Dielectric investigations of the dynamic glass transition in nanopores

Arndt, M. B.; Stannarius, Ralf; Gorbatschow, W.; Kremer, Friedrich

doi:10.1103/physreve.54.5377

Cited by 226 publications

(316 citation statements)

References 48 publications

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“…The idea is that there is an immobile particle layer which is bound to the surface (slow process) and the remaining liquid is then often accelerated with respect to the bulk [24,26,30,36,37]. A systematic study of supported and free standing polymer films by ellipsometry [40,41,42,43] supports this interpretation.…”

Section: Introductionsupporting

confidence: 50%

“…Depending on the investigated liquid, the realization of the confinement and even the experimental method, there is a great variety in experimental findings. Some give evidence for an accelerated dynamics in confinement [21,25,26,29,30,36], i.e. the glass transition temperature T g goes down, while others see an increase of T g [24,37,38,39].…”

Section: Introductionmentioning

confidence: 99%

“…One large class of experiments are done on molecular liquids (such as salol, glycol, ortho-terphenyl) confined to porous material such as Vycor glass, sol-gel glass and zeolithes with pore sizes ranging from several hundred to only a few nanometer. Typical experimental methods are differential scanning calorimetry (DSC) [21,22,23], dielectric spectroscopy [24,25,26,27,28,29], neutron scattering [30,31], solvation dynamics [32,33,34] and many more.…”

Section: Introductionmentioning

confidence: 99%

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The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

2004

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We present the results of molecular dynamics computer simulations of a binary Lennard-Jones liquid confined between two parallel rough walls. These walls are realized by frozen amorphous configurations of the same liquid and therefore the structural properties of the confined fluid are identical to the ones of the bulk system. Hence this setup allows us to study how the relaxation dynamics is affected by the pure effect of confinement, i.e. if structural changes are completely avoided. We find that the local relaxation dynamics is a strong function of z, the distance of the particles from the wall, and that close to the surface the typical relaxation times are orders of magnitude larger than the ones in the bulk. Because of the cooperative nature of the particle dynamics, the slow dynamics also affects the dynamics of the particles for large values of z. Using various empirical laws, we are able to parameterize accurately the z−dependence of the generalized incoherent intermediate scattering function Fs(q, z, t) and also the spatial dependence of structural relaxation times. These laws allow us to determine various dynamical length scales and we find that their temperature dependence is compatible with an Arrhenius law. Furthermore, we find that at low temperatures time and space dependent correlation function fulfill a generalized factorization property similar to the one predicted by mode-coupling theory for bulk systems. For thin films and/or at sufficiently low temperatures, we find that the relaxation dynamics is influenced by the two walls in a strongly non-linear way in that the slowing down is much stronger than the one expected from the presence of only one confining wall. Finally we study the average dynamics of all liquid particles and find that the data can be described very well by a superposition of two relaxation processes that have clearly separated time scales. Since this is in contrast with the result of our analysis of the local dynamics, we argue that a correct interpretation of experimental data can be rather difficult.

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Section: Introductionsupporting

confidence: 50%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

2004

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“…Indeed, evidence of fluidity at the molecular level is mounting. A divergence of molecular relaxation times was not observed in dielectric measurements, neither in pores [20] nor in a sandwich geometry [21,22]. Loss of translational fluidity was not observed in molecular dynamics computer simulations (translational diffusion parallel to confining boundaries slowed by only 1-2 orders of magnitude) [23,24], nor in tracer diffusion experiments (translational diffusion slowed by only 2 -4 orders of magnitude) [2].…”

Section: Prl 93 236105 (2004) P H Y S I C a L R E V I E W L E T T E mentioning

confidence: 87%

How Confined Lubricants Diffuse During Shear

et al. 2004

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The translational diffusion of a fluorescent dye embedded at a dilute concentration in a confined fluid was compared at rest and during shear. The fluid, octamethylcyclotetrasiloxane (OMCTS), was confined between step-free muscovite mica to thickness 3-4 layers. Fluorescence correlation spectroscopy showed that the time scales of intensity-intensity autocorrelation functions were essentially the same during shear and at rest, except they were faster during shear by a factor of 2 to 5. This dynamical probe of how liquids order in molecularly thin films fails to support the hypothesis that shear produced a melting transition. DOI: 10.1103/PhysRevLett.93.236105 PACS numbers: 68.35.Af, 68.08.-p A curious fact about liquids is that they wrap themselves around solid surfaces to form layers; the simple reason is that space must be filled. Those molecules closest to the surface form the best-defined layer but the number of layers is believed to extend several molecular dimensions, to a distance normal to the surface approximately the correlation length of the bulk fluid. The layered structures on opposed surfaces interfere with one another when the spacing between two solids is less than twice this length [1]. Questions about confined fluid structure involve deep scientific puzzles. On the applied side, they also pertain directly to understanding the physics of porous media, of dense suspensions of colloidal particles in fluids, and of friction and lubrication.Here we employ a fluorescence-based method to contrast the rate of Brownian diffusion during shear and at rest. Previously we introduced a method of few-molecule fluorescence to study confined fluids [2,3] but that study was restricted to fluids at rest. Here we describe what happens while sliding one surface past the other. The premise was that if the hypothesis of shear melting held, shear should induce speed up of the translational diffusion of a nonadsorbing fluorescent dye.The fluid, containing dilute dye, was confined to a thickness of 3-4 molecular dimensions between parallel single crystals of muscovite mica. A large amount of prior study probed the friction of such systems. The hypothesis of a shear-induced phase transition, from solid to fluid, has been advanced based on the observation that, in most such experiments, static friction gives way to kinetic friction in molecularly thin liquid films [4]. The generality of this interpretation has been cast into doubt by the observation that fluids confined between rough metal surfaces behave similarly [5] and especially by recent experiments showing that the solidity of molecularly thin films in mica-based experiments depends on a particular method of surface preparation [6,7]. As arguments about experimental protocols are surely of no interest outside a specialized community, it is opportune to address the important question of order in confined fluids by an independent technique. Fluorescence correlation spectroscopy (FCS) is usually used to study molecules dissolved in bulk solution [8,9]. As implement...

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“…In particular, the behaviour of water confined in nanometer-sized volumes has drawn the attention of several studies over the last decades, in view of its central importance in biological, geological and technological contexts [5,6].…”

Section: Introductionmentioning

confidence: 99%