Super Arrhenius temperature dependent viscosity due to liquid-liquid phase separation in the super-cooled Kob-Andersen model

Abstract

In this paper, we introduce a new order parameter called the weighted coordination number (WCN) to study the liquid-liquid (LL) phase separation, using the temperature-dependent coarsening of the LL interface as a possible mechanism for the glass transition. The well-established glass-forming Kob-Andersen binary Lennard-Jones system is used for our studies. The gas-liquid binodal line is reconstructed using the WCNs, and the same approach is extended to study the liquid-liquid binodal line. Systems of various densities are instantaneously quenched from high to low temperatures where a liquid-liquid separation is observed. Densities and the composition of each liquid state are used to check the level rule, along with density and pressure profiles, demonstrating local equilibrium of liquid-liquid phase separation. The transition from the liquid-liquid phase separation in the supercooled region to the glass transition region is modeled by adopting a Markov Network Model to estimate the temperature dependent viscosity using liquid-liquid interfacial information from the classification.

Figures (12)

• Figure Fig. 1: The radial distribution function with weighted Gaussians placed at each notable feature for a KA binary LJ system at $T^{*}=0.43$, $\rho^{*} =1.12$. The Gaussian widths are chosen so the the height at the intersection points is below 0.25.
• Figure Fig. 2: (a) 2D projection of PC space for KA binary LJ system at $T^{*}=0.3$, $\rho^{*} =1.12$. (b) Corresponding 2D projection in configuration space using K-means clustering information from PC space.
• Figure Fig. 3: (a) 2D projection of PC space for KA binary LJ system at $T^{*}=0.8$, $\rho^{*} =0.5$ with interfacial particles. (b) 2D projection of configuration space using K-means clustering information from PC space with k=3 to visualize interfacial particles.
• Figure Fig. 4: Isotherms of the Kob-Andersen system. Monotonicity ends below $T^{*}=0.6$, signifying a first-order phase transition from liquid-liquid metastability to liquid-liquid unstablebility. Minima of non-monotonic isotherms, $\rho^{*}_{s}(T)$, represent densities in which gas nucleation ceases and liquid-liquid phase separation occurs.
• Figure Fig. 5: Phase diagram of the KA binary LJ potential. All systems are equilibrated at $T^{*}=2.0$ before instantaneously quenching to the target temperature. The bulk density for gas-liquid systems was varied to obtain ideal separations, targeting spherical liquid clusters when possible. States were identified using WCN classification, which is then used to obtain average densities over 5ns. Densities at which the liquid-liquid separation was examined include $T^{*}=\{0.3,0.4,0.43,0.46,0.5,0.52,0.55,0.575,0.58,0.585\}$, and the lever rule was verified at key temperatures. Open circles are from Testard and filled squares are from our simulations. The blue curve is the liquid-liquid binodal line from our simulations with detailed data listed in Table \ref{['table:binodal']}. The gas-liquid spinodal line (green), taken from ref Sastry-eos, is estimated using Restricted Monte Carlo. Diamonds indicate end of gas nucleation and beginning of $l$-$l$ separation.