Unstable Mixtures Research Articles

Zero Sound in a Mixture of a Single-Component Fermion Gas and a Bose-Einstein Condensate

The resonant dynamics of mediated interactions supports zero sound in a cold atom degenerate mixture of a single-component fermion gas and a Bose-Einstein condensate. We characterize the onset of instability in the phase separation of an unstable mixture and we find a rich collective mode structure for stable mixtures with one undamped mode that exhibits an avoided crossing and a Landau-damped mode that terminates.

Separation of n-hexane–ethyl acetate mixtures by azeotropic batch distillation with heterogeneous entrainers

In this article, a systematic study of the separation of the n-hexane–ethyl acetate mixture with an entrainer by heterogeneous azeotropic batch distillation is performed. Based upon the thermodynamic behaviour of the ternary mixtures, potential entrainers partially miscible with one or two original azeotropic components are chosen. In all cases, the entrainer adds a heterogeneous binary or ternary azeotrope that is the lowest boiling point in the ternary diagram. Therefore, it leaves the column by the overhead stream which is subcooled to get two liquid phases in the decanter. The phase with the highest amount of the original component is removed as distillate product whereas the entrainer-rich phase is continuously refluxed to the column. Considering methanol, acetonitrile, water and nitromethane as heterogeneous entrainers, screening was performed based on the composition of the unstable heteroazeotropic mixture, the ratio of both liquid phases in the condensed top vapour and the purity of the distillate product determined by the liquid–liquid envelope at the decanter temperature. The process feasibility analysis is validated by using rigorous simulation with the batch process simulator ProSimBatch. Simulation results are then corroborated in a bench experimental column for the selected entrainer, showing several advantages of heterogeneous batch distillation (HBD) compared to homogeneous systems.

Inert Gas and Fuel Gas Influence on the Pressure Limits of Stability of Acetylene

Chemically unstable gases obtained in many processes of, for example, the chemical and petrochemical industries, can decompose explosively also in the absence of oxidizing agents. In order to avoid hazards resulting from explosion reactions, safety measures need to be developed. Key to such methods is the knowledge of stability limits of unstable gases. In the present paper, an experimental method for determining the pressure limits of stability of unstable gases and gas mixtures is presented. It has proved useful when examining C2H2/inert gas and C2H2/fuel gas mixtures.

Crystallization of Organic Compounds in Reversed Micelles. III. Solubilization of Aspartame

The artificial sweetener, aspartame, is a dipeptide, consisting of a hydrophobic phenylalanine methyl ester entity and a hydrophilic aspartyl residue. In this work, its solubilization in five different types of water in oil (w/o) microemulsions (MEs) was investigated. The stabilizing surfactants and cosurfactants of the MEs were ME 1, sodium di-2-ethylhexyl sulfosuccinate (AOT) and no cosurfactant; ME 2, maleic anhydride α-olefin copolymer and dimethyl amino ethanol; ME 3, sucrose ester monostearate and n-butanol; ME 4, l-α-phosphatidylcholine and n-butanol; and ME 5, mono- and diglycerides of fatty acids and l-α-phosphatidylcholine. The maximum amount that could be dissolved (boundary concentration) was determined by adding powdered aspartame to a heated ME and cooling to a specified temperature under controlled conditions. The solutions that remained clear for at least 4 days were regarded as stable, and those of the stable solutions with the highest aspartame concentrations were taken as having the boundary concentrations. From the solubility data, the distribution of the aspartame molecules between the w/o interface and the water pools was calculated, and the results were correlated with the molecular structure and ionic state of the surfactant. The results show that aspartame can be solubilized in all the investigated MEs to an extent, exceeding by far its solubility in pure water, and that overall solubilization is most efficient in water/isooctane MEs stabilized with AOT (ME 1). While the aspartame solubility in the water pools was comparable in all the investigated MEs, the aspartame/surfactant molar ratio at the w/o interface was found to decrease with decreasing polarity of the stabilizing surfactant at the interface. In addition to the solubilization studies, the effect of aspartame on some properties of ME 1 was investigated. It was found that aspartame lowers the interfacial tension at the water/isooctane/AOT interface and, under certain conditions, stabilizes unstable mixtures of water, isooctane, and AOT. The results of small-angle X-ray scattering measurements show that the ME droplet sizes increase in the presence of aspartame molecules and their shapes change from near spherical to ellipsoidal. It was concluded that aspartame acts as a cosurfactant in water/isooctane MEs stabilized with AOT.

