Engineering topics

Moldover, M. R.

Publications and source records attributed to Moldover, M. R..

At least 19 records

NASA NTRS2002

Acoustic Experiment to Measure the Bulk Viscosity of Near-Critical Xenon in Microgravity

We plan a rigorous test of the theory of dynamic scaling by accurately measuring the bulk viscosity of xenon in microgravity 50 times closer to the critical temperature T(sub c) than previous experiments. The bulk viscosity zeta (or "second viscosity" or "dilational viscosity") will be determined by measuring the attenuation length of sound alpha lambda and also measuring the frequency-dependence of the speed of sound. For these measurements, we developed a unique Helmholtz resonator and specialized electro-acoustic transducers. We describe the resonator, the transducers, their performance on Earth, and their expected performance in microgravity.

Gillis, K. A.↗

NASA NTRS1999

Electrostriction of Near-Critical SF6 in Microgravity

We used interferometry to measure the electric-field-induced (i.e., "electrostrictive") increase of the density of sulfur hexafluoride near its critical point. The results at the temperatures T(sub c) + 5 mK, T(sub c) + 10 mK, and T(sub c) + 30 mK with T(sub c) = 319 K agree with a calculation based on the Clausius-Mossotti relation and the restricted cubic model equation of state. These measurements were performed in microgravity so that the electrostrictive density changes (less than or equal to 3.5% in this work) would not be complicated by stratification of the fluid's density induced by the Earth's gravity.

Zimmerli, G. A.↗

NASA NTRS1994

Electric field effects on a near-critical fluid in microgravity

The effects of an electric field on a sample of SF6 fluid in the vicinity of the liquid-vapor critical point is studied. The isothermal increase of the density of a near-critical sample as a function of the applied electric field was measured. In agreement with theory, this electrostriction effect diverges near the critical point as the isothermal compressibility diverges. Also as expected, turning on the electric field in the presence of density gradients can induce flow within the fluid, in a way analogous to turning on gravity. These effects were observed in a microgravity environment by using the Critical Point Facility which flew onboard the Space Shuttle Columbia in July 1994 as part of the Second International Microgravity Laboratory Mission. Both visual and interferometric images of two separate sample cells were obtained by means of video downlink. The interferometric images provided quantitative information about the density distribution throughout the sample. The electric field was generated by applying 500 Volts to a fine wire passing through the critical fluid.

Zimmerli, G.↗

NASA NTRS1993

Scientific Objectives of the Critical Viscosity Experiment

In microgravity, the Critical Viscosity Experiment will measure the viscosity of xenon 15 times closer to the critical point than is possible on earth. The results are expected to include the first direct observation of the predicted power-law divergence of viscosity in a pure fluid and they will test calculations of the value of the exponent associated with the divergence. The results, when combined with Zeno's decay-rate data, will strengthen the test of mode coupling theory. Without microgravity viscosity data, the Zeno test will require an extrapolation of existing 1-g viscosity data by as much as factor of 100 in reduced temperature. By necessity, the extrapolation would use an incompletely verified theory of viscosity crossover. With the microgravity viscosity data, the reliance on crossover models will be negligible allowing a more reliable extrapolation. During the past year, new theoretical calculations for the viscosity exponent finally achieved consistency with the best experimental data for pure fluids. This report gives the justification for the proposed microgravity Critical Viscosity Experiment in this new context. This report also combines for the first time the best available light scattering data with our recent viscosity data to demonstrate the current status of tests of mode coupling theory.

Berg, R. F.↗

NASA NTRS1990

Critical exponent for the viscosity of carbon dioxide and xenon

The viscosities of carbon dioxide and xenon have been measured near their critical points and the critical exponent y characterizing the asymptotic divergence has been determined. Both fluids yielded exponents in the range y = 0.041 + or - 0.001 and thus also fell in the range y = 0.042 + or - 0.002 from an earlier study of four binary liquids. This agreement between experiments is the first evidence that pure fluids and binary liquids are in the same dynamic universality class. A recent theoretical value for y is 0.032. The 30 percent discrepancy is much greater than the combined errors from experiment and theory. The torsion oscillator viscometer operated at low frequency and low shear rate to avoid systematic errors caused by critical slowing down. Far from T(c) the analysis accounted for the crossover from critical to noncritical temperature dependence, where the latter was obtained from previously published correlations. Corrections for gravitational stratification were included close to T(c).

Berg, R. F.↗

NASA NTRS1986

Viscometer for low frequency, low shear rate measurements

A computer-controlled torsion-oscillator viscometer with low 0.5 Hz frequency and very low 0.05/s shear rate is designed to precisely study shear-sensitive fluids such as microemulsions, gels, polymer solutions and melts, colloidal solutions undergoing coagulation, and liquid mixtures near critical points. The viscosities are obtained from measurements of the logarithmic decrement of an underdriven oscillator. The viscometer is found to have a resolution of 0.2 percent when used with liquid samples and a resolution of 0.4 percent when used with a dense gaseous sample. The design is compatible with submillikelvin temperature control.

