Revealing the pure confinement effect in glass-forming liquids by dynamic mechanical analysis J. Koppensteiner ∗ and W. Schranz † Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria M.A. Carpenter Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK (Dated: October 31, 2018) Abstract The dynamic mechanical response of mesoporous silica with coated inner surfaces confining the glass forming liquid salol is measured as a function of temperature and frequency (1-100 Hz) for various pore sizes (2.4-7.5 nm). Compared to former results on natural pores, a distinct acceleration of dynamics due to the removal of surface-related retardation of molecular dynamics is found now, which can be fitted by a homogeneous relaxation using an unmodified Vogel-Fulcher-Tammann relation. This lubrication effect leads to a stronger decrease of the glass transition temperature T$_{g}$ with decreasing pore size. The present data allow to quantify and separate competing side effects as surface bondings and negative pressure from the pure confinement induced acceleration of molecular dynamics with decreasing pore size. We analyze the dynamic elastic susceptibility data in terms of a recently proposed procedure [C. Dalle-Ferrier et al., Phys. Rev. E 76 , 041510 (2007)], which relates the number N$_{corr}$ of molecules that are dynamically correlated to the three-point dynamic susceptibility. Assuming that N$_{corr}$ is representative of the size ξ of a dynamically correlated region, i.e. N$_{corr}$ ∝ ξ 3 we find that ξ grows when approaching the glass transition to about 3 nm, a size which was also reported previously using other techniques. PACS numbers: 64.70.P- 61.20.Lc 62.25.-g arXiv:1001.3804v1 [cond-mat.mes-hall] 21 Jan 2010 $^{∗}$Electronic address: johannes.koppensteiner@univie.ac.at $^{†}$Electronic address: wilfried.schranz@univie.ac.at 1