ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Confinement effects on phase behaviour of amorphous solids

A full understanding of the slowing down of transport upon approaching the glass transition is one of the grand challenges of condensed matter theory. A recent focus has been to introduce competing mechanisms that lead to glass transition phase diagrams exhibiting non-monotonic behaviour. Indeed, quite new theoretical work predicts that if a hard sphere glass is sandwiched between two planar hard walls, its glass transition may be an oscillatory function of the distance between the walls. Such a prediction is quite interesting as is means that a purely geometric effect may be used to induce the transition from the liquid to the amorphous solid state.

Top: The static structure factor of a polydisperse hard sphere system at a constant packing fraction of 0.49 for different wall-to-wall separations as indicated. The inset shows the corresponding density profile. Bottom: The liquid-glass phase diagram. The symbols are constant-diffusivity lines. Note that the phase transition line (solid line with closed circles) corresponds to zero diffusion constant.

In order to test this theoretical prediction, we have performed event driven molecular dynamics simulations of a model hard sphere glass confined between two planar walls. The confinement introduces a new length scale competing with the local near ordering for plate separations comparable to the diameter of the particles. We evaluate the mean-square displacements and the diffusion coefficient to assess the slowing down of dynamics in confinement and to establish a glass transition phase diagram. For all packing fractions investigated, we find non-monotonic variations of diffusion coefficient with wall-to-wall separation. Extrapolating this data to the limit of vanishing diffusion coefficient, we extract the phase diagram for the glass transition of the confined model (Fig.2). The results obtained from our simulations are qualitatively in line with theoretical predictions. These findings may used to tailor properties of amorphous solids without any change of the composition or chemistry but only by changing the geometric confinement.

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