Structural origin of enhanced slow dynamics near a wall in glass-forming systems

摘要

Spatial confinement is known to induce a drastic change in the viscosity, relaxation times, and flow profile of liquids near the glass (or jamming) transition point. The essential underlying question is how a wall affects the dynamics of densely packed systems. Here we study this fundamental problem, using experiments on a driven granular hard-sphere liquid and numerical simulations of polydisperse and bidisperse colloidal liquids. The nearly hard-core nature of the particle-wall interaction provides an ideal opportunity to study purely geometrical confinement effects. We reveal that the slower dynamics near a wall is induced by wall-induced enhancement of 'glassy structural order', which is a manifestation of strong interparticle correlations. By generalizing the structure-dynamics relation for bulk systems, we find a quantitative relation between the structural relaxation time at a certain distance from a wall and the correlation length of glassy structural order there. Our finding suggests that glassy structural ordering may be the origin of the slow glassy dynamics of a supercooled liquid. We expect that when a densely packed system (even a crowd of people) is confined by a rigid wall, the local mobility near the wall becomes significantly lower than that near the centre. Our intuition tells us that motion towards the wall may be severely suppressed. However, the very origin of this phenomenon is not understood very clearly. This problem has been a long-standing subject of research, and is now attracting renewed interest because of its relevance to nanomaterials synthesis~1, nanotribology~2, micro- and nanofluidics~(3,4), and thin-film technology. An important example is a dense liquid (for example, a supercooled liquid) confined to nanoscale dimensions~(5,6). Recent fabrication techniques of thin films and nanoporous materials have enabled detailed experimental investigations of this problem and revealed a shift in the glasstransition temperature T_g for various liquids~(7-11).