Kinetics of phase separation in the driven lattice gas: Self-similar pattern growth under anisotropic nonequilibrium conditions

The driven lattice gas (DLG) evolving at low temperature helps understanding the kinetics of pattern formation in unstable mixtures under anisotropic conditions. We here develop a simple theoretical description of kinetics in Monte Carlo simulations of the DLG. A Langevin continuum analog is also studied which is shown to exhibit the same behavior. We demonstrate that pattern growth is mainly a consequence of single-particle processes and that, after a short transient time, in which a surface evaporation/condensation mechanism is important, hole diffusion in the bulk becomes dominant. Consequently, there is a unique relevant length that behaves $l(t) \sim t^{1/3}$ for macroscopic systems except at some very early (perhaps unobservable) time. This implies sort of self-similarity, namely, the spatial pattern looks alike, but for a (non-trivial) change of scale at different times. We also characterize the structure factor, in which we identify Guinier and Porod regions, and its scaling form with both time and size. The underlying anisotropy turns out to be essential in determining the macroscopically-emergent peculiar behavior.

Surface-directed phase separation with off-critical composition: analytical and numerical results.

We study the interplay of wetting and phase separation in an unstable binary mixture (AB) with off-critical composition, placed in contact with a surface which prefers the component A. We consider surface potentials V(z) approximately z(-n), where z is the distance from the surface, and present analytical arguments and detailed numerical results to elucidate wetting-layer kinetics for arbitrary mixture compositions. If the preferred component is the minority phase, the wetting-layer thickness exhibits a potential-specific behavior at early times tau, R1 approximately tau(1/(n+2)), before crossing over to the universal growth law, R1 approximately tau(1/3). On the other hand, if the preferred component is the majority phase, there is a crossover from potential-specific growth (as before) to a slower growth regime.

Alcohol, Communities and Researchers: Theorizing the Relationships in an Unstable mixture

The growing importance of local communities as sites for preventive alcohol-related initiatives has generated interest in the role of research associated with community action. To date, however, this has not led to a theoretically informed account because of a failure to account reflexively for researchers' own practices and the use of conceptually inadequate models of “community.” This paper proposes an analytical framework, derived from the author's association with alcohol initiatives in several northern Australian towns and drawing on Bourdieu's concept of the social field. It is argued that the initiation of local action generates a complex social field in which stakeholders pursue their interests by mobilizing forms of capital: economic, social, political, and symbolic. Research, as well as the conveyance of valued information, is imbued with symbolic and political capital. The analysis concludes with four propositions relating to how, by whom, and with what consequences research is likely to be utilized in these settings.

The failure mechanism of gaseous detonations: experiments in porous wall tubes

To clarify the important role of the transverse wave structure in real detonations, we conducted experiments in porous wall tubes, as to attenuate the detonation’s transverse waves. Flow visualization and measurements of the attenuation and failure processes for three typical unstable mixtures (oxy-acetylene, oxy-methane, and oxy-propane mixtures), characterized by their highly irregular frontal structure, revealed the ongoing competition between transverse wave elimination at the porous wall and re-amplification of triple points within the reaction zone. At critical conditions, when these two effects balance, a unique failure limit is found, namely d∗ ≈ 4λ. Below this limit, the detonations fail. These experiments thus illustrate that transverse wave interactions are essential in the ignition and propagation mechanism for such unstable detonations. In comparison, experiments with argon-diluted detonations displaying a regular cellular structure with weaker transverse waves indicate that their transverse waves do not play a significant role in their propagation mechanism. The failure of these stable detonations is due to the global curvature mechanism caused by the mass divergence into the porous wall, leading to the slow distribution of frontal curvature. The results obtained in diluted and undiluted mixtures are also compared with a model taking into account the mass divergence at the permeable tube walls. Very good agreement is found for the argon-diluted mixtures, while the agreement for the unstable mixtures is very poor. This further indicates that the weak transverse wave structure in argon-diluted detonations, unlike the undiluted ones, does not significantly contribute to the ignition mechanism. Hence these argon-diluted detonations can be well approximated by a ZND reaction zone structure.

