Molecular Dynamics Simulation of the Capillary Leveling of a Glass-Forming Liquid

Abstract

Motivated by recent experimental studies probing i) the existence of a mobile layer at the free surface of glasses, and ii) the capillary leveling of polymer nanofilms, we study the evolution of square-wave patterns at the free surface of a generic glass-forming binary Lennard-Jones mixture over a wide temperature range, by means of molecular dynamics simulations. The pattern's amplitude is monitored and the associated decay rate is extracted. The evolution of the latter as a function of temperature exhibits a crossover between two distinct behaviours, over a temperature range typically bounded by the glass-transition temperature and the mode-coupling critical temperature. Layer-resolved analysis of the film particles' mean-squared displacements further shows that diffusion at the surface is considerably faster than in the bulk, below the glass-transition temperature. The diffusion coefficient of the surface particles is larger than its bulk counterpart by a factor that reaches 10 5 at the lowest temperature studied. This factor decreases upon heating, in agreement with recent experimental studies.

Heterogeneous Glass Transition Behavior of Poly(Ethylene oxide)/Silica Nanocomposites via Atomistic MD Simulations

“…This behavior implies that dynamic heterogeneities tend to disappear upon heating the system well above its T g . This trend lies in accordance with findings from other experimental and simulation studies in thin polymer films and binary Lennard-Jones liquids. − …”

UsingMwDependence of Surface Dynamics of Glassy Polymers to Probe the Length Scale of Free-Surface Mobility

“…Experimental studies of the surface relaxation of glassy films have been performed to provide a more quantitative picture, , but none of these have yet been able to quantitatively provide the size of the liquid-like layer, h *. However, we note that experiments and numerical simulations on the surface leveling of glassy films provide a quantitative determination of the surface mobility h */3η, where η is the viscosity of the liquid-like layer. A key step in improving our understanding of the surface properties of glassy polymers is to find a way to independently determine h * and η.…”

Non-monotonic dynamic correlations beneath the surface of glass-forming liquids

“…This opens an interesting link between the study of such films and our fundamental understanding of the glass transition in the bulk, in analogy to the situation that the diverging width of a gas-liquid interface at its critical point is governed by the same diverging correlation length in the bulk [19]. Furthermore the free-surface dynamics is an important factor in the preparation of glassy films [20][21][22], and more specifically ultra-stable glasses [23][24][25][26][27] and nanostructured materials [28] that are produced by depositing atoms layer-by-layer on an amorphous substrate.…”