Experimental evidence of non-Gaussian fluctuations near a critical point

The orientation fluctuations of the director of a liquid crystal are measured, by a sensitive polarization interferometer, close to the Fréedericksz transition, which is a second-order transition driven by an electric field. We show that, near the critical value of the field, the spatially averaged order parameter has a generalized Gumbel distribution instead of a Gaussian one. The latter is recovered away from the critical point. The relevance of slow modes is pointed out. The parameter of the generalized Gumbel distribution is related to the effective number of degrees of freedom.     

Properties of the adsorption layer in a binary liquid mixture near the critical point

A method for the determination of parameters of the surface layer (thickness and refraction index) at the liquid-vapor interface in binary liquid mixtures was developed. The parameters of the surface layer for the C₇H₁₄-C₇F₁₄ liquid mixture in the vicinity of the critical point, which was studied by means of ellipsometry in [1], were calculated using the proposed method. The temperature dependences of the thickness and refraction index that were determined in the homogeneous isotropic layer approximation at the interface liquid-vapor, show structural peculiarities that were not observed earlier. Their appearance is explained by the possible influence of hydrodynamic processes at the boundary.     

Contraction and reswelling of a polymer chain near the critical point of a binary liquid mixture

We have studied the conformation change of a flexible linear polymer chain near the critical point of a binary liquid mixture using fluorescence correlation spectroscopy, which measured the hydrodynamic radius of the chains. Our results indicate that as the critical temperature (Tc) is approached, the chain size decreases. The polymer attains its most compact conformation when the correlation length of the critical fluctuations becomes comparable to the coil size. At very close to Tc, the polymer reexpands dramatically. To our knowledge, this is the first experimental evidence supporting the prediction of Brochard and de Gennes that a polymer chain will collapse and subsequently reswell on approaching Tc.     

Structure of liquid solutions near the critical point

Specific features of the structure of the critical state of binary liquid solutions leading to an anomalous behavior of the Rayleigh line due to a dramatic increase in concentration and density fluctuations are considered. It is shown that an experimental treatment must deal with two fluctuation regions near the critical point of solvent vaporization. In the first region, one can achieve a sufficient degree of accuracy by using theories like self-consistent field theory. In the second region, which is closer to the critical point than the first region, scaling theory of second-order phase transitions may be applied. It is found that the anomalous behavior of the Rayleigh line associated with kinetic coefficients is determined by the equilibrium thermodynamic properties and by the radius of fluctuation correlation (r_c). A general theory is developed for calculating thermodynamic potentials, especially the chemical potential and its concentration derivative in the fluctuation region. The results of these calculations are compared with the experimental data briefly described in the paper. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 655–668, July–August, 1998.     

Gravity influence on thermal relaxation near the critical point

Holographic interferometry measurements on a critical fluid sample performed during the D1 Spacelab flight reveal unexpected large density inhomogeneities under microgravity conditions. They are sustained by adiabatic temperature gradients and grow when approaching the critical point due to the diverging thermal expansion coefficient. Substantially increased relaxation time scales are found.     

Nuclear spin relaxation in molecular fluids near the critical point

The Watson version of the molecular vibration-rotation hamiltonian is expanded in terms of the scalar invariants of the inverse of the moment of inertia tensor using the generalized Lucas polynomials. The resultant expansion consists of the matrices Î, , ² with coefficients depending on the normal coordinates. This is in contrast to the usual expansion as a power series in the matrix .     

Experimental evidence for fast heterogeneous collective structural relaxation in a supercooled liquid near the glass transition

We have extended the exploration of microscopic dynamics of supercooled liquids to small wave numbers Q corresponding to the scale of intermediate range order, by developing a new experimental approach for precise data correction for multiple scattering noise in inelastic coherent neutron scattering. Our results in supercooled Ca0.4K0.6(NO3)(1.4) reveal the first direct experimental evidence, after a decade of controversy, that the so-called picosecond process around the glass transition corresponds to a predicted first, faster stage of the structural relaxation. In addition, they show that this process takes the spatial form of fast heterogeneous collective flow of correlated groups of atoms.     

