Law of binary mixtures has remained an interesting problem and has attracted considerable attention over the last three to four decades. A Mode Coupling Theory (MCT) approach has been presented from our group to address the issue of the non-monotonic composition dependence of viscosity in binary mixtures. Two models, structure former (Model I) and structure breaker (Model II), were introduced to understand the positive as well as negative deviation from ideality. The theory was also verified using molecular dynamics simulations.
Recently, we have looked at the non-ideality of viscosity in binary mixture from the perspective of potential energy landscapes using the structure former and structure breaker models of binary mixture. For both the models, the average inherent structure energy shows an inverse correlation with viscosity. The computed the configurational entropy also shows an inverse correlation with viscosity (See the figure below).Figure caption:
Results for Structure former (All quantities are plotted as a
function of solute mole fraction, xB)
(a) Viscosity; dashed line indicates the ideal Raoult's Law.
(b) Average inherent structure energy, < eIS >
(c) Configurational entropy per particle
coefficients of solvent (A) and solute (B); green circles are solute
self-diffusivity and blue circles are solvent self-diffusivity values.