Berg, R. F.↗

NASA NTRS1986

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

The interfacial tension of the binary liquid mixtures triethylamine/heavy water, triethylamine/water, and cyclohexane/methanol near their consolute temperatures is determined experimentally by measuring the densities of the phases and the capillary lengths. The results are presented in tables and graphs and compared with published data obtained by other methods and with theoretical predictions. The findings are shown to be in good agreement with previous experiments, confirming the discrepancy between temperature-independent universal amplitude ratios calculated from such data and those predicted by renormalization-group calculations or Monte Carlo simulations of Ising models.

Moldover, M. R.↗

NASA NTRS1986

A search for the prewetting line

This paper describes efforts to locate the prewetting line in a binary liquid system (isopropanol-perfluoromethylcyclohexane) at the vapor-liquid interface. Tight upper bounds were placed on the temperature separation (0.2 K) between the prewetting line and the line of bulk liquid phase separation. The prewetting line in systems at equilibrium was not detected. Experimental signatures indicative of the prewetting line occurred only in nonequilibrium situations. Several theories predict that the adsorption of one of the components (the fluorocarbon, in this case) at the liquid-vapor interface should increase abruptly, at a temperature sightly above the temperature at which the mixture separates into two liquid phases. A regular solution calculation indicates that this prewetting line should have been easily detectable with the instruments used in this experiment. Significant features of the experiment are: (1) low-gradient thermostatting, (2) in situ stirring, (3) precision ellipsometry from the vapor-liquid interface, (4) high resolution differential index of refraction measurements using a novel cell design, and (5) computer control.

Schmidt, J. W.↗

NASA NTRS1985

The liquid-vapor interface of a binary liquid mixture near the consolute point

The present investigation is concerned with measurements of the reflectance of polarized light by the liquid-vapor interface above liquid mixtures of a fluorocarbon and an alcohol in the vicinity of the consolute point of the liquid mixture. The denser fluorocarbon is preferentially adsorbed at the interface. The denser fluorocarbon is preferentially adsorbed at the interface. The data are consistent with a simple model of the dielectric constant vs height profile in the interface region. Liquid mixtures of isopropanol (i-C3H7OH) and perfluoromethylcyclohexane (C7F14) near their consolute point (Tc = 363 K) are utilized in the conducted experiments. The primary quantity measured is the ellipticity as a function of temperature and liquid composition. Attention is given to details regarding the experimental techniques, model dielectric constant profiles, and the calculation of the ellipticity.

Schmidt, J. W.↗

NASA NTRS1985

Light Scattering Tests of Fundamental Theories of Transport Properties in the Critical Region

The objective of this program is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light scattering and photon correlation spectroscopy. Such experiments have been severely limited on Earth by the presence of gravity which causes large density gradients in the sample when the compressibility diverges approaching the critical point. The goal is to measure decay rates deep in the critical region where the scaled wavevector is the order of 1000. This will require loading the sample to 0.01% of the critical density and taking data as close as 3 microKelvin to the critical temperature (Tc = 289.72 K). Other technical problems have to be addressed such as multiple scattering and the effect of wetting layers. The ability to avoid multiple scattering by using a thin sample (100 microns) was demonstrated, as well as a temperature history which can avoid wetting layers satisfactory temperature control and measurement, and accurate sample loading. Thus the questions of experimental art are solved leaving the important engineering tasks of mounting the experiment to maintain alignment during flight and automating the state-of-the-art temperature bridges for microcomputer control of the experiment.

Gammon, R. W.↗

NASA NTRS1985

Precise Viscosity Measurements Very Close to Critical Points

A torsion oscillator is being developed to measure the viscosity of fluids at moderate temperatures (0-100 C) and pressures (0-10 MPa) at very low frequencies (0.5 Hz) and very low shear rates (0.05/sec). These conditions are required to measure the shear viscosity of fluids extremely close to the critical point. The oscillator, thermostat, vacuum system, and instrumentation were assembled and are undergoing debugging. Preliminary measurements indicate that the system has a noise corresponding to 0.2% of the viscosity of methanol-cyclohexane mixtures near the consolute point.

Moldover, M. R.↗

NASA NTRS1985

Experimental and Theoretical Studies of Wetting and Multilayer Adsorption

The recent work with partially miscible binary liquid mixtures has established that the structure of the liquid-vapor interface can undergo a first-order phase transition from incomplete to complete wetting of the vapor as the temperature is raised. A discontinuity in the change of interfacial tension as a function of temperature at the phase transition has been predicted to occur in many systems and to play an important role in the growth of uniform composites from alloy melts at monotectic points. These measurements are the first to establish the order of the transition. Studies of capillary rise in SF6 in a unique interferometer have led to the first measurements of the thickness of wetting layers (or equivalently, multilayer adsorbed films) on a solid surface near a liquid-vapor critical point. Instabilities in wetting layers were observed. A theory for the instabilities is being developed and will be checked by both static and dynamic optical experiments. The effect of gravity on the apparent thickness of interfaces (as measured by ellipsometry) is under study.

