@article { ISI:000381384300031, title = {Composition dependence of the glass forming ability in binary mixtures: the role of demixing entropy}, journal = {Journal of Chemical Physics}, volume = {145}, number = {3}, year = {2016}, month = {JUL}, pages = {Article Number: 034503}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {

We present a comparative study of the glass forming ability of binary systems with varying composition, where the systems have similar global crystalline structure (CsCl+fcc). Biased Monte Carlo simulations using umbrella sampling technique show that the free energy cost to create a CsCl nucleus increases as the composition of the smaller particles is decreased. We find that systems with comparatively lower free energy cost to form CsCl nucleus exhibit more pronounced pre-crystalline demixing near the liquid/crystal interface. The structural frustration between the CsCl and fcc crystal demands this demixing. We show that closer to the equimolar mixture, the entropic penalty for demixing is lower and a glass forming system may crystallize when seeded with a nucleus. This entropic penalty as a function of composition shows a non-monotonic behaviour with a maximum at a composition similar to the well known Kob-Anderson (KA) model. Although the KA model shows the maximum entropic penalty and thus maximum frustration against CsCl formation, it also shows a strong tendency towards crystallization into fcc lattice of the larger {\textquoteleft}{\textquoteleft}A{\textquoteright}{\textquoteright} particles which can be explained from the study of the energetics. Thus for systems closer to the equimolar mixture although it is the requirement of demixing which provides their stability against crystallization, for KA model it is not demixing but slow dynamics and the presence of the {\textquoteleft}{\textquoteleft}B{\textquoteright}{\textquoteright} particles make it a good glass former. The locally favoured structure around {\textquoteleft}{\textquoteleft}B{\textquoteright}{\textquoteright} particles is quite similar to the CsCl structure and the incompatibility of CsCl and fcc hinders the fcc structure growth in the KA model. Although the glass forming binary systems studied here are quite similar, differing only in composition, we find that their glass forming ability cannot be attributed to a single phenomenon. Published by AIP Publishing.

}, issn = {0021-9606}, doi = {10.1063/1.4958630}, author = {Nandi, Ujjwal Kumar and Banerjee, Atreyee and Chakrabarty, Suman and Bhattacharyya, Sarika Maitra} } @article {45717, title = {Fickian yet non-Gaussian behaviour: A dominant role of the intermittent dynamics}, journal = {Journal Of Chemical Physics}, volume = {146}, year = {2017}, month = {APR }, pages = {134504}, type = {Article}, abstract = {

We present a study of the dynamics of small solute particles in a solvent medium where the solute is much smaller in size, mimicking the diffusion of small particles in crowded environment. The solute exhibits Fickian diffusion arising from non-Gaussian Van Hove correlation function. Our study shows that there are at least two possible origins of this non-Gaussian behaviour: the decoupling of the solute-solvent dynamics and the intermittency in the solute motion, the latter playing a dominant role. In the former scenario when averaged over time long enough to explore different solvent environments, the dynamics recovers the Gaussian nature. In the case of intermittent dynamics the non-Gaussianity remains even after long averaging and the Gaussian behaviour is obtained at a much longer time. Our study further shows that only for an intermediate attractive solute-solvent interaction the dynamics of the solute is intermittent. The intermittency disappears for weaker or stronger attractions. Published by AIP Publishing.

}, issn = {0021-9606}, doi = {10.1063/1.4979338}, author = {Acharya, Sayantan and Nandi, Ujjwal Kumar and Bhattacharyya, Sarika Maitra} } @article {46246, title = {Fickian yet non-Gaussian behaviour: A dominant role of the intermittent dynamics (vol 146, 134504, 2017)}, journal = {Journal of Chemical Physics}, volume = {151}, year = {2019}, month = {AUG}, type = {Article}, doi = {10.1063/1.5120313}, author = {Acharya, Sayantan and Nandi, Ujjwal Kumar and Bhattacharyya, Sarika Maitra} } @article {47043, title = {Connecting real glasses to mean-field models}, journal = {Journal of Chemical Physics}, volume = {154}, year = {2021}, month = {MAR}, pages = {094506}, type = {Article}, abstract = {

We propose a novel model for a glass-forming liquid, which allows us to switch in a continuous manner from a standard three-dimensional liquid to a fully connected mean-field model. This is achieved by introducing k additional particle-particle interactions, which thus augments the effective number of neighbors of each particle. Our computer simulations of this system show that the structure of the liquid does not change with the introduction of these pseudo-neighbors and by means of analytical calculations, and we determine the structural properties related to these additional neighbors. We show that the relaxation dynamics of the system slows down very quickly with the increase in k and that the onset and the mode-coupling temperatures increase. The systems with high values of k follow the mode-coupling theory power law behavior for a larger temperature range compared to the ones with lower values of k. The dynamic susceptibility indicates that the dynamic heterogeneity decreases with the increase in k, whereas the non-Gaussian parameter is independent of it. Thus, we conclude that with the increase in the number of pseudo-neighbors, the system becomes more mean-field-like. By comparing our results with previous studies on mean-field-like systems, we come to the conclusion that the details of how the mean-field limit is approached are important since they can lead to different dynamical behavior in this limit.

}, issn = {0021-9606}, doi = {10.1063/5.0038749}, author = {Nandi, Ujjwal Kumar and Kob, Walter and Bhattacharyya, Sarika Maitra} } @article {47100, title = {Effective structure of a system with continuous polydispersity}, journal = {Journal of Chemical Physics}, volume = {154}, year = {2021}, month = {JAN }, pages = {034503}, type = {Article}, abstract = {

In a system of N particles, with continuous size polydispersity, there exists an N(N - 1) number of partial structure factors, making it analytically less tractable. A common practice is to treat the system as an effective one component system, which is known to exhibit an artificial softening of the structure. The aim of this study is to describe the system in terms of M pseudospecies such that we can avoid this artificial softening but, at the same time, have a value of M \<\< N. We use potential energy and pair excess entropy to estimate an optimum number of species, M-0. We then define the maximum width of polydispersity, Delta sigma (0), that can be treated as a monodisperse system. We show that M-0 depends on the degree and type of polydispersity and also on the nature of the interaction potential, whereas Delta sigma (0) weakly depends on the type of polydispersity but shows a stronger dependence on the type of interaction potential. Systems with a softer interaction potential have a higher tolerance with respect to polydispersity. Interestingly, M-0 is independent of system size, making this study more relevant for bigger systems. Our study reveals that even 1\% polydispersity cannot be treated as an effective monodisperse system. Thus, while studying the role of polydispersity by using the structure of an effective one component system, care must be taken in decoupling the role of polydispersity from that of the artificial softening of the structure.

}, issn = {0021-9606}, doi = {10.1063/5.0038812}, author = {Patel, Palak and Nandi, Manoj Kumar and Nandi, Ujjwal Kumar and Bhattacharyya, Sarika Maitra} } @article {47771, title = {Thermodynamics and its correlation with dynamics in a mean-field model and pinned systems: a comparative study using two different methods of entropy calculation}, journal = {Journal of Chemical Physics}, volume = {156}, year = {2022}, month = {JAN }, pages = {014503}, type = {Article}, abstract = {A recent study introduced a novel mean-field model system where each particle over and above the interaction with its regular neighbors interacts with k extra pseudo-neighbors. Here, we present an extensive study of thermodynamics and its correlation with the dynamics of this system. We surprisingly find that the well-known thermodynamic integration (TI) method of calculating the entropy provides unphysical results. It predicts vanishing of the configurational entropy at temperatures close to the onset temperature of the system and negative values of the configurational entropy at lower temperatures. Interestingly, well below the temperature at which the configurational entropy vanishes, both the collective and the single-particle dynamics of the system show complete relaxation. Negative values of the configurational entropy are unphysical, and complete relaxation when the configurational entropy is zero violates the prediction of the random first-order transition theory (RFOT). However, the entropy calculated using the two-phase thermodynamics (2PT) method remains positive at all temperatures for which we can equilibrate the system, and its values are consistent with RFOT predictions. We find that with an increase in k, the difference in the entropy computed using the two methods increases. A similar effect is also observed for a system where a randomly selected fraction of the particles are pinned in their positions in the equilibrated liquid. We show that the difference in entropy calculated via the 2PT and TI methods increases with pinning density.}, issn = {0021-9606}, doi = {10.1063/5.0065668}, author = {Nandi, Ujjwal Kumar and Patel, Palak and Moid, Mohd and Nandi, Manoj Kumar and Sengupta, Shiladitya and Karmakar, Smarajit and Maiti, Prabal K. and Dasgupta, Chandan and Bhattacharyya, Sarika Maitra} }