In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

https://iopscience.iop.org/article/10.1088/0953-8984/17/15/R01/pdf

Effects of confinement on material behaviour at the nanometre size scale

We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials) The most commonly used molecular simulation, theoretical and experimental methods are first presented We also provide a brief description of the most widely used porous materials The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed We also address how confinement affects the glass transition

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Effects of confinement on freezing and melting.

As one increases the concentration of a colloidal suspension, the system exhibits a dramatic increase in viscosity. Beyond a certain concentration, the system is said to be a colloidal glass; structurally, the system resembles a liquid, yet motions within the suspension are slow enough that it can be considered essentially frozen. For several decades, colloids have served as a valuable model system for understanding the glass transition in molecular systems. The spatial and temporal scales involved allow these systems to be studied by a wide variety of experimental techniques. The focus of this review is the current state of understanding of the colloidal glass transition, with an emphasis on experimental observations. A brief introduction is given to important experimental techniques used to study the glass transition in colloids. We describe features of colloidal systems near and in glassy states, including increases in viscosity and relaxation times, dynamical heterogeneity and ageing, among others. We also compare and contrast the glass transition in colloids to that in molecular liquids. Other glassy systems are briefly discussed, as well as recently developed synthesis techniques that will keep these systems rich with interesting physics for years to come.

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The Physics of the Colloidal Glass Transition

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance. The metastability of these systems, which gives rise to various rearrangement processes at elevated temperatures, calls for an understanding of their diffusional behavior. From the fundamental point of view, these metallic glasses are the paradigm of dense random packing. Since the recent discovery of bulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid state and to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article the authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. They consider in detail the experimental techniques, the temperature dependence of diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, the isotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown that diffusion in metallic glasses is significantly different from diffusion in crystalline metals and involves thermally activated, highly collective atomic processes. These processes appear to be closely related to low-frequency excitations. Similar thermally activated collective processes were also found to mediate diffusion in the supercooled liquid state well above the caloric glass transition temperature. This strongly supports the mode-coupling scenario of the glass transition, which predicts an arrest of liquidlike flow already at a critical temperature well above the caloric glass transition temperature.

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Diffusion in Metallic Glasses and Supercooled Melts

Supercooled liquids and glasses are important for current and developing technologies. Here we provide perspective on recent progress in this field. The interpretation of supercooled liquid and glass properties in terms of the potential energy landscape is discussed. We explore the connections between amorphous structure, high frequency motions, molecular motion, structural relaxation, stability against crystallization, and material properties. Recent developments that may lead to new materials or new applications of existing materials are described.

Perspective: Supercooled liquids and glasses

Using molecular-dynamics simulations we investigate the relaxation dynamics of a supercooled liquid close to a rough as well as to a smooth wall. For the former situation the relaxation times increase strongly with decreasing distance from the wall, whereas in the second case they strongly decrease. We use this dependence to extract various dynamical length scales and show that they grow with decreasing temperature. Finally, we calculate the frequency-dependent average susceptibility of such confined systems and show that they compare very well with experimental data.

http://iopscience.iop.org/article/10.1209/epl/i2002-00182-9/pdf

Cooperative motion and growing length scales in supercooled confined liquids

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 parametrize accurately the z dependence of the generalized incoherent intermediate scattering function Fs(...

The relaxation dynamics of a supercooled liquid confined by rough walls

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 F_s(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. ....

http://arxiv.org/pdf/cond-mat/0309025.pdf

We use molecular dynamics computer simulations to investigate the relaxation dynamics of a binary Lennard-Jones liquid confined in a narrow pore. We find that this dynamics is strongly influenced by the confinement in that time correlation functions are much more stretched than in the bulk. By investigating the motion of the particles as a function of their distance from the wall, we can show that this stretching is due to a spatial heterogeneity in the dynamics in that the relaxation time depends strongly on this distance.

The relaxation dynamics of a simple glass former confined in a pore

We use molecular-dynamics computer simulations to study the relaxation dynamics of a confined simple liquid. Two types of confining walls are considered: A rough wall and a smooth wall. The simulation is set up in such a way that the static properties of the confined system are identical to the ones of the bulk. Nevertheless, we find that upon cooling the relaxation dynamics of the confined systems differ strongly from the one of the bulk. In particular, we find that close to the rough/smooth wall this dynamics is slowed down/accelerated by orders of magnitude. Using these results we are able to extract a dynamical length scale of the system and we show that this length shows an Arrhenius dependence.

Peter Scheidler

Papers

Cooperative motion and growing length scales in supercooled confined liquids

The relaxation dynamics of a supercooled liquid confined by rough walls

The relaxation dynamics of a supercooled liquid confined by rough walls

The relaxation dynamics of a simple glass former confined in a pore

The relaxation dynamics of a confined glassy simple liquid.