Moldover, M. R.↗

NASA NTRS1985

Wetting layers and dispersion forces for a fluid in contact with a vertical wall

Ellipsometric measurements of the thickness of the wetting layer of sulfur hexafluoride that forms on a fused-silica surface partially immersed in the liquid are presented. The components of the experiment included a He-Ne laser (0.6328 micron), polarizer, and quarter-wave plate with 45-deg azimuth. An optical-grade fused-silica prism was polished to wavelength/20, and strain-induced birefringence was accounted for. It was found that the layer thickness decreases from 40 to 20 nm with a height increase from 0.7 to 3 mm above the liquid-vapor interface. The thickness is independent of the temperature between 10 and 0.1 K below the critical temperature (319 K). The results are compared in detail to the theory of Dzyaloshinskii et al. (1961), and are noted to coincide closely.

Kayser, R. F.↗

NASA NTRS1985

Interfacial tension of fluids near critical points and two-scale-factor universality

Measurements of interfacial tension in the critical region of a wide variety of fluids are reviewed. The data are analyzed using theoretical values of the critical exponents to obtain amplitude ratios. Many of the data are consistent within a few percent of sigma(0). Comparison of these amplitude ratios with ratios from a field-theory calculation and from a simulation suggests that the theoretical values are about half the experimental ones. Systematic errors which are the source of this discrepancy are discussed. The data sources and processing procedures of this study are also discussed, pointing out that for liquid-vapor systems the capillary length often has a remarkably simple temperature dependence.

Moldover, M. R.↗

NASA NTRS1984

Turbidity very near the critical point of methanol-cyclohexane mixtures

The turbidity of a critical mixture of methanol and cyclohexane has been measured extremely close to the consolute point. The data span the reduced-temperature range between 10 to the -7th and 10 to the -3d, which is two decades closer to Tc than previous measurements. In this temperature range, the turbidity varies approximately as 1nt, as expected from the integrated form for Ornstein-Zernike scattering. A thin cell (200-micron optical path) with a very small volume (0.08 ml) was used to avoid multiple scattering. A carefully controlled temperature history was used to mix the sample and to minimize the effects of critical wetting layers. The data are consistent with a correlation-length amplitude of 3.9 plus or minus 1.0 A, in agreement with the value 3.5 A calculated from two-scale-factor universality and heat-capacity data from the literature.

Kopelman, R. B.↗

NASA NTRS1984

Capillary rise, wetting layers, and critical phenomena in confined geometry

Interferometric procedures have been used to measure the thickness of the wetting layers of liquid SF6 which intrude between SF6 vapor and the walls of an interferometer. Close to the critical temperature the measurements show that the layers become thicker as the height at which they are observed approaches the height of the bulk liquid-vapor meniscus. The functional form of the thickness increase agrees with the dependence expected from models in which the layers' thicknesses are governed by a competition between gravitational and van der Waals forces. The layers are a factor of three thicker than a simple estimate based on such models. No evidence is found of a transition from three-dimensional critical behavior towards two-dimensional critical behavior in the available temperature and size ranges. This contrasts with the interpretation of experiments in binary liquid mixtures carried out with comparable size and temperature resolution.

Moldover, M. R.↗

NASA NTRS1984

Wetting, multilayer adsorption, and interface phase transitions

When two fluid phases coexist near their mutual critical point, one fluid phase forms a layer which intrudes between the other fluid phase and any third phase that happens to be present. As the two fluid phases are taken away from their critical point, a phase transition often occurs such that the intruding layer vanishes. Recent evidence is presented that the transition from the complete wetting (intruding layer) configuration to the incomplete wetting (three-phase contact) configuration is a first-order phase transition. The thickness of the intruding layers has been measured for diverse systems. These data are not fully understood. The intruding layers are extreme examples of multilayer adsorption. Certain theories predict that a 'prewetting' transition from high adsorption to low adsorption is associated with the transition from complete wetting to incomplete wetting. Present search for this transition is inconclusive.

Moldover, M. R.↗

NASA NTRS1983

First-order wetting transition at a liquid-vapor interface

Evidence from reflectance and contact angle measurements is presented that three-phase mixtures of i-C3H7OH-C7F14 exhibit a first-order wetting phase transition at the liquid-vapor interface at 38 C. Equilibration phenomena support this interpretation. Ellipsometry was used to measure the apparent thickness of the intruding layer in the three-phase mixture. At temperatures slightly above the wetting temperature T(w), the intruding layer's thickness is several hundred angstroms and its variation with temperature is extremely weak. Below T(w), three-phase contact can occur between the vapor and both the upper and lower liquid phases; one of the angles which characterizes this contact has a very simple temperature dependence. The thickness of the intruding layer, monitored as the solutions approached equilibrium, is found to depend quite weakly on the height spanned by the upper liquid phase in the vicinity of a first-order wetting transition.

Schmidt, J. W.↗