Confined viscous liquids: Interfacial versus finite size effects

Confining a supercooled liquid to spaces of severalnanometer in diameter can lead to dramatic changes in the relaxationdynamics of the material. In many cases, the effect is reported as aconfinement induced shift of the glass transition temperature T_g. Both positive and negative values for ΔT_g have been observed and the length scale of the confininggeometry is considered the main variable. We review the dynamics ofglass-forming liquids in both hard and soft confinement of <10 nm spaces, with focus on results from solvation dynamics experiments.It is shown that the interface is instrumental in determinig the dynamics, giving rise to reaxation time gradients across the cooperativity length scale of the liquid. Depending on the interfacial conditions, dynamics can become faster or slower for the same liquid, same size of confinement, and identical experimental technique used. No indications of true finite size effects are observed, and the pore or droplet size is relevant only indirectly through the relative number of molecules near the surface.