Nobility, Visibility, and Publicity in Colley Cibber's Apology

Just what kind of book is An Apology for the Life of Colley Cibber? While this question has not perhaps been burning in the minds of most readers for some time, those who do pick up the Apology are often just as frustrated by its heterogeneity as some of its first readers were in 1740. Cibber’s famous nemesis Henry Fielding, for instance, was quick to register his dissatisfaction with the Apology’s failure to stick to its task: “As to the Book . . . all agreed it was a very entertaining one; that several Parts of it were really excellent, and that if he had not, from the Warmth of his Imagination, run into Nonsense, nor from the Coldness of either his Circumstances or his Principles had crawled out of his Way into Politicks, his Book would have been perfect in its Kind.”1 Fielding does not really tell us, though, what “Kind” of book it could be, at the moderate temperature he implicitly advocates; this problem remains with us. Cibber’s editor B. R. S. Fone is typical in his tripartite division of the book’s aims: “Cibber offers mainly three things in the Apology: an autobiographical account of his life, a history of the theater as he saw it from 1690 to 1733, and a collection of portraits, anecdotes, and observations about actors and acting.”2 Some readers have valued one aspect more than the others; most are happy to take the book as an unstable mixture of all three—or more. We may very well ask how Cibber imagined it all coming together. For his part, he claims at least two goals:

Institutions and Political Change in Russia by Neil s> Robinson (review)

