arXiv subjects

Vinay Vaibhav

Publications and source records attributed to Vinay Vaibhav.

12 recordsLinked to original sources

arXiv2026

Glassy dynamics, crossover temperature and density scaling in fragile glass-formers

We investigate the slowing down of dynamics in a glass-forming mixture interacting via an inverse-power-law (IPL) potential using a combination of theory and large-scale molecular dynamics simulations. We measure the static pair-correlation function, configurational entropy, inherent-structure energy, and structural relaxation time. We employ a theoretical framework to calculate the structural relaxation time $τ_α$, which is found to be in very good agreement with the simulation results. The theory identifies a local structural order which defines the cooperativity of the relaxation and brings forth a fluctuation induced parameter $ψ( T )$ and a crossover temperature $T_a$ that characterize the density and temperature dependence of the glassy dynamics. Furthermore, we determine a crossover temperature using independent dynamical and thermodynamic criteria and compare with the theoretically predicted crossover temperature $T_a$. Relaxation dynamics is shown to obey density-temperature scaling, similar to thermodynamic properties, in terms of a variable $Γ$ formed by an appropriate combination of density and temperature, characteristic of IPL interactions. Finally, we show that, when the excess thermodynamic and dynamic quantities obtained at different densities are plotted as functions of the reduced temperature $T/T_a$ (or $T_a/T$), the data collapse onto master curves with excellent agreement between theory and simulation. These scaling relations provide a unified description of the thermodynamics and dynamics in IPL systems, enabling the prediction of relaxation behavior over a wide range of densities from data at a single state point.

cond-mat.soft↗

arXiv2026

Theory of distribution skewness effect on polydisperse random close packing

We investigate the random close packing density, $ϕ_\textrm{RCP}$, of polydisperse hard sphere systems using a theoretical framework based on the equilibrium model of crowding. We derive a closed-form solution for $ϕ_\textrm{RCP}$ in terms of the moments of the diameter distribution, enabling an analytical exploration of the effects of polydispersity ($δ$) and skewness ($S$) on packing density. For a binary mixture, it is possible to explore a broader range of dependence of $ϕ_\textrm{RCP}$ on $δ$ for a given $S$ or on $S$ for a given $δ$. We show that the dependencies of $ϕ_\textrm{RCP}$ on skewness for a variety of continuous distributions collapse onto a theoretical master curve obtained for the binary mixture case. By correcting the theory so that it obeys known exact limiting behaviours for extreme size asymmetry, our analytical predictions not only agree with previously obtained numerical results, but also predict previously unexplored regions of the $ϕ_\textrm{RCP}$ parameter space.

cond-mat.soft↗

arXiv2026

Atomistic Framework for Glassy Polymer Viscoelasticity Across Twenty Frequency Decades

Glassy polymers are central to engineering applications, yet their viscoelastic response over broad frequency and temperature ranges remains difficult to characterize. We extend non-affine deformation theory by incorporating a time-dependent memory kernel within the Generalized Langevin Equation for atomistic non-affine motions, yielding frequency-dependent mechanical response. Applied to poly(methyl methacrylate) (PMMA), the method captures the shear modulus and relaxation spectrum across more than twenty decades in frequency, from hundreds of terahertz to the millihertz regime, thus bridging polymer mechanics from ordinary to extreme scales. Our predictions show quantitative consistency with independent estimates from oscillatory-shear molecular dynamics, Brillouin scattering, ultrasonic spectroscopy, Split-Hopkinson testing, and dynamic mechanical analysis (DMA), demonstrating a unified theoretical-computational route for multiscale characterization of polymer glasses.

cond-mat.soft↗

arXiv2025

Persistently Non-Gaussian Metastable Liquids

Particles undergoing Fickian diffusion within smooth energy landscapes exhibit Gaussian statistics. However, this Gaussian behavior is often elusive in complex liquids, where particle dynamics within spontaneously fluctuating or spatio-temporally heterogeneous environments lead to a breakdown of ergodicity and time-reversal symmetry. This is usually caused by extreme particle movements or sudden dynamical arrest. Such situations are prevalent in dense metastable liquids exhibiting slow flow or cooperative movements, facilitated by cage-breaking. We investigate the dynamics of glassy systems driven by either thermal, external, or environmental fluctuations. Despite their differences, our findings reveal that particle motion is universally affected by large deviations, resulting in non-Gaussian tails persisting over multiple decades in time. We further discuss the underlying dynamical aspects.

cond-mat.soft↗

arXiv2025

Viscosity of polymer melts using non-affine theory based on vibrational modes

Viscosity, a fundamental transport and rheological property of liquids, quantifies the resistance to relative motion between molecular layers and plays a critical role in understanding material behavior. Conventional methods, such as the Green-Kubo (GK) approach, rely on time integration of correlation functions, which becomes computationally intensive near the glass transition due to slow correlation decay. A recently proposed method based on non-affine lattice dynamics (NALD) and instantaneous normal mode analysis offers a promising alternative for estimating the viscosity. In this study, we apply the NALD approach to compute the viscosity of the Kremer-Grest polymer system over a range of temperatures and compare these results with those from the GK method and non-equilibrium molecular dynamics simulations. Our findings reveal that all vibration modes, including the instantaneous normal modes, contribute to the viscosity. This work presents an efficient framework for calculating viscosity across diverse systems, including near the glass transition where the GK method is no longer applicable. Also, it opens the avenue to understanding the role of different vibrational modes linked with structure, facilitating the design of materials with tunable rheological properties.

cond-mat.soft↗

arXiv2025

Experimental identification of topological defects in 2D colloidal glass

Topological defects are singularities within a field that cannot be removed by continuous transformations. The definition of these irregularities requires an ordered reference configuration, calling into question whether they exist in disordered materials, such as glasses. However, recent work suggests that well-defined topological defects emerge in the dynamics of glasses, even if they are not evident in the static configuration. In this study, we reveal the presence of topological defects in the vibrational eigenspace of a two-dimensional experimental colloidal glass. These defects strongly correlate with the vibrational features and spatially correlate with each other and structural "soft spots", more prone to plastic flow. This work experimentally confirms the existence of topological defects in disordered systems revealing the complex interplay between topology, disorder, and dynamics.

cond-mat.soft↗

arXiv2024

Random close packing of binary hard spheres favors the stability of neutron-rich atomic nuclei

In spite of the success of the Bethe-Weizsäcker mass formula in its modern numerical and predictive implementations, the common-knowledge principle that it is electrostatics which, ultimately, favors neutron-rich nuclei still presents unclear aspects. For example, while it is true that the Coulomb interaction promotes the tendency towards neutron-rich nuclei, the opposite effects of Majorana exchange forces and Pauli exclusion are known to counteract this tendency. We show that a recent analytical progress in the mathematical description of random close packing of spheres with different sizes provides a missing contribution to the theoretical description of the $Z$ versus $N$ slope in the nuclides chart. In particular, the theory suggests, on geometric grounds and with a physically-reasoned assumption that the excluded-volume size of neutrons is 20\% larger than that of protons, that the most stable nuclei are those with ratio $Z/N\approx 0.75$. This new ``geometric'' random-packing contribution to the semi-empirical mass formula may be the missing aspect of nuclear structure that tilts the balance towards neutron-rich nuclei in the Segrè stability chart.

nucl-th↗

arXiv2024

Timescale bridging in atomistic simulations of epoxy polymer mechanics using non-affine deformation theory

Developing a deep understanding of macroscopic mechanical properties of amorphous systems which lack structural periodicity, has posed a key challenge, not only at the level of theory but also in molecular simulations. Despite significant advancements in computational resources, there is a vast timescale disparity, more than 6 orders of magnitude, between mechanical properties probed in simulations compared to experiments. Using the theoretical framework of non-affine lattice dynamics (NALD), based on the instantaneous normal modes analysis determined through the dynamical matrix of the system, we study the viscoelastic response of a cross-linked epoxy system of diglycidyl ether of bisphenol A (DGEBA) and poly(oxypropylene) diamine, over many orders of magnitude in deformation frequency, below the glass transition temperature. Predictions of the elastic modulus are satisfactorily validated against the non-equilibrium molecular dynamics simulations in the high-frequency regime, and against experimental data from dynamic mechanical analysis at frequencies $ \sim 1 {\rm Hz}$, hence successfully bridging the timescale gap. The comparison shows that non-affine displacements at the atomic level account for nearly two orders of magnitude reduction in the low-frequency elastic modulus of the polymer glass, compared to affine elasticity estimates. The analysis also reveals the role of internal stresses (as reflected in the instantaneous normal modes), which act as to strengthen the mechanical response.

cond-mat.soft↗

arXiv2023

Entropic Timescales of Dynamic Heterogeneity in Supercooled Liquid

Non-Gaussian displacement distributions are universal predictors of dynamic heterogeneity in slowly varying environments. Here, we explore heterogeneous dynamics in supercooled liquid using molecular dynamics simulations and show the efficiency of the information-theoretic measure in quantifying dynamic heterogeneity over the widely used moment-based quantifications of nonGaussianity. Our analysis shows that the heterogeneity quantified by the negentropy is significantly different from the one obtained using the conventional approach that considers deviation from Gaussianity up to lower-order moments. Further, we extract the timescales of dynamic heterogeneity using the two methods and show that the differential changes diverge as the system experiences strong intermittency near the glass transition.

cond-mat.soft↗

arXiv2022

Rheological response of a glass-forming liquid having large bidispersity

Using extensive numerical simulations, we investigate the flow behaviour of a model glass-forming binary mixture whose constituent particles have a large size ratio. The rheological response to applied shear is studied in the regime where the larger species are spatially predominant. We demonstrate that the macroscopic rigidity that emerges with increasing density occurs in the regime where the larger species undergo a glass transition while the smaller species continue to be highly diffusive. We analyse the interplay between the timescale imposed by the shear and the quiescent relaxation dynamics of the two species to provide a microscopic insight into the observed rheological response. Finally, by tuning the composition of the mixture, we illustrate that the systematic insertion of the smaller particles affects the rheology by lowering of viscosity of the system.

cond-mat.soft↗

arXiv2022

Finite-size effects in the diffusion dynamics of a glass-forming binary mixture with large size ratio

Extensive molecular dynamics (MD) computer simulations of an equimolar glass-forming AB mixture with large size ratio are presented. While the large A particles show a glass transition around the critical density of mode coupling theory $ρ_c$, the small B particles remain mobile with a relatively weak decrease of their self diffusion coefficient $D_{\rm B}$ with increasing density. Surprisingly, around $ρ_c$, the self-diffusion coefficient of the A particles, $D_{\rm A}$, also starts to follow a rather weak dependence on density. We show that this is due to finite-size effects that can be understood from the analysis of the collective interdiffusion dynamics.

cond-mat.soft↗

arXiv2017

Equilibrium properties of blackbody radiation with an ultraviolet energy cut-off

We study various equilibrium thermodynamic properties of blackbody radiation (i.e. a photon gas) with an ultraviolet energy cut-off. We find that the energy density, specific heat etc. follow usual acoustic phonon dynamics as have been well studied by Debye. Other thermodynamic quantities like pressure, entropy etc. have also been calculated. The usual Stefan-Boltzmann law gets modified. We observe that the values of the thermodynamic quantities with the energy cut-off is lower than the corresponding values in the theory without any such scale. The phase-space measure is also expected to get modified for an exotic spacetime appearing at Planck scale, which in turn leads to the modification of Planck energy density distribution and the Wien's displacement law. We found that the non-perturbative nature of the thermodynamic quantities in the SR limit (for both the case with ultravilolet cut-off and the modified measure case), due to nonanalyticity of the leading term, is a general feature of the theory accompanied with an ultraviolet energy cut-off. We have also discussed the possible modification in the case of Big Bang and the Stellar objects and have suggested a table top experiment for verification in effective low energy case.

gr-qc↗