Finally, we take full advantage of the additional information that is available in single-molecule approaches by selecting a population of ≈70 molecules and following the rotational diffusion of each individual one as a function of temperature. We then measure individual molecules at various temperatures slightly above T g (190 K), following the experimental scheme of Deschenes and Vanden Bout ( 10). For small ensembles of typically 10–30 molecules, fluctuations of the anisotropy provide an average rotational diffusion time ( 27, 28). We first characterize the system at the ensemble level by fluorescence anisotropy correlation spectroscopy (FACS). Because of its intermolecular hydrogen bonds, glycerol is a stronger glass former than the fragile polymers and molecular glass formers investigated previously by single-molecule techniques. Here, we study the relaxation of supercooled glycerol above and close to T g (190 K) by analyzing the rotational diffusion of fluorophores at the ensemble and single-molecule levels. The nature of boundaries between liquid domains and the mechanism of the exchanges remain mysterious. Deschenes and Vanden Bout ( 10) view the supercooled liquid as a mosaic of heterogeneous regions, each of which behaves as a homogeneous liquid. At longer times, single-molecule tumbling rates were found to undergo sudden jumps, possibly indicating environmental exchanges. This result suggests that spatial inhomogeneities are the main source of nonexponential relaxation. ( 15) for the glass-forming polymer poly(methyl acrylate), is that reorientation times are broadly distributed from molecule to molecule, with nearly single-exponential correlation functions for each single molecule on a short time scale. Their surprising result, confirmed by Schob et al. Deschenes and Vanden Bout ( 10) have studied rotational diffusion of single fluorophores in the molecular liquid ortho-terphenyl. Each individual fluorophore is a local probe for Brownian motion and relaxation of the glass-forming host in its immediate surroundings. Its fluctuations are a signature of rotational diffusion of the probe. Linear dichroism, which characterizes the ratio of fluorescence intensities emitted by a single molecule on two orthogonal polarizations, is followed in real time. Single-molecule fluorescence is a well established method for studying rotational diffusion ( 23– 26). NMR and dielectric relaxation point to times comparable to the molecular rotation times ( 16, 22), whereas light scattering and fluorescence show inhomogeneities with exceedingly slow relaxation ( 9, 12, 19– 21). The typical mechanisms, length scales, and time scales of environmental exchanges remain largely unclear. Heterogeneous regions with different relaxation dynamics are therefore expected to exchange with one another to restore ergodicity, by processes called environmental exchanges ( 6, 9, 10, 12, 14– 16). These observations have to be reconciled with the common view of a supercooled liquid, which is supposed to remain a normal ergodic liquid until the glass transition. Inhomogeneities have been found in the particular case of supercooled glycerol by dielectric hole burning ( 16), NMR ( 17), x-ray ( 18), light scattering ( 19), and stimulated Brillouin gain spectroscopy experiments ( 20, 21). Indeed, spatially inhomogeneous dynamics ( 5) have been observed with a variety of techniques in many glass formers, including simple liquids ( 6– 10) and polymers ( 11– 15). In their view, glass formation arises from dynamical arrest of cooperative domains, and their spread in sizes leads to an overall nonexponential relaxation. The necessity for cooperative motion at larger and larger scales on approaching the glass transition was recognized a long time ago by Adam and Gibbs ( 4).
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