arXiv subjects

Gan Ren

Publications and source records attributed to Gan Ren.

10 recordsLinked to original sources

arXiv2026

The validity of the Stokes-Einstein relation in ionic liquids

Ionic liquids (ILs) exhibit supercooled liquids behavior even above room temperature and the Stokes-Einstein (SE) relation is often considered to be invalid in ILs as that in supercooled liquids. However, the conclusion is usually drawn based on some variants of SE relation. In this work, we have systematically investigated the validity of the Stokes-Einstein relation in ILs by performing molecular dynamics simulations of coarse-grained IL within the temperature range of 400-800 K. Both the original SE formulation, and two commonly employed variants, were examined to assess their applicability and consistency. These three formulas yield distinctly different results, indicating that the two variants are not reliable substitutes for the original SE relation. The inconsistency suggests the fact that the r in Stokes law varies with conditions. The breakdown arises from the ion correlations introduced by the strong electrostatic interactions. This proposition is further confirmed by simulations.

cond-mat.soft↗

arXiv2026

Coupling and decoupling between translational and rotational dynamics in a tetrahedral molecular liquid

The translational dynamics usually decouples earlier than the rotational in supercooled liquids as the temperature decreases, whereas the inverted scenario remains scarce reported. In this work, starting from the coarse-grained ortho-terphenyl model, we build a rigid tetrahedral structure model. It exhibits an earlier decoupling in rotation than in translation. The Stokes-Einstein-Debye relation breaks down while the Stokes-Einstein relation remains intact. The decoupling happens at approximately 2Tg similar as that observed in supercooled water. The rotation shows more heterogeneous dynamics than the translation at all temperatures. Our results suggest that the steric hindrance plays an important role in determining the decoupling between the translational and rotational dynamics.

cond-mat.soft↗

arXiv2024

The Stokes-Einstein-Debye relation in ortho-terphenyl liquid

The Stokes-Einstein-Debye (SED) relation is proposed to be breakdown in supercooled liquids by many studies. However, the conclusions are usually drawn by testing some variants of the SED relation rather than its original form. In this work, the rationality of the SED relation and its variants is examined by performing molecular dynamics simulations with the Lewis-Wahnstrom model of ortho-terphenyl (OTP). The results indicate the original SED relation is valid for OTP but the three variants are all breakdown. The inconsistency between SED relation and its variants is attributed to the adopted assumptions and approximations, heterogeneous dynamics and the interactions among molecules. Therefore, care should be taken when employing its variants to judge the validity of the SED relation in supercooled liquids.

cond-mat.soft↗

arXiv2023

Testing the variants of the Stokes-Einstein relation in the framework of self-consistent generalized Langevin equation theory

The two functional forms, D~1/tau and D~T/tau, are usually adopted as the variants of the Stokes-Einstein relation; where D is the diffusion constant, tau the relaxation time and T the temperature. The self-consistent generalized Langevin equation (SCGLE) theory is presented as an analytical tool to predict the long time dynamics of colloids and molecular liquids. In this work, taking truncated Lennard-Jones-like liquids as an example, the rationality of the two variants were tested in the framework of the SCGLE theory. Our results indicate that D~1/tau is a good variant of the Stokes-Einstein relation in the framework of SCGLE theory; however, D~T/tau is not a good one but taking a fractional from as D~(T/tau)^zeta with an exponent zeta is not equal to 1.0 even the Stokes-Einstein relation is established in SCGLE theory.

cond-mat.soft↗

arXiv2022

The effective hydrodynamic radius is not a constant

The effective hydrodynamic radius is usually assumed to be a constant in testing the Stokes-Einstein relation by its variants. We have performed molecular dynamics simulations with ortho-terphenyl and Kob-Andersen model to examine the assumption and found the effective hydrodynamic radius is not a constant but decreases with decreasing temperature. The variant of Stokes-Einstein relation is not established but Stokes-Einstein relation is valid by considering the changes of the effective hydrodynamic radius. We propose the assumption should be seriously evaluated when using variants to test Stokes-Einstein relation.

cond-mat.soft↗

arXiv2022

The Properties of doping ZrCo

ZrCo alloy is promising to substitute uranium for handling hydrogen isotope storage in thermonuclear reactor. The alloying substitution of Zr in ZrCo with Hf or Ti can enhance the ability of anti-disproportionation. In this work, Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co were considered in the framework of density functional theory, aimed to investigate the properties of the alloying substitution of Zr with Hf or Ti in ZrCo. Our results found that the optimized lattice constants of the alloying substitutions, Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co, are smaller than ZrCo. The thermodynamic stability reduces in the order Zr$_{0.75}$Hf$_{0.25}$Co > ZrCo > Zr$_{0.75}$Ti$_{0.25}$Co, as demonstrated by the enthalpy of formation. The valence electrons are mainly localized at the ion core and the chemical bonds are polarized in Zr$_{0.75}$Hf$_{0.25}$Co and Zr$_{0.75}$Ti$_{0.25}$Co analogous to ZrCo.

cond-mat.mtrl-sci↗

arXiv2021

Conservation of the Stokes-Einstein Relation in Supercooled Water

The Stokes-Einstein (SE) relation is commonly regarded as being breakdown in supercooled water. However, this conclusion is drawn upon testing the validities of some variants of the SE relation rather than its original form, and it appears conflicting with the fact that supercooled water is in its local equilibrium. In this work, we show by molecular dynamics simulation that both the Einstein and Stokes relations are indeed conserved in supercooled water. The inconsistency between the original SE relation and its variants comes from two facts: (1) the substitutes of the shear viscosity in the SE variants are wavevector-dependent, so it is only a cursory approximation; (2) the effective hydrodynamic radius actually decreases with decreasing temperature, instead of being a constant as assumed in the SE variants. Besides supercooled water, this inconsistency may also exist in other supercooled liquids.

cond-mat.soft↗

arXiv2016

Statistical Theory of Initiation of Explosives by Impact

When a given weight dropped onto an explosive charge, explosion or not is probabilistic for certain impact energy and the frequency of explosion is always increase with increasing impact energy. Based on experimental results and recently theoretical work, we propose that the hot spot formation is attributed to the activated molecules decomposition and the number of molecules initiation is proportional to the impact energy but not the dropped weight heating as the previous hot spot theory. A theoretical model based on two states model has been put forward for this phenomena. It is shown that the activated molecules to form a hot spot determine the probabilistic nature of initiation by impact. It is shown a good agreement tested with Hexogen (RDX) experimental impact data.

cond-mat.soft↗

arXiv2015

Saturated Sodium Chloride Solution under an External Static Electric Field: a Molecular Dynamics Study

The behavior of saturated aqueous sodium chloride solutions under a constant external electric field (E) was studied by molecular dynamics (MD) simulation. Our dynamic MD simulations have indicated that the irreversible nucleation process towards crystallization is accelerated by a moderate E, but retarded or even prohibited under a stronger E, which can be understood by the competition between self-diffusion and drift motion. The former increases with E resulting in the acceleration of the nucleation process, and the latter tears oppositely charged ions more apart under a stronger E leading to the deceleration of nucleation. Moreover, our steady-state MD simulations have indicated that a first-order phase transition happens in saturated solutions only when the applied E is below a certain threshold Ec, and the ratio of crystallized ions does not change with the electric field. The magnitude of Ec increases with concentration, because larger clusters are easy to form in a more concentrated solution and require a stronger E to dissociate them.

cond-mat.soft↗

arXiv2014

Reversible Transient Nucleation in Ionic Solutions as the Precursor of Ion Crystallization

Molecular dynamics simulations for aqueous sodium chloride solutions were carried out at various concentrations. Supplementary to the Debye-Hückel theory, reversible transient nucleation of ions was observed even in dilute solutions. The average size of formed ion clusters and the lifetime of ion pairs increase with concentration until the saturation point, when ion clusters become stable and individual ions adjust their positions to form ordered lattice structures, leading to irreversible ion crystallization, which is beyond the description of the classical nucleation theory.

cond-mat.soft↗