REVIEWS I79 that civil society can have. This makesthe outcome of change uncertainsince it is hardfor new democraciesto maintainlegitimacyand theirabilityto resist restorationistor authoritarianalternativesis thus not guaranteed. Elections create some legitimacy for governments,but do not necessarilylegitimatethe constitutionalstructuresthatthey inhabit.The legitimationof these structures depends on their allowing for revision and learning, in other words depends on the design of constitutionsthemselvesallowingfor change and adaptation, but should not be an open ended process. This observationmarksa watershedin the book since afterit Arato focuses more and more on the dynamics of constitutionaldevelopment. Both Arato's discussion of the role of civil society in the East European transitionand the paradoxes that it produces, and of the strugglesfor legitimacy that surround constitutions are informative. However, the informed analysis of events and the desire for civil society to have some greater role in the process of democratizationthatArato startswith driftapartsomewhere in the middle of the book. Civil society scarcelygets a mention in the last hundred or so pages of the book, which are concerned with a sophisticated, but narrower, argument about constitutionalismin theory and in East European practice. Consequently, one is left with a sense of how things should be ratherthan an argument as to why they should be that way. This gives the book its uneven feel; it is as though there are two books in one cover competing with one another, and in the end the narrower concerns of constitutionalismprevail. Putting this to one side, however, there is still much to be learnt from the book, especially from its questioning of assumptions about the processes of change in EasternEurope. Department ofGovernment andSociety NEIL ROBINSON University ofLimerick Robinson, Neil (ed.). Institutions and PoliticalChangein Russia.Macmillan, Basingstoke and London, and St. Martin's Press, New York, 2000. xii + 235 pp. Tables. Figures. Notes. Bibliographicalreferences. Index. ?42.50. THIS collection of essays is an attempt to introduce new dimensions to the analysis of institutions. The introductorychapter by Robinson startswith a discussionof 'new institutionalism',set apartfromthe 'old institutionalism'by 'its concern to describe actual behaviour, rather than just map formal organizationalstructures'.Institutionsin thiscontext are definedas 'rules,the existence and enforcement of which both enable and constrainbehaviour by actors within institutions'. This definition covers both formal and informal institutionswhich can exist in tandem, reinforcing, subverting or reshaping each other. For Robinson, this creates an ambiguity in 'what actually constitutesan "institution"', especiallywhen appliedto the processof radical politicalchange. It is, indeed, confusing to deal with a 'ship' which, to quote Neurath's metaphor, 'sailorshave to rebuildon the open sea, without ever being able to dismantle it in dry dock and reconstructit from the best components'. And it i8o SEER, 8o, I, 2002 is also daunting to monitor the interplay of the future-orientedreforms and the legacy of the communist past, which accounts for the tendency of postcommunist institutional studies to be focused on the design of formal institutions. Therefore, Robinson's and especially Gordon's points that the relegation of informal institutionsto a secondary role, particularlyin case of Russia, is problematic, and that by no means the Soviet legacy can be viewed as a tabula rasaare highly relevant.Unfortunately,these conclusions are taken no further,and no new ways areofferedforhandlingthe influenceof informal institutions in order to create new institutional frameworks for a 'market democracy'. In fact, apart from Wyman's account of the level of trustpeople have in institutions, which is indicative of the institutional environment in general, most of the chapters of the book focus on the formal institutionsin the making viewed as a ratherchaotic process. Neil Robinson overviewsthe development of the Russian presidencyfroin its founding in I99I till the very end of the Yeltsin era placing particular emphasis on the role played by Russia's first president in the 'politics of institutionalchaos'. The legislativeprocess'schronicdisorganizationbecomes evident fromthe analysisof the activitiesof the lower house of parliament,the State Duma, by PaulChaistyandJeffreyGleisner.The ConstitutionalCourt, the Supreme Court and the Procuracy,accordingto BillBowring,have found it difficult to establish effective hierarchies in the judiciary and agree on mutual competencies in the legal system. Gelman provides a convincing account of a diverse and unstable mixture of types of political regimes in Russia'sregions and republics. Gelman, Gordon, and to some extent Shearman and Sussex, indicate that...

Phase separation in milk protein and amylopectin mixtures

The phase separation behaviour of milk protein (colloidal casein) and amylopectin mixtures was studied. The phase boundary between stable and unstable mixtures was determined by visual observation and confocal microscopy (CSLM). In the latter case, an image analysis procedure (the variance measure) was used. The phase boundary was also calculated using depletion theory as developed by Vrij, A. (1976). Polymers at interfaces and the interactions in colloidal dispersions. Pure and Applied Chemistry, 48, 4, 471–483. Good agreement was obtained between calculated and measured phase boundaries. The CSLM study revealed the appearance of protein aggregate structures when phase separation occurred. Qualitatively, the appearance of the structures was similar to that of rennet- or acid-induced milk protein aggregates. At high concentrations, the aggregates form a network with gel-like properties. The occurrence of aggregate structures has not been reported before for phase separating milk protein–polysaccharide mixtures. So far, emulsion-like microstructures have been observed with milk proteins concentrated in spherical droplets. The observed microstructural difference could be explained using earlier experimental observations reported by Sperry, P. R. (1984). Morphology and mechanism in latex flocculated by volume restriction. Journal of Colloid and Interface Science, 99, 1, 97–108 and a theoretical description of phase separation in colloid–polymer mixtures developed by Lekkerkerker, H. N. W., Poon, W. C. K., Pusey, P. N., Stroobants, A., and Warren, P. B. (1992). Phase behaviour of colloid+polymer mixtures. Europhysics Letters, 20, 6, 559–564 and is ascribed to a low value of the polymer–colloid size ratio.

