Diffusion in ordinary and supercooled liquids

Summary We discuss a simplified approach for the recollision part of the memory function for self-motion in ordinary and supercooled liquids. This part accounts for the substantial decrease of the diffusion coefficient with respect to the binary prediction, an effect which becomes larger and larger on cooling the system. The quantitative results compare satisfactorily with previous and new simulation data obtained for a liquid rubidium model both near the triple point and in moderately supercooled states. Paper presented at the workshop ?Highlights on Simple Liquids”, held in Turin at ISI on 1–3 May, 1989.     

The stability curve for short wavelength density fluctuations of deuterium in niobium

By means of coherent neutron scattering the temperature dependence of density fluctuations of deuterium in niobium has been measured for three concentrations (2.3, 9.8, and 18% D/Nb). Depending on the scattering vector, the extrapolation to the stability curve for these fluctuations yields temperatures, which are from 2 to 7 times lower than the stability curve for macroscopic fluctuations. Both suppression and anisotropy of the short wavelength stability curves are attributed to the existence of elastic interaction between deuterium atoms.     

Viscoelasticity and metastability limit in supercooled liquids

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter lambda, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where lambda is larger than a critical value lambda(c) the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if lambda相似文献   

Hydrophobic nanoconfinement suppresses fluctuations in supercooled water

We perform very efficient Monte Carlo simulations to study the phase diagram of a water monolayer confined in a fixed disordered matrix of hydrophobic nanoparticles between two hydrophobic plates. We consider different hydrophobic nanoparticle concentrations c. We adopt a coarse-grained model of water that, for c = 0, displays a first-order liquid-liquid phase transition (LLPT) line with negative slope in the pressure-temperature (P-T) plane, ending in a liquid-liquid critical point at about 174 K and 0.13 GPa. We show that upon increase of c the liquid-gas spinodal and the temperature of the maximum density line are shifted with respect to the c = 0 case. We also find dramatic changes in the region around the LLPT. In particular, we observe a substantial (more than 90%) decrease of isothermal compressibility, thermal expansion coefficient and constant-pressure specific heat upon increasing c, consistent with recent experiments. Moreover, we find that a hydrophobic nanoparticle concentration as small as c = 2.4% is enough to destroy the LLPT for P ≥ 0.16 GPa. The fluctuations of volume apparently diverge at P ≈ 0.16 GPa, suggesting that the LLPT line ends in an LL critical point at 0.16 GPa. Therefore, nanoconfinement reduces the range of P-T where the LLPT is observable. By increasing the hydrophobic nanoparticle concentration c, the LLPT becomes weaker and its P-T range smaller. The model allows us to explain these phenomena in terms of a proliferation of interfaces among domains with different local order, promoted by the hydrophobic effect of the water-hydrophobic-nanoparticle interfaces.     

Pulsed dielectric spectroscopy of supercooled liquids

Pulsed dielectric spectroscopy is introduced as a technique for selectively emphasizing specific components of the non-exponential dielectric response of matter. Samples studied include supercooled liquid propanol, propylene carbonate, and poly(lauryl-methacrylate). It is shown that particular sequences of pulses can be used to emphasize the fast response regime, to produce a cross-over or memory effect, or to eliminate the response of selected components. Furthermore, for materials characterized by broad distributions of relaxation times, the technique facilitates the investigation of a relatively narrow band from that distribution. It is also shown that the time domain spectroscopy can be combined with conventional frequency domain techniques to provide the characterization of dielectric response over an extraordinarily broad spectral range.     

Heterodyne detected transient gratings in supercooled molecular liquids

We present an analysis, based on a phenomenological set of Generalised Navier-Stokes equations, of Heterodyne Detected Transient Gratings on supercooled molecular liquids of anisotropic molecules. This set of equations generalises equations proven in Franosch, Latz and Pick [24] for the same type of liquids. It also takes into account the three different sources generated by the laser pumping process pertinent for these experiments. We give analytical expressions for the response functions that can be measured using the different polarisation of the experimental set-up. Specialising to the case of parallel polarisation (where longitudinal phonons are launched), we show that each response function is a sum of the same seven elementary response functions (ERFs) whose time and temperature evolutions are individually analysed. We also show that the response functions corresponding to two of the sources can be directly connected to the Laplace Transform of a light scattering signal. The ERFs generated by the heat-absorption process, which is the third source, are of a different nature. They do not have the same time and temperature behaviours and they can provide, inter alia, unique information on the rotation-translation coupling function characteristic of these liquids.Received: 5 January 2004, Published online: 29 June 2004PACS: 64.70.Pf Glass transitions - 78.47. + p Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter - 61.25.Em Molecular liquidsR. Gupta: Present address: Dept of Physics,1110 W Green St, Urbana, Ill 61801, USA.     

Heterogeneities in supercooled liquids: a density-functional study

A metastable state, characterized by a low degree of mass localization, is identified using density-functional theory (DFT). This free energy minimum, located through the proper evaluation of competing terms in the free energy functional, is independent of the specific form of the DFT used. Computer simulation results on particle motion indicate that this heterogeneous state corresponds to the deeply supercooled state.     

Long wavelength photodetectors

The role of long wavelength systems (1.0μm<λ< 10.0μm) in fiber optics communications is evaluated. For high-bit rate optical telecommunications at 1.3 μm or 1.5 μm, GaInAs p-i-n detectors have emerged as the preferred choice because of their low noise, excellent sensitivity, and high temperature stability. Ge and HgCdTe photodiodes offer nearly equivalent performance and a somewhat more advanced production technology. Beyond 2 μm, HgCdTe would seem to be the clear choice for photodetector applications. Avalanche photodiodes in long wavelength optical fiber links may find uses in situations involving high bit rate transmission using low cost components. Although avalanche gain will always improve the system sensitivity, the LED/APD combination is only half as sensitive as a laser/p-i-n system, as well as being bandwidth limited. Nontelecommunications applications involving data base and fiber guidance systems are discussed, and a prospective look is taken at the uses, such as power transmission, of ultra low-loss fibers in the 2 to 10 μm region of the optical spectrum.     

Atomic mechanism of glass formation in supercooled monatomic liquids

Atomic mechanism of glass formation in supercooled monatomic liquids is monitored via analyzing the spatial arrangement of solid-like atoms. The supercooled states are obtained by cooling from the melt using molecular dynamics (MD) simulation. Solid-like atoms, detected via Lindemann-like freezing criterion, are found throughout the liquid. Their number increases with decreasing temperature and they form clusters. In the deeply supercooled region, all solid-like atoms form a single percolation cluster which spans throughout the system. The number of atoms in this cluster increases steeply with further cooling. Glass formation in supercooled liquids occurs when a single percolation cluster of solid-like atoms involves the majority of atoms in the system to form a relatively rigid glassy solid. By analyzing the temperature dependence of static and dynamic properties, we identify three characteristic temperatures of glass formation in supercooled liquids including the Vogel–Fulcher temperature.     

Saddles in the energy landscape probed by supercooled liquids

We numerically investigate the supercooled dynamics of two simple model liquids exploiting the partition of the multidimensional configuration space in basins of attraction of the stationary points (inherent saddles) of the potential energy surface. We find that the inherent saddle order and potential energy are well-defined functions of the temperature T. Moreover, by decreasing T, the saddle order vanishes at the same temperature (T(MCT)) where the inverse diffusivity appears to diverge as a power law. This allows a topological interpretation of T(MCT): it marks the transition from a dynamics between basins of saddles (T > T(MCT)) to a dynamics between basins of minima (T < T(MCT)).