Dynamic decoupling and local atomic order of a model multicomponent metallic glass-former

Machine learning predicts the glass transition of two-dimensional colloids besides medium-range crystalline order

We employ only the positions of colloidal particles and construct machine learning (ML) models to test the presence of structural order in glass transition for two kinds of two-dimensional (2D) colloids: 2D polydisperse colloids (PC) with medium-range crystalline order (MRCO) and 2D binary colloids (BC) without MRCO. ML models predict the glass transition of 2D colloids successfully without any information on MRCO. Even certain ML models trained with BC predict the glass transition of PC successfully, thus suggesting that universal structural characteristics would exist besides MRCO.

Dynamics of highly polydisperse colloidal suspensions as a model system for bacterial cytoplasm

There are various kinds of macromolecules in bacterial cell cytoplasm. The size polydispersity of the macromolecules is so significant that the crystallization and the phase separation could be suppressed, thus stabilizing the liquid state of bacterial cytoplasm. On the other hand, recent experiments suggested that the macromolecules in bacterial cytoplasm should exhibit glassy dynamics, which should be also affected significantly by the size polydispersity of the macromolecules. In this work, we investigate the anomalous and slow dynamics of highly polydisperse colloidal suspensions, of which size distribution is chosen to mimic Escherichia coli cytoplasm. We find from our Langevin dynamics simulations that the diffusion coefficient (D_{tot}) and the displacement distribution functions (P(r,t)) averaged over all colloids of different sizes do not show anomalous and glassy dynamic behaviors until the system volume fraction ϕ is increased up to 0.82. This indicates that the intrinsic polydispersity of bacterial cytoplasm should suppress the glass transition and help maintain the liquid state of the cytoplasm. On the other hand, colloids of each kind show totally different dynamic behaviors depending on their size. The dynamics of colloids of different size becomes non-Gaussian at a different range of ϕ, which suggests that a multistep glass transition should occur. The largest colloids undergo the glass transition at ϕ=0.65, while the glass transition does not occur for smaller colloids in our simulations even at the highest value of ϕ. We also investigate the distribution (P(θ,t)) of the relative angles of displacement for macromolecules and find that macromolecules undergo directionally correlated motions in a sufficiently dense system.

Tracer Shape and Local Media Structure Determine the Trend of Translation-Rotation Decoupling in Two-Dimensional Colloids

The translational diffusion of tracers in glass-forming materials often violates the Stokes-Einstein relation while their rotation follows the Debye-Stokes-Einstein relation faithfully, thus decoupling translational and rotational diffusion. In this Letter, we show by performing molecular dynamics simulations for two-dimensional (2D) colloids that the tracer shape and the local media structure are critical such that rotational diffusion is either suppressed or enhanced depending on the tracer shape. For square tracers dissimilar in structure to the local media structure of 2D colloids, the translation-rotation decoupling occurs and the rotational diffusion is enhanced relative to the translation. For sufficiently large diamond tracers similar in structure to the local media structure, tracers undergo rotational hopping motions and their rotation is suppressed relative to the translation. For distorted-diamond tracers, the decoupling is marginal. Translational diffusion does not change significantly with the tracer shape and obeys the Stokes-Einstein relation.