  Collective motion over increasing length scales is a signature of the
vitrification process of liquids. We demonstrate the emergence of distinct
static and dynamic length scales probed near the free surface in fully
equilibrated glass-forming liquid films, and their connection to the bulk
properties of the system. In contrast to a monotonically growing static
correlation length, the dynamic correlation length that measures the extent of
surface-dynamics acceleration into the bulk, displays a striking non-monotonic
temperature evolution that is robust also against changes in detailed
interatomic interaction. The maximum of dynamic correlations defines a
cross-over temperature $T_*$ that we show to agree with a morphological change
of cooperative rearrangement regions (CRR) of fast particles in the bulk
liquids. The cross-over occurs at a temperature larger than the critical
temperature Tc of mode-coupling theory (MCT). We link it to the point where
fast-particle motion decouples from structural relaxation that can be defined
rigorously within a recent extension of MCT, the stochastic $\beta$-relaxation
theory (SBR).

Statistical Mechanics