http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Recent advancements in experimental studies of wetting phenomena have helped to bridge the gap between the progress made in theory and simulation over the past decade, and the experimental evidence or verification of the theoretical predictions. These developments include new measurements of the equilibrium thickness of precursor wetting films on solid and liquid substrates and at the liquid/gas interface, experimental studies of critical adsorption, as well as measurements of the dynamics of wetting and spreading and the nucleation of wetting layers in simple and complex systems. There have also been some recent results on dewetting of solid substrates by liquid films.

Wetting: Statics and dynamics

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

GenX is a versatile program using the differential evolution algorithm for fitting X-ray and neutron reflectivity data. It utilizes the Parratt recursion formula for simulating specular reflectivity. The program is easily extensible, allowing users to incorporate their own models into the program. This can be useful for fitting data from other scattering experiments, or for any other minimization problem which has a large number of input parameters and/or contains many local minima, where the differential evolution algorithm is suitable. In addition, GenX manages to fit an arbitrary number of data sets simultaneously. The program is released under the GNU General Public License.

GenX: an extensible X-ray reflectivity refinement program utilizing differential evolution

Hollow-core photonic crystal fibres have excited interest as potential ultra-low loss telecommunications fibres because light propagates mainly in air instead of solid glass. We propose that the ultimate limit to the attenuation of such fibres is determined by surface roughness due to frozenin capillary waves. This is confirmed by measurements of the surface roughness in a HC-PCF, the angular distribution of the power scattered out of the core, and the wavelength dependence of the minimum loss of fibres drawn to different scales.

/pdf/ultimate-low-loss-of-hollow-core-photonic-crystal-fibres-2k1vmtnkiy.pdf

Ultimate low loss of hollow-core photonic crystal fibres.

We present the results of a high-resolution neutron diffraction experiment with a fully deuterated methane hydrate type I at temperatures of 2, 100, and 150 K. Precise crystallographic parameters of the ice-like D 2 O lattice and the thermal parameters of the encaged methane molecules have been obtained. The parameters of the host lattice differ only slightly from values found for hydrates with asymmetric guests included, which leads to the conclusion that the host lattice of structure I is only a little adaptive. At low temperatures (2 K) the methane molecules in both types of cages present in structure I occupy positions in the center of the cages. At higher temperatures the thermal parameters in both types of cages reflect the surrounding cage geometries or more precisely the translational potentials of the cages. The orientational scattering length density of the CD 4 molecules has been analyzed in terms of a multipole expansion with symmetry adapted functions [Press and Huller, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. A29, 252 (1972); Press, ibid. A29, 257 (1972)]. In both types of cages we found only small modulations of a spherically symmetric scattering density accounting for almost free rotations of the methane molecules. The large and asymmetric cage leads to a somewhat more pronounced modulation of the orientational density than in the small dodecahedral cage. The orientational probability distribution function (PDF) remains nearly unchanged from 2 to 150 K. At 200 K we observed the time-resolved decomposition of the hydrate structure I into hexagonal ice Ih.

The structure of deuterated methane–hydrate

The tunnel splitting of the librational ground state of a NH+4 ion in (NH4)2 SnCl6 has been analyzed by ’’incoherent’’ inelastic neutron scattering. Ground state splittings of 2.96±0.04 and 1.51±0.03 μeV have been observed at T=6 K and are interpreted as transitions between A?T and T?E spin symmetry states, respectively. With increasing temperature the observed tunnel splitting shifts towards smaller energies but fails to show significant lifetime broadening. The first excited librational state is observed at 13.4±0.5 meV. An excitation with E?30 meV probably corresponds to the second excited librational state. Above 70 K classical reorientational motion gives rise to quasielastic scattering. From the temperature dependence of the quasielastic linewidth an activation energy EA=590±30 K is obtained. A three dimensional rotational potential is discussed in view of the above data. The temperature dependence of the tunneling states is compared with current theories.

Rotational states of the NH+4 ion in (NH4)2SnCl6 by inelastic neutron scattering

In this paper it is shown that diffuse-scattering experiments within the region of total external reflection can be explained quantitatively using the distorted-wave Born approximation for layer systems. Three Si/Ge samples with different degrees of complexity were investigated. The simultaneous analysis of the specular reflected intensity and the diffuse scattering leads to one consistent set of interface and layer parameters, which is able to fit both the shapes and the locations of all dynamic peaks in the off-specular scans and the characteristics of the reflected intensity. Therefore the distorted-wave Born approximation seems to give a correct and complete description of the diffuse scattering in the region of total external reflection.

/pdf/x-ray-diffraction-from-si-ge-layers-diffuse-scattering-in-1wbp9wk0pb.pdf

X-ray diffraction from Si/Ge layers: Diffuse scattering in the region of total external reflection.

The thermal conductivity of a gas hydrate is much lower than that of ice and more intriguingly, in spite of the crystalline structure, the temperature profile resembles an amorphous solid. Here we present results from neutron incoherent inelastic scattering (IINS) experiments and theoretical relaxation time calculations to demonstrate unambiguously the coupling of guest and water framework vibrations. The coupling is the consequence of symmetry forbidden crossings of the localized guest motions with the acoustic branches of the lattice vibrations. This phonon interaction leads to an energy exchange mechanism that explains the anomalous thermal behaviour.

Coupling of localized guest vibrations with the lattice modes in clathrate hydrates

The anomalous glass-like thermal conductivity of crystalline clathrates has been suggested to be the result of the scattering of thermal phonons of the framework by ‘rattling’ motions of the guests in the clathrate cages. Using the site-specific 83Kr nuclear resonant inelastic scattering spectroscopy in combination with conventional incoherent inelastic neutron scattering and molecular-dynamics simulations, we provide unambiguous evidence and characterization of the effects on these guest–host interactions in a structure-II Kr clathrate hydrate. The resonant scattering of phonons led to unprecedented large anharmonic motions of the guest atoms. The anharmonic interaction underlies the anomalous thermal transport in this system. Clathrates are prototypical models for a class of crystalline framework materials with glass-like thermal conductivity. The explanation of the unusual molecular dynamics has a wide implication for the understanding of the thermal properties of disordered solids and structural glasses.

Werner Press

Papers

The structure of deuterated methane–hydrate

Rotational states of the NH+4 ion in (NH4)2SnCl6 by inelastic neutron scattering

X-ray diffraction from Si/Ge layers: Diffuse scattering in the region of total external reflection.

Coupling of localized guest vibrations with the lattice modes in clathrate hydrates

Anharmonic motions of Kr in the clathrate hydrate.