We have studied the high-frequency dynamics of salol by inelastic x-ray scattering over a wide temperature range between 50 and 450 K, across the glass transition. We find that salol efficiently realizes the mechanism of dynamical arrest described by the mode-coupling theory, as manifested by a cusp singularity in the behaviour of the non-ergodicity parameter and a $Q$ dependence of the critical non-ergodicity parameter that is in phase with the static structure factor. These results confront positively the mode-coupling theory with liquids with local order.
Static granular packs have been studied in the last three decades in the frame of a modified equilibrium statistical mechanics that assumes ergodicity as a basic postulate. The canonical example on which this framework is tested consists in the series of static configurations visited by a granular column subjected to taps. By analyzing the response of a realistic model of grains, we demonstrate that volume and stress variables visit different regions of the phase space at low tap intensities in different realizations of the experiment. We show that the tap intensity beyond which sampling by tapping becomes ergodic coincides with the forcing necessary to break all particle-particle contacts during each tap. These results imply that the well-known reversible branch of tapped granular columns is only valid at relatively high tap intensities.
Grazing incidence x-ray surface scattering has been used to investigate liquid surfaces down to the molecular scale. The free surface of water is well described by the capillary wave model (<z(q)z(-q)> ~ q-2 spectrum) up to wavevectors > 10^8 m^-1. At larger wavevectors near-surface acoustic waves must be taken into account. When the interface is bounded by a surfactant monolayer, it exhibits a bending stiffness and the bending rigidity modulus can be measured. However, bending effects generally cannot be described using the Helfrich Hamiltonian and the characteristic exponent in the roughness power spectrum can smaller than 4. Finally, upon compression, tethered monolayers formed on a subphase containing divalent ions are shown to buckle in the third dimension with a characteristic wavelength on the order of 10^8 m^-1.
We present an x-ray study of freely suspended hexatic films of the liquid crystal 3(10)OBC. Our results reveal spatial inhomogeneities of the bond-orientational (BO) order in the vicinity of the hexatic-smectic phase transition and the formation of large scale hexatic domains at lower temperatures. Deep in the hexatic phase up to 25 successive sixfold BO order parameters have been directly determined by means of angular x-ray cross-correlation analysis (XCCA). Such strongly developed hexatic order allowed us to determine higher order correction terms in the scaling relation predicted by the multicritical scaling theory over a full temperature range of the hexatic phase existence.
Metallic liquid silicon at 1787K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally-localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.
We present results of temperature resolved scattering studies of a liquid crystalline block copolymer undergoing an order-disorder transition (ODT) in the presence of magnetic fields and time-resolved measurements during isothermal field annealing at sub-ODT temperatures. In each case, field interactions produced strongly textured mesophases with the cylindrical microdomains aligned parallel to the field. We find there is no measurable field-induced shift in the ODT temperature ($T_{ODT}$) which suggests that selective melting does not play a role in mesophase alignment during isothermal experiments. Our data indicate instead that sub-ODT alignment occurs by slow, large scale grain rotation whereas alignment during cooling from the disordered melt is rapid and driven by the nucleation of weakly ordered but preferentially aligned material. We identify an optimum sub-cooling that maximizes alignment during isothermal field annealing. This is corroborated by a simple model incorporating the competing effects of an exponentially decreasing mobility and divergent, increasing magnetic anisotropy on cooling below $T_{ODT}$. The absence of measurable field-effects on $T_{ODT}$ is consistent with rough estimates derived from the relative magnitudes of the free energy due to field interaction and the enthalpy of the isotropic-LC transition.
L. Comez
,D. Fioretto
,J. Gapinski
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(2019)
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"Inelastic x-ray scattering reveals the ergodic to nonergodic transition of salol, a liquid with local order"
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Daniele Fioretto
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