This paper investigates stability behavior in a variant of a generalized Jackson queueing network. In our network, some customers use a join-the-shortest-queue policy when entering the network or moving to the next station. Furthermore, we allow interarrival and service times to have general distributions. For networks with two stations we derive necessary and sufficient conditions for positive Harris recurrence of the network process. These conditions involve only the mean values of the network primitives. We also provide counterexamples showing that more information on distributions and tie-breaking probabilities is needed for networks with more than two stations, in order to characterize the stability of such systems. However, if the routing probabilities in the network satisfy a certain homogeneity condition, then we show that the stability behavior can be explicitly determined, again using the mean value parameters of the network. A byproduct of our analysis is a new method for using the fluid model of a queueing network to show non-positive recurrence of a process. In previous work, the fluid model was only used to show either positive Harris recurrence or transience of a network process.
Enzyme activities were determined for lactate dehydrogenase (LDH) powder produced by lyophilization, and two fast freezing processes, spray freeze-drying (SFD) and spray freezing into liquid (SFL) nitrogen. The 0.25 mg/mL LDH aqueous feed solutions included either 30 or 100 mg/mL trehalose. The SFL process produced powders with very high enzyme activities upon reconstitution, similar to lyophilization. However, the specific surface area of 13 m(2)/g for SFL was an order of magnitude larger than for lyophilization. In SFD activities were reduced in the spraying step by the long exposure to the gas-liquid interface for 0.1-1 s, versus only 2 ms in SFL. The ability to produce stable high surface area submicron particles of fragile proteins such as LDH by SFL is of practical interest in protein storage and in various applications in controlled release including encapsulation into bioerodible polymers. The SFL process has been scaled down for solution volumes < 1 mL to facilitate studies of therapeutic proteins. (c) 2006 Published by Elsevier B.V.
Large amount of chemicals and highly purified-water are needed in microelectronic manufacture. The ability of solutions to penetrate tiny spaces will become significantly more challenging as the feature size of semiconductor devices decreases to nanoscale dimensions and the functional complexity of integrated circuitries (ICs) ever increases. Supercritical fluids (SCFs) possess a unique combination of properties (no surface tension and gas-like viscosity) that can potentially be exploited for application in microelectronics manufacturing and processing in response to needs for material-compatible cleaning systems, small-dimension developing solvents, and low chemical-use processes. Recent microelectronics processes for cleaning and rinsing of patterned porous low-k dielectrics and drying of photoresist in CO2-based solvents are the main focus of this review. Additional topics in supercritical fluid processing include spin coating of photoresists, development with nanoscale dimensions, metal deposition and silylation.
Thin film functional hybrid materials composed of inorganic nanocrystals sequestered within a self-assembled template are important for a diverse range of applications, from sensors to device electronics. The properties of these materials can be "tailored" by control of composition over various length scales; the major processing challenges are associated with understanding and controlling external factors, such as confinement and interfacial interactions, that affect the self-organization process. Spin-cast polystyrene-b-poly(1,1’,2,2’-tetrahydroperflurooctyl methacrylate) (PS-b-PFOMA) diblock copolymer films can form a micellar structure, with a PFOMA core (PS corona), and are induced to undergo a transition by annealing in supercritical CO2; consequently, the PS segments form the core with a PFOMA corona. We show that functionalized Au nanocrystals, initially dispersed within the corona (the PS phase) of the micelles, follow the morphological inversion and become sequestered within the core, now composed of PS chains; the nanoparticles segregate primarily at the PS/PFOMA interface within the core. These inversion experiments were performed on nanocomposite films with thicknesses h <= 150 nm. Therefore, only one or two layers of micelles spanned the entire film. The nanoparticles were not distributed uniformly throughout the films but remained primarily near the substrate. Several competing factors determine the overall distribution of nanoparticles: the van der Waals interaction between nanoparticles and interfaces, favorable ligand-block enthalpic interactions, and the conformational entropy of the host chains.
Tertiary amine esters, a new class of surfactants for CO2-based dispersions, stabilize carbon dioxide in water macroemulsions for several hours even at a CO2 density as low as 0.74 g/mL (70 bar) at 298 K. The combination of a weakly hydrophilic tertiary amine, which is protonated by carbonic acid, and branched ester tails provides proper values of the hydrophilic-CO2-philic balance (HCB) for emulsion stabilization. The surfactant nitrilotripropane-1,2-diyl tripivalate (tBu-TIA) lowered the CO2-water interfacial tension to 2.6 mN/m as a result of the stubby architecture (low aspect ratio) of the surfactant tail, which helps block contact between water and carbon dioxide. The high level of methylation produces a smaller interfacial tension and greater emulsion stability relative to nitrilotripropane-1,2-diyl triacetate (TIA). Relative to the high-pressure CO2-water system with a pH 3.3, an increase in pH with the addition of NaOH decreases interfacial activity and reduces emulsion stability, as the surfactant is deprotonated. The adsorption isotherm shows a high interfacial area per surfactant molecule (400 A(2)) as a result of the stubby structure of the surfactant. The extremely low aspect ratio of this surfactant compared to other hydrocarbon surfactants shields water from CO2 at the interface, resulting in a lower interfacial tension, and minimizes interactions between surfactