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Structure & Dynamics of Supercooled Polydisperse Liquids

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.

References:

  1. R. K. Murarka and B. Bagchi, Phys. Rev. E 67, 51504 (2003).
  2. S. E. Abraham, S. M. Bhattacharrya and B. Bagchi, Phys. Rev. Lett. 100, 167801 (2008).
  3. S. E. Abraham and B. Bagchi, Phys. Rev. E In Press (2008).

Binary liquid mixtures: anomalous composition dependence

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.

Binary mixture

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).

Structure former

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

(d) Self-diffusion coefficients of solvent (A) and solute (B); green circles are solute self-diffusivity and blue circles are solvent self-diffusivity values.

References:

  1. G. Srinivas, A. Mukherjee and B. Bagchi, J. Chem. Phys. 114, 6220 (2001).
  2. A. Mukherjee, G. Srinivas and B. Bagchi, Phys. Rev. Lett.  86, 5926 (2001)
  3. A. Mukherjee and B. Bagchi, J. Phys. Chem. B 105, 9581 (2001).
  4. S. E. Abraham, D. Chakrabarti and B. Bagchi, J. Chem. Phys.  126, 74501 (2007).