Physicochemical assessments of parenteral lipid emulsions: light obscuration versus laser diffraction

The United States Pharmacopeia (USP) has proposed a new Chapter 〈729〉 entitled ‘Globule Size Distribution in Intravenous Emulsions’ that is intended to identify methods for analyzing the stability of lipid emulsions. We studied the differences between particle-sizing instruments when analyzing the physicochemical stability of a parenteral nutrition mixture compounded with intravenous lipid emulsion, known as an all-in-one mixture. As the growth of lipid droplets, i.e. coalescence, signals an irreversible change in emulsion stability, we focused our investigation on the large diameter tail (>5 μm) of the globule size distribution. Of the four proposed methods, droplet size was studied over a range of mixture stabilities using a low osmolality parenteral nutrition formula employing both light scattering and light obscuration techniques. In addition, the same mixtures were also freshly prepared, and then spiked with a known amount of 5 μm latex spheres. The response obtained from the light obscuration technique was linear and detected both unstable and latex-spiked mixtures in every case for droplets or particles >5 μm. The results of the laser diffraction method were non-linear and overestimated, was less sensitive or missed entirely, globules or particles in the large diameter tail of the dispersion. The results demonstrate that light obscuration is superior to laser diffraction in identifying unstable intravenous fat emulsions.

The role of unsteadiness in direct initiation of gaseous detonations

An analytical model is presented for the direct initiation of gaseous detonations by a blast wave. For stable or weakly unstable mixtures, numerical simulations of the spherical direct initiation event and local analysis of the one-dimensional unsteady reaction zone structure identify a competition between heat release, wave front curvature and unsteadiness. The primary failure mechanism is found to be unsteadiness in the induction zone arising from the deceleration of the wave front. The quasi-steady assumption is thus shown to be incorrect for direct initiation. The numerical simulations also suggest a non-uniqueness of critical energy in some cases, and the model developed here is an attempt to explain the lower critical energy only. A critical shock decay rate is determined in terms of the other fundamental dynamic parameters of the detonation wave, and hence this model is referred to as the critical decay rate (CDR) model. The local analysis is validated by integration of reaction-zone structure equations with real gas kinetics and prescribed unsteadiness. The CDR model is then applied to the global initiation problem to produce an analytical equation for the critical energy. Unlike previous phenomenological models of the critical energy, this equation is not dependent on other experimentally determined parameters and for evaluation requires only an appropriate reaction mechanism for the given gas mixture. For different fuel–oxidizer mixtures, it is found to give agreement with experimental data to within an order of magnitude.

Molybdenum(VI) speciation in sulfidic waters:: Stability and lability of thiomolybdates

Molybdenum in sedimentary rocks and sediments has evoked interest as a paleo-environmental indicator of reducing conditions. This application demands better understanding of Mo deposition mechanisms and in turn, requires improved knowledge of Mo speciation in anaerobic natural waters. In sulfidic solution, molybdate undergoes sulfidation in four steps that conserve MoVI and lead to tetrathiomolybdate: MoOxS4−x2− + H2S(aq) ⇄ MoOx−1S5−x2− + H2O(l), 1 ≤ x ≤ 4. Equilibrium constants (K(4−x)(5−x)) and rate constants (k(4−x)(5−x)) have been measured by UV-visible spectroscopy. At 25°C, I = 0.8–2.2 M, log K01 = 5.19 ± 0.03, log K12 = 4.80 ± 0.12, log K23 = 5.00 ± 0.13, log K34 = 4.88 ± 0.28. The monothio to trithio-intermediates have negligible stability fields. If H2S(aq) increases slowly enough to maintain near-equilibrium conditions, a sharp switch will occur from MoO42− to MoS42− at ∼11 μM H2S(aq) (298 K, 1 atm). Sulfide actuation of the switch depends on aH2S4, so a ∼3-fold change in aH2S produces a ∼100-fold change in MoO42−/MoS42−. Conversion of the predominant anion from a hard to a soft base alters Mo’s geochemical behavior, increasing its susceptibility to scavenging. Because each successive sulfidation reaction is ∼1 order of magnitude slower than the previous one, dithio- ⇄ trithio- and trithio- ⇄ tetrathiomolybdate equilibria might not be achieved in seasonally or intermittently sulfidic waters. In such environments, unstable mixtures of intermediate thiomolybdates can predominate, contradicting thermodynamic predictions. The sulfidation reactions are acid catalyzed. Anoxic sediment porewaters, which are enriched relative to overlying waters in Brønsted acids, will promote conversion of molybdate to thiomolybdates. This may account for observations that Mo deposition in euxinic environments occurs often by diagenetic pathways rather than by water column scavenging. Slow hydrolysis and oxidation of MoS42− suggests that it could serve as a reservoir of fixed, but acid-labile sulfide in oxic waters.

