We have formulated a quantitative, force-level theory of activated relaxation in real space that covers in a unified manner the apparent Arrhenius, dynamic crossover and deeply supercooled regimes. The irreversible re-arrangement event has a mixed local-nonlocal spatial character and involves coupled cage-scale large amplitude hopping and a longer range collective elastic distortion of the surrounding liquid. This results in two inter-related, but physically distinct, contributions to the activation barrier. At relatively high temperatures the barrier determined by local packing constraints is dominant. With sufficient cooling or densification, the barrier associated with the spontaneous elastic fluctuation required to sterically allow large amplitude hopping dominates the increase of the alpha relaxation time due to a growing microscopic jump distance and emergent elasticity in the cold liquid. Connections between short and long time dynamics are predicted. To treat laboratory molecular liquids, a quasi-universal approach is constructed based on an a priori mapping of the specific chemical system to an effective hard sphere fluid with an identical amplitude of long wavelength density fluctuations (dimensionless compressibility). The theory is devoid of fit parameters, and captures the key features of the alpha time over 14 decades for diverse molecular liquids. Free-standing thin films are described by accounting for both the reduction of caging constraints due to nearest neighbor loss close to a vapor interface and suppression of collective elastic effects due to geometric confinement and interfacial softening. The spatial gradient of the relaxation time is predicted as a function of film thickness and temperature, and its consequences for experiments which probe accelerated relaxation, surface diffusion, mobile layers and film-averaged glass transition temperatures are established.<br/> <br/>

<b>Award Address </b>(Joel Henry Hildebrand Award in the Theoretical and Experimental Chemistry of Liquids sponsored by ExxonMobil Research and Engineering Company). Collective elasticity mediated activated relaxation in supercooled molecular liquids and thin films

251st National Meeting (2016)

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Award Address (Joel Henry Hildebrand Award in the Theoretical and Experimental Chemistry of Liquids sponsored by ExxonMobil Research and Engineering Company). Collective elasticity mediated activated relaxation in supercooled molecular liquids and thin films