## I. INTRODUCTION 

Glass-forming materials have been produced by mankind for more than 6000 years. Despite several decades of intense research the transition of a liquid into its glassy state is still lacking a universal theory explaining both the increase of viscosity η and molecular relaxation rates by 14 orders of magnitude [1, 2] without creating any long range order. A widely used explanation which goes back to Adam and Gibbs [3] is based on the assumption of cooperative rearrangement of molecules ("cooperative rearranging regions, CRR"), forming compact clusters of a typical size ξ . Such a subsystem of molecules can rearrange into another configuration independently of its environment. 

The size of these groups of molecules is considered to grow to some nm as T$_{g}$ is approached [4]. E.g. random first order transition theory of glasses [5, 6] predicts ξ = r$_{0}$ 0 . 51 ( ln τ τ $_{0}$) 2 3 . At T$_{g}$ where τ ≈ 100 s one obtains for typical values of τ$_{0}$ ≈ 10 − 12 s and r$_{0}$ ≈ 1 nm, ξ ( T$_{g}$ ) ≈ 5 nm. Measuring such dynamic heterogeneities is one of the most important but at the same time difficult issues in the field of glass formation. Several experimental setups like specific heat spectroscopy [4, 7], multidimensional NMR [8–10] and non-linear dynamic susceptibility measurements [11], etc. have been used to determine a possible growing length scale accompanying the glass transition. All these results agree in the fact, that the obtained cooperative regions are of the order of several nm near T$_{g}$ , and are displaying a weak temperature dependence [12]. Recent computer simulations also confirm a rather modest growth of the dynamically correlated regions when approaching the glass transition from above [13]. Very recently in a groundbreaking work Biroli et al. [14] found direct evidence for a growing dynamical length in supercooled liquids by applying inhomogeneous mode-coupling theory. Based on this theory these authors have recently developed a method to quantify the size of the correlated regions [11] by analyzing three-point dynamic susceptibilites. For a large number of supercooled liquids they could indeed confirm growing of the correlated regions when approaching the glass transition [15]. 

However, direct measurements of ξ are extremely difficult, sometimes even impossible and one has to resort to indirect investigations. A widely used approach is confining the glass-forming liquid spatially, either in the form of thin films or by using mesoporous 