Synthesis of 4-cyanophenyl and 4-nitrophenyl 2-azido-2-deoxy-1,5-dithio-β- d-arabino- and -β- d-lyxopyranosides possessing antithrombotic activity

Tri- O-acetyl-5-thio- d-ribopyranosyl bromide was converted into 3,4-di- O-benzoyl-1,5-anhydro-5-thio- d- erythro-pent-1-enitol (3,4-di- O-benzoyl-5-thio- d-ribal), the azidonitration of which afforded an unstable mixture of 2-azido-3,4-di- O-benzoyl-2-deoxy-1- O-nitro-5-thio- d-pentopyranoside isomers. This was converted without separation into the corresponding 1- O-acetyl derivatives from which an α,β anomeric mixture of the 1- O-acetyl-2-azido-3,4-di- O-benzoyl-2-deoxy-5-thio- d-arabinopyranose isomers could be isolated in high yield. Glycosidation of this mixture with 4-cyano- or 4-nitrobenzenethiol, using trimethylsilyl triflate or boron trifluoride etherate, respectively, as promoters gave the corresponding β anomers exclusively. Zemplén debenzoylation afforded 4-cyanophenyl as well as 4-nitrophenyl 2-azido-2-deoxy-1,5-dithio-β- d-arabinopyranoside, respectively. When 1- O-acetyl-2-azido-3,4-di- O-benzoyl-2-deoxy-5-thio- d-lyxopyranose was used as glycosyl donor only the corresponding β anomers, i.e., 4-cyanophenyl as well as 4-nitrophenyl 2-azido-2-deoxy-1,5-dithio-β- d-lyxopyranosides, could be isolated after Zemplén debenzoylation in high yield. All four 1,5-dithioglycosides possess significant oral antithrombotic activity.

Stretch effects on the burning velocity of turbulent premixed hydrogen/air flames

Unsteady stretch effects on the laminar burning velocity of unsteady premixed hydrogen/air flames are studied using two-dimensional direct numerical simulation (DNS) with detailed chemical kinetics and transport. Preferential diffusion-unsteady stretch interactions are studied by varying the fuel mass equivalence ratio from fuel-lean to fuel-rich conditions. The correlation of the burning velocity with strain, curvature, and stretch is determined along with other flame propagation statistics. Different definitions of the burning velocity are compared. It is found that definitions based on heat release and fuel consumption lead to different correlations of burning velocity with curvature for some fuel-lean conditions. For fuel-lean and fuel-rich conditions corresponding to unstable and stable mixtures, classical diffusive-thermal effects are recovered in correlations of burning velocity with strain and curvature. It is also observed that the theory for flame propagation in the limit of weak stretch is more widely applicable to turbulent stretch conditions. As such, the strain contribution to the Markstein number is computed from the DNS data and compared against experimental and numerical counterflow data. Good agreement between all three approaches is found in terms of the crossover value of the equivalence ratio from negative to positive Markstein numbers and for fuel-lean stoichiometries. It is found from DNS that strong stretch-preferential diffusion interactions exist in the turbulent flame, with computed Markstein numbers varying between −5.34 and 2.85 and area-weighted mean burning velocities as high as 2.43 times the laminar velocity. It is further observed that over a wide range of mixture stoichiometries, as the ratio of the characteristic turbulence to flame transit time decreases, the flame response to stretch is attenuated, resulting in a marked decrease in Markstein number. This is consistent with linear theory and recent unsteady counterflow computations.

