Non-monotonic effect of confinement on the glass transition

Abstract

The relaxation dynamics of glass forming liquids and their structure are influenced in the vicinity of confining walls. This effect has mostly been observed to be a monotonic function of the slit width. Recently, a qualitatively new behaviour has been uncovered by Mittal and coworkers, who reported that the single particle dynamics in a hard-sphere fluid confined in a planar slit varies in a non-monotonic way as the slit width is decreased from five to roughly two particle diametres (Mittal et al 2008 Phys. Rev. Lett. 100 145901). In view of the great potential of this effect for applications in those fields of science and industry, where liquids occur under strong confinement (e.g. nano-technology), the number of researchers studying various aspects and consequences of this non-monotonic behaviour has been rapidly growing. This review aims at providing an overview of the research activity in this newly emerging field. We first briefly discuss how competing mechanisms such as packing effects and short-range attraction may lead to a non-monotonic glass transition scenario in the bulk. We then analyse confinement effects on the dynamics of fluids using a thermodynamic route which relates the single particle dynamics to the excess entropy. Moreover, relating the diffusive dynamics to the Widom's insertion probability, the oscillations of the local dynamics with density at moderate densities are fairly well described. At high densities belonging to the supercooled regime, however, this approach breaks down signaling the onset of strongly collective effects. Indeed, confinement introduces a new length scale which in the limit of high densities and small pore sizes competes with the short-range local order of the fluid. This gives rise to a non-monotonic dependence of the packing structure on confinement, with a corresponding effect on the dynamics of structural relaxation. This non-monotonic effect occurs also in the case of a cone-plate type channel, where the degree of confinement varies with distance from the apex. This is a very promising issue for future research with the possibility of uncovering the existence of alternating glassy and liquid-like domains.

Complex dynamics induced by strong confinement – From tracer diffusion in strongly heterogeneous media to glassy relaxation of dense fluids in narrow slits

“…This multiple reentrance is attributed to a complex competition between the layering induced by the walls and the local caging. This scenario has been corroborated by event-driven molecular dynamics simulations for slightly polydisperse hard-sphere systems [115,117] upon measuring the self-diffusion coefficients parallel to the walls.…”

“…Here we do not repeat the equations of motion but refer to a recent review [117], rather we focus on the nonequilibrium state diagram.…”

Anisotropic Dynamics of Binary Particles in Confined Geometries

“…In recent years, those fundamental microscopic theories that can describe dynamic responses in inhomogeneous fluids have begun to emerge . In particular, the generalized Langevin and model‐coupling theories have recently provided some important insights into these issues .…”

Diverging Time Scale in the Dimensional Crossover for Liquids in Strong Confinement

“…There, the simplest geometry to investigate the effects of strong confinement is a slit where fluid particles are restricted to a narrow space between two smooth parallel plates, but also tubes or spherical confinements have been realized experimentally [24][25][26]. Computer simulations and experiments for the planar confinement have revealed an exotic equilibrium phase behavior due to commensurable stacking [27][28][29][30][31][32][33][34] as well as the hexatic phases in the limit of quasi-2D confinement [35,36]. Confinement induced order-disorder phase transitions for certain nonpolar liquids have also been reported in several experiments [37], but the interpretation has been challenged in favor of a glass transition [38][39][40].…”

“…For instance, the role of local order has been elucidated within a remarkable empirical scaling of the diffusivity or structural relaxation times with the excess entropy [43][44][45][46]. Complementarily, a microscopic theory for the dynamics in confinement is the mode-coupling theory that predicts a multiple-reentrant in a glassy phase as the plate separation is varied [33,47,48] From a more fundamental point of view one would like to know how the dimensional crossover from a 3D bulk liquid to a (quasi-)2D system occurs. For the thermodynamic and structural properties it has been shown recently that a small parameter emerges such that the convergence to a 2D system including leading corrections can be proven [49].…”