Probability distribution of singly and multiply scattered light near the critical point of a binary mixture

The probability density function p(I) is theoretically and experimentally investigated in the case of single, double, and multiple light scattering near a second-order transition (demixing of the binary mixture nitrobenzene and n-hexane). Experimentally we find no differences between the statistics of singly, doubly or multiply scattered light, and as expected from theory, p(I) shows an exponential variation. These results justify the use of either heterodyne or homodyne techniques for studying single or multiple scattering close to a critical point. We also describe a particularly simple way of determining whether homodyne or heterodyne detection occurs.     

Spectrum of depolarized light scattered from nitroethane-isooctane mixture near its critical point

We present the results of a study of the spectrum of depolarized light scattered from the critical nitroethane—isooctane mixture which has a very weak critical opalescence. Near the critical point only slight narrowing of the spectrum is observed.     

Spectral measurements of the depolarized multiple scattered light from a binary mixture near its critical point

We report the first spectral measurements of the depolarized multiple scattered light from a binary mixture near its critical point. Spectral analysis of the depolarized light allows to separate rigorously the multiple scattering from the light scattered by the anisotropy fluctuations. The results are in very good agreement with a recent theory by Oxtoby-Gelbart.     

The equation of state of metastable liquid xenon near the critical point

This paper presents the results of measurements of xenon density near the critical liquid-vapour point in stable and metastable states. The spinodal has been approximated. The equation of state of metastable liquid is discussed.     

A scaling equation of state near the critical point and the stability boundary of a liquid

A new scaling equation of state is proposed to describe the equilibrium thermodynamic properties of liquids near the critical point. In distinction from the existing scaling equations, which are parametric, the new equation is nonparametric and is expressed directly in terms of the physical quantities (pressure, temperature, and so on). It creates a number of advantages for the traditional representation and data processing. The equation gives rise to a binodal, spinodal, and a curve of thermal capacity divergence (pseudospinodal). The equation is expressed in terms of reduced variables (the ratio of the deviation of a thermodynamic variable from its critical value to the critical value) and contains 3 system-dependent adjustable constants. With the help of this equation, we conducted an approximation of the experimental PVT data in the critical region of ⁴He, C₂H₄, and H₂O with a pressure error of 0.4% and carried out a calculation of the C _v ⁴He thermal capacity with no more than 4% error using a three-system constant determined from the PVT data.     

Topological crossovers near a quantum critical point

We study the temperature evolution of the single-particle spectrum ε-(p) and quasiparticle momentum distribution n(p) of homogeneous strongly correlated Fermi systems beyond a point where the necessary condition for stability of the Landau state is violated, and the Fermi surface becomes multi-connected by virtue of a topological crossover. Attention is focused on the different non-Fermi-liquid temperature regimes experienced by a phase exhibiting a single additional hole pocket compared with the conventional Landau state. A critical experiment is proposed to elucidate the origin of NFL behavior in dense films of liquid ³He.     

On heat fluctuations near a critical point

The spatial dependence of the heat fluctuations is examined from a thermodynamic potential for the probability of fluctuations. The corresponding correlation length is seen to diverge near the critical point.     

Experimental gravity induced coexistence curve of methyl fluoride near the critical point

Abstract

The top part of the coexistence curve of methyl fluoride in the critical region has been determined first by direct observation of the disappearance of the meniscus of the fluid situated in a high-pressure light scattering cell and subsequently from the measured maximum depolarization ratio of an appreciable number of isochores close to the critical point. The experimental data obtained from direct observations yield a coexistence curve which nearly coincides with that found recently from pVT measurements. However, the data obtained from light scattering experiments describe a gravity induced coexistence curve which lies above the previous one and which shows a minimum at the critical point. The complete experimental data have been used to evaluate the critical parameters p_c, ρ_c and T_c and the critical exponent β.     

Non-newtonian effect near the critical point

In the presence of a shear flow the order parameter fluctuations are distorted and suppressed by the shear and there appears a new correlation length independent of temperature sufficiently near the critical point.