A Laser Light Scattering Study of Transport and Critical Phenomena

The intensity and Rayleigh linewidth have been measured as a function of temperature and scattering angle for light scattered by concentration fluctuations near the critical point of the binary liquid system 2,6-lutidine-water. From the intensity data it is found that γ = 1.26 ± 0.02 and ν = 0.61 ± 0.07. From the linewidth data the mutual diffusion coefficients were calculated as a function of temperature. It is found that the diffusion coefficient decreases as the critical point is approached. The behavior of the linewidth as a function of k∈ was compared with the Kawasaki theory without the nonlocal viscosity and vertex corrections. General agreement with some systematic deviations is observed. The shear viscosity anomaly in the same system was also studied in detail by measuring the shear viscosities as a function of temperature near the critical point. Results of analyses indicate that the viscosity is at most weakly divergent, with an exponent ø ≃ ±0.001. Light scattering techniques have been employed to measure the mutual diffusion coefficient D as a function of concentration in ten binary mixtures and the thermal diffusivity χ in nine pure liquids and one binary mixture. The diffusion coefficient was also measured at one or two concentrations for four binary mixtures. The values obtained are in excellent agreement with the available literature data determined by more classical methods. Under most circumstances light scattering is found to offer a fast and accurate way of determining χ and D. The turbidity τ and the decay rate Γ of the density fluctuations have been measured as a function of temperature on the critical isochore of ethane near the critical point. From the turbidity data absolute values of isothermal compressibilities and correlation lengths were calculated. The isothermal compressibility KT and the correlation length ∈ are found to behave as: KT = 1.24 ± 0.11 x 10-3(ΔT/Tc)-1.225 ± 0.02atm-1 ∈ = 1.64 ± 0.20 (ΔT/Tc)-0.664 ± 0.02 Å. From the Γ data thermal diffusivities, thermal conductivities and excess thermal conductivities were calculated as a function of temperature. It is found that the thermal diffusivity does not exhibit a simple power law behavior whereas the excess thermal conductivity does with an exponent of ψ = 0.605 ± 0.02. The singular part of the decay rate Γs, was compared with the Kawasaki expression with the nonlocal viscosity correction. It is observed that the nonlocal viscosity correction together with the vertex and the correlation function corrections improve the agreement between the theory and the experiment. The results for the isothermal compressibility, the thermal conductivity and the excess thermal conductivity are in very good agreement with the available literature data.</p

A Study of Critical Phenomena in Krypton

A detailed experimental study of equilibrium critical phenomena in krypton was made. Using the method of angle of minimum deviation the refractive index was measured along the coexistence curve, along 16 isotherms above and along 11 isotherms below the critical temperature. The range of the temperature measurements in terms of t the reduced temperature difference from Tc was -6.8 x 10-2 ≦ t ≦ -5.7 x 10-5 and 3.8 x 10-5 ≦ t ≦ 4.8 10-2. The measurements were planned so that the region very near the critical point was covered in most detail. The refractive index was related to the density through the Lorentz-Lorenz relation. After proper weight assignment, the data were analyzed in terms of the asymptotic power laws. The following values of the critical parameters, exponents and coefficients were determined: Tc = 209.286 ± 0.010°K, Pc = 54.213 ± 0.003 atm., LLc = 0.070588 ± 0.000006, β = 0.3571 ± 0.0008, B = 1.840 ± 0.001 γ = 1.182 ± 0.008, Γ = 0.0835 ± 0.0011; γG’ =1.15 ± 0.01, ΓG’ = 0.021 ± 0.001, γL’ = 1.13 ± 0.01, ΓL’ = 0.025 ± 0.001; δ = 4.25 ± 0.25. The law of the rectilinear diameter was obeyed with its slope = 0.0918 ± 0.0004. The reduced chemical potential differences and the reduced density differences were calculated. The chemical potential was observed to show antisymmetry for -2 x 10-3 ≦ t &#60; 4.8 x 10-2 and -0.3 &#60; ΔLL &#60; 0.3. The data in this range were analyzed using Widom's equation of state and the closed-form(29) of h(x). The proposed equation was found to fit the experimental data very well. The predictions of the linear model(32) were also checked and were observed to be consistent with the experimental results.</p

Salt-induced changes of colloidal interactions in critical mixtures

We report on salt-dependent interaction potentials of a single charged particle suspended in a binary liquid mixture above a charged wall. For symmetric boundary conditions (BC) we observe attractive particle-wall interaction forces which are similar to critical Casimir forces previously observed in salt-free mixtures. However, in case of antisymmetric BC we find a temperature-dependent crossover from attractive to repulsive forces which is in strong contrast to salt-free conditions. Additionally performed small-angle x-ray scattering experiments demonstrate that the bulk critical fluctuations are not affected by the addition of salt. This suggests that the observed crossover can not be attributed alone to critical Casimir forces. Instead our experiments point towards a possible coupling between the ionic distributions and the concentration profiles in the binary mixture which then affects the interaction potentials in such systems.Comment: 5 pages, 4 Figure

Nonadditivity of critical Casimir forces

In soft condensed matter physics, effective interactions often emerge due to the spatial confinement of fluctuating fields. For instance, microscopic particles dissolved in a binary liquid mixture are subject to critical Casimir forces whenever their surfaces confine the thermal fluctuations of the order parameter of the solvent close to its critical demixing point. These forces are theoretically predicted to be nonadditive on the scale set by the bulk correlation length of the fluctuations. Here we provide direct experimental evidence of this fact by reporting the measurement of the associated many-body forces. We consider three colloidal particles in optical traps and observe that the critical Casimir force exerted on one of them by the other two differs from the sum of the forces they exert separately. This three-body effect depends sensitively on the distance from the critical point and on the chemical functionalisation of the colloid surfaces

Direct measurement of critical Casimir forces

When fluctuating fields are confined between two surfaces, long- range forces arise. A famous example is the quantum electrodynamical Casimir force that results from zero- point vacuum fluctuations confined between two conducting metal plates. A thermodynamic analogue is the critical Casimir force: it acts between surfaces immersed in a binary liquid mixture close to its critical point and arises from the confinement of concentration fluctuations within the thin film of fluid separating the surfaces. So far, all experimental evidence for the existence of this effect has been indirect. Here we report the direct measurement of critical Casimir force between a single colloidal sphere and a flat silica surface immersed in a mixture of water and 2,6- lutidine near its critical point. We use total internal reflection microscopy to determine in situ the forces between the sphere and the surface, with femtonewton resolution. Depending on whether the adsorption preferences of the sphere and the surface for water and 2,6- lutidine are identical or opposite, we measure attractive and repulsive forces, respectively, that agree quantitatively with theoretical predictions and exhibit exquisite dependence on the temperature of the system. We expect that these features of critical Casimir forces may result in novel uses of colloids as model systems