Spinodal decomposition of an ABv model alloy: Patterns at unstable surfaces

We develop mean-field kinetic equations for a lattice gas model of a binary alloy with vacancies (ABv model) in which diffusion takes place by a vacancy mechanism. These equations are applied to the study of phase separation of finite portions of an unstable mixture immersed in a stable vapor. Due to a larger mobility of surface atoms, the most unstable modes of spinodal decomposition are localized at the vapor-mixture interface. Simulations show checkerboard-like structures at the surface or surface-directed spinodal waves. We determine the growth rates of bulk and surface modes by a linear stability analysis and deduce the relation between the parameters of the model and the structure and length scale of the surface patterns. The thickness of the surface patterns is related to the concentration fluctuations in the initial state.

Influence of the Al2O3‐contents in the ternary‐systems (FeO, MnO, CaO, MgO)‐ Al2O3‐SiO2 on the course of the mixture gaps in the ternary space

In accordance with the previous phase equilibria in the ternary systems (FeO, MnO, CaO, MgO)‐ Al2O3‐SiO2 even small Al2O3‐contents should be sufficient to close the wide mixture gaps in the binary edge systems of FeO‐SiO2, MnO‐SiO2, CaO‐SiO2 and MgO‐SiO2, which are stable in the temperature range from 1700 to 2100‐2200°C. Thermodynamically this acceptance is not impossible but rather improb‐able. Based on phase‐theoretical considerations and the previous results of literature about the course of an unstable mixture gap in the system SiO2‐Al2O3 two thermodynamically founded versions were developed as possible, resulting phase equilibria in the SiO2‐rich, but experimentally not exactly examined part of the mentioned ternary‐systems, and were added to the experimentally secured sections of these systems unproblematically.

Thermodynamics of nitrogen-rich uranium sesquinitride formation by reaction of uranium monocarbide with ammonia

According to our thermodynamic analysis, the flowing NH 3 possesses high nitrogen and hydrogen activities due to its instability with respect to the dissociation equilibrium, NH 3⇄(1/2)N 2+(3/2)H 2. As the extent of dissociation of NH 3 into N 2 and H 2, α, is lower, nitrogen and hydrogen activities exhibited by unstable gaseous mixture of NH 3, N 2 and H 2 become higher. The reaction experiments of UC with a stream of NH 3 at 600, 700, 800 and 900°C were performed at two different values of α. Some runs employed 1 g UC as the starting material and the others used 2 g UC. At each of these four temperatures two runs were performed under the same experimental conditions except for the difference of the amount of used UC. The experimental results that the extent of dissociation, α, is larger in the case of using 2 g UC than in the case of using 1 g UC may suggest that the extent of NH 3 dissociation is greatly influenced by the amount of UC as the starting material. It was found that the gaseous phase consisting of NH 3, N 2 and H 2 with lower α yields U 2N 3+ x with higher N/U ratio at a given temperature. These experimental results are consistent with our thermodynamic analysis.

Droplet Breakup and Shear-Induced Mixing in Critical Polymer Blends

Phase-contrast microscopy, small-angle light scattering, and fluorescence microscopy have been combined in situ to study domain deformation, breakup, and homogenization in unstable mixtures of polystyrene (PS) and polybutadiene (PB) under shear flow. Close to the critical point, mixing of the two components toward a single homogeneous phase occurs via repeated deformation and fragmentation of minority-phase droplets, and the data are in good agreement with the mode-coupling renormalization-group (MCRG) theory of a simple binary mixture under shear flow. Well into the two-phase region of diluted high-molecular-weight PS/PB blends, however, the data suggest a tendency for Tc(γ) to saturate at very high shear rates.