[Paper Review] Phase separation kinetics of a symmetric binary mixture of glass-forming liquids

Yuri Oku, Kyohei Takae+1 more|arXiv (Cornell University)|Apr 5, 2020

Material Dynamics and Properties7 references4 citations

TL;DR

This study uses molecular dynamics simulations to investigate phase separation kinetics in a symmetric binary mixture of glass-forming liquids with identical A-A and B-B Lennard-Jones interactions and purely repulsive A-B interactions. It reveals that below the glass transition temperature, domain growth becomes logarithmically slow or slower, driven by aging-like microscopic dynamics, with coarsening kinetics decoupled from hydrodynamic effects and consistent with dynamic scaling despite extreme slowdown.

ABSTRACT

Mixtures of glass-forming fluids sometimes exhibit glass-glass phase separation at low temperatures. Here, we use a molecular dynamics simulation to study one of the simplest examples of the glass-glass phase separation. We consider a mixture composed of type A and B particles, in which the A-A and B-B interactions are the identical Lennard-Jones interactions and the A-B interaction is repulsive only. To avoid crystallization, we also introduce the polydispersity in the particle sizes for each component. We study the phase separation kinetics of this model at a 50:50 concentration at various temperatures. We find that hydrodynamic coarsening takes place when the temperature is higher than the onset temperature of the glassy dynamics. At lower temperatures, diffusive coarsening is observed over a long duration, and a further slower coarsening appears within a shorter time. Below the glass transition temperature, the domain growth does not stop but becomes logarithmically slow or even slower than logarithmic. By analyzing two-time correlation functions, we show that these slow coarsening processes are accompanied by a slowing down of the microscopic dynamics, which has qualitative similarities with the aging dynamics without phase separation. Based on the results, we discuss a possible link between the slow coarsening and the aging-like microscopic slowing down in the glass-glass phase separation.

Motivation & Objective

• To understand phase separation kinetics in a symmetric binary mixture of glass-forming liquids under controlled conditions.
• To investigate how glassy dynamics, particularly the onset of slow dynamics and glass transition, affect domain growth during phase separation.
• To distinguish between hydrodynamic and diffusive coarsening mechanisms in the presence of glassy dynamics.
• To examine the relationship between microscopic relaxation dynamics and macroscopic domain evolution in the glassy regime.
• To assess whether dynamic scaling holds during ultra-slow coarsening in the glassy state.

Proposed method

• Employed molecular dynamics (MD) simulations of a binary mixture with A-A and B-B interactions modeled by identical Lennard-Jones potentials.
• Introduced size polydispersity to prevent crystallization while maintaining symmetry.
• Used semi-grand canonical Monte Carlo simulations to determine coexistence temperatures and phase boundaries.
• Defined key temperatures: $T_{\rm onset}$ (onset of glassy dynamics) and $T_{\rm g,sim}$ (simulation glass transition temperature).
• Measured domain size via structure factor first moment and chord length methods to validate dynamic scaling.
• Analyzed two-time correlation functions to probe microscopic relaxation dynamics and aging behavior.

Experimental results

Research questions

• RQ1 How does the phase separation kinetics evolve across different temperature regimes, especially below the glass transition temperature?
• RQ2 What is the nature of domain growth in the glassy regime—does it follow diffusive, hydrodynamic, or a novel coarsening law?
• RQ3 To what extent is the slow coarsening in the glassy state driven by microscopic dynamics resembling aging in pure glass formers?
• RQ4 Does dynamic scaling hold for domain structures in the ultra-slow coarsening regime?
• RQ5 How does the presence of a concentration field affect microscopic relaxation compared to a homogeneous glass-forming system?

Key findings

• At $T > T_{\rm onset}$, domain growth follows hydrodynamic coarsening with $\xi \propto t^{2/3}$, indicating dominant hydrodynamic transport.
• In the regime $T_{\rm onset} > T > T_{\rm g,sim}$, a transient weak power-law growth is observed before crossover to diffusive coarsening $\xi \propto t^{1/3}$.
• Below $T_{\rm g,sim}$, domain growth becomes logarithmically slow or slower than logarithmic, with no sign of saturation within simulation times.
• Despite extreme slowdown, the structure factor exhibits dynamic scaling with a single scaling function, indicating self-similar domain morphology.
• Microscopic dynamics show strong waiting-time dependence below $T_{\rm onset}$, resembling aging in pure glass formers, but with faster relaxation than in the homogeneous case.
• The slow coarsening is decoupled from hydrodynamic effects and is instead linked to the aging-like slowdown of local dynamics, as supported by a modified Cahn-Hilliard model.