Michael R. Moldover

An Interface Phase Transition: Complete to Partial Wetting

When two fluid phases are near a critical point, one of them will be excluded from contact with any third phase that happens to be present by a wetting film of the other critical phase. A simple and quite general strategy that may be used to induce a phase transition from complete wetting of the third phase to incomplete wetting is to add a new component to the fluid phases chosen to drive the two phases away from their critical point. This strategy is illustrated for methanol-cyclohexane mixtures.

Acoustic gas thermometry

We review the principles, techniques and results from primary acoustic gas thermometry (AGT). Since the establishment of ITS-90, the International Temperature Scale of 1990, spherical and quasi-spherical cavity resonators have been used to realize primary AGT in the temperature range 7 K to 552 K. Throughout the sub-range 90 K < T < 384 K, at least two laboratories measured (T − T90). (Here T is the thermodynamic temperature and T90 is the temperature on ITS-90.) With a minor exception, the resulting values of (T − T90) are mutually consistent within 3 × 10−6 T. These consistent measurements were obtained using helium and argon as thermometric gases inside cavities that had radii ranging from 40 mm to 90 mm and that had walls made of copper or aluminium or stainless steel. The AGT values of (T − T90) fall on a smooth curve that is outside ±u(T90), the estimated uncertainty of T90. Thus, the AGT results imply that ITS-90 has errors that could be reduced in a future temperature scale. Recently developed techniques imply that low-uncertainty AGT can be realized at temperatures up to 1350 K or higher and also at temperatures in the liquid-helium range.

Gas‐filled spherical resonators: Theory and experiment

Gas‐filled spherical resonators are excellent tools for routine measurement of thermophysical properties. The radially symmetric gas resonances are nondegenerate and have high Q’s (typically 2000–10 000). Thus they can be used with very simple instrumentation to measure the speed of sound in a gas with an accuracy of 0.02%. We have made a detailed study of a prototype resonator filled with argon (0.1–1.0 MPa) at 300 K, with the objective of discovering those phenomena which must be understood to use gas‐filled spherical resonators to measure the thermodynamic temperature and the universal gas constant R. The resonance frequencies fN and half‐widths gN were measured for nine radially symmetric modes and nine triply‐degenerate nonradial modes with a precision near 10−7 fN. The data were used to develop and test theoretical models for this geometrically simple oscillating system. The basic model treats the following phenomena exactly for the case of a geometrically perfect sphere: (1) the thermal boundary la...

Ab Initio Values of the Thermophysical Properties of Helium as Standards

Recent quantum mechanical calculations of the interaction energy of pairs of helium atoms are accurate and some include reliable estimates of their uncertainty. We combined these ab initio results with earlier published results to obtain a helium-helium interatomic potential that includes relativistic retardation effects over all ranges of interaction. From this potential, we calculated the thermophysical properties of helium, i.e., the second virial coefficients, the dilute-gas viscosities, and the dilute-gas thermal conductivities of 3He, 4He, and their equimolar mixture from 1 K to 104 K. We also calculated the diffusion and thermal diffusion coefficients of mixtures of 3He and 4He. For the pure fluids, the uncertainties of the calculated values are dominated by the uncertainties of the potential; for the mixtures, the uncertainties of the transport properties also include contributions from approximations in the transport theory. In all cases, the uncertainties are smaller than the corresponding experimental uncertainties; therefore, we recommend the ab initio results be used as standards for calibrating instruments relying on these thermophysical properties. We present the calculated thermophysical properties in easy-to-use tabular form.

Critical exponent for the viscosity of four binary liquids

We have measured the viscosity of four binary mixtures near their consolute points: (1) methanol+cyclohexane, (2) isobutyric acid+water, (3) nitroethane+3‐methylpentane, and (4) 2‐butoxyethanol+water. The viscosity data are consistent with the power‐law divergence: η∼‖T−Tc‖−y, with an apparent viscosity exponent in the range 0.0404<y<0.0444. Recent theoretical estimates for y are near 0.032, which is outside the experimental range. The value of y is independent of whether the critical point is an upper or a lower consolute point and of whether the approach toward Tc is at constant pressure or at constant volume. Our torsion oscillator viscometer is unique in its simultaneous low frequency (∼1 Hz) and low shear rate (∼0.1 s−1), allowing its use close to the critical point before encountering non‐Newtonian fluid behavior associated with critical slowing down. Nevertheless, we find quantitative evidence for viscoelasticity near the critical point.

Acoustic thermometry: new results from 273 K to 77 K and progress towards 4 K

We used a quasi-spherical cavity as an acoustic and microwave resonator to measure the thermodynamic temperatures, T, of the triple points of equilibrium hydrogen, neon, argon and mercury and to measure the difference T − T90, in the range 7 K to 273 K. (T90 is the temperature on the International Temperature Scale of 1990 (ITS-90).) In the range 7 K to 24.5 K, our preliminary values of T − T90 agree with recent results from dielectric-constant gas thermometry and achieve uncertainties that are comparable to or smaller than those achievable using the interpolating constant volume gas thermometer as currently defined on the ITS-90. In the range 90 K to 273 K, the present results for T − T90 obtained using a helium-filled, copper-walled, quasi-spherical cavity agree with earlier results obtained using argon-filled, steel-walled or aluminium-walled, spherical cavities. The agreement confirms our understanding of both acoustic and microwave cavity resonators and demonstrates that resonators function as primary thermometers spanning wide temperature ranges. The mutually consistent acoustic thermometry data from several laboratories imply that the values of (T − T90)/T90 are 5 times larger than the uncertainty of T/T90 near 150 K and near 400 K. They also imply that the derivative dT/dT90 is too large by approximately 10−4 near 273.16 K and that dT/dT90 has a discontinuity of 4 × 10−5 at 273.16 K.

Thermophysical properties of gaseous refrigerants from speed of sound measurements. I. Apparatus, model, and results for 1,1,1,2‐tetrafluoroethane R134a

The speed of sound in gaseous 1,1,1,2‐tetrafluoroethane (R134a) has been obtained between 233.16 and 340 K from measurements of the frequency of the radial acoustic resonances of a gas‐filled spherical cavity. Perfect‐gas heat capacities and second and third acoustic virial coefficients have been calculated from the results. The acoustic virial coefficients are used to estimate the density virial coefficients B(T) and C(T) and an effective square‐well potential. The estimates of B(T) are consistent with B(T) deduced from high‐quality equation‐of‐state measurements; those for C(T) are slightly inconsistent. The apparatus and its calibration with argon are described.

Universal amplitude ratios and the interfacial tension near consolute points of binary liquid mixtures

The densities of the coexisting phases and the capillary length have been measured to obtain the interfacial tension (σ) near the consolute temperatures Tc of the three binary liquid mixtures: triethylamine+water, triethylamine+heavy water, and methanol+cyclohexane. Our data are combined with data from the literature to test predictions for three temperature‐independent ‘‘universal’’ ratios: U+1=σ(ξ+)2/(kBTc) and Y(±)=σ(αt2C±s/kB) −2/3/(kBT0). [Here ξ+ is the correlation length, C±s is the singular part of the heat capacity per unit volume, α=0.11 is the exponent characterizing the specific heat divergence, and t≡(T−Tc)/Tc]. Near Tc, the new experimental values of Y(+) range from 5.5–5.8 in agreement with the value 5.6 obtained by Moldover [Phys. Rev. 31, 1022 (1985)] in a review of earlier experiments. However, the experimental values of Y(+) are inconsistent with either the value Y(+)=4.4±0.4 obtained from a recent simulation of the simple‐cubic Ising model or the value Y(+)=2.6–3.0 obtained from a one‐...

Partially halogenated hydrocarbons CHFClCF3, CF3CH3, CF3CHFCHF2, CF3CH2CF3, CHF2CF2CH2F, CF3CH2CHF2, CF3OCHF2: critical temperature, refractive indices, surface tension and estimates of liquid, vapor and critical densities

Abstract For seven partially halogenated hydrocarbons, designated by the refrigeration industry as the refrigerants R124, R143a, R236ea, R236fa, R235ca, R245fa, and E125, we determined the critical temperatures Tc (ITS-90), the capillary rise, and the refractive indices of the liquid and vapor phases in the temperature range from 20°C to their critical points. Independently measured liquid densities at lower temperatures, when combined with our refractive indices, produce estimates of the vapor and liquid densities at higher temperatures up to the critical point. The densities combined with the capillary rise data give the surface tensions, σ, up to the critical points. The surface tensions of the seven refrigerants in this study together with nine others measured previously can be represented by the scaled equation σ = 2.51 (1 + 0.609ω) k b T c ( N a V c ) 2 3 t 1.26 (1 + 0.348t 1 2 −0.487t) where t ≡ (T c −T) T c is the reduced temperature, and kb, a, Vc, and ω are the Boltzmann and Avagadro constants, the critical volume, and the acentric factor, respectively. This equation yields values for σ within 5% of the measured values for all 16 candidate replacement refrigerants.

First-order wetting transition at a liquid-vapor interface

In certain binary solutions the lower of the two liquid phases forms a layer which intrudes between the upper liquid phase and the vapor. We find that such intruding layers form above binary solutions of a fluorocarbon (C7F14) and an alcohol (i‐C3H7OH). As the temperature of C7F14–i‐C3H7OH solutions is increased, the intruding layer abruptly appears at a characteristic wetting temperature TW=311 K. This temperature is well below the consolute temperature (363 K). At temperatures slightly above Tw the intruding layer’s thickness (measured by ellipsometry) is several hundred angstroms and its variation with temperature is extremely weak. Below Tw, the layer’s thickness may be zero and is no greater than 20 A when a naive slab model is used to interpret the data. Below Tw, three‐phase contact can occur between the vapor and both the upper and the lower liquid phases. Our measurements show that one of the angles (θ) which characterizes this three‐phase contact has a very simple temperature dependence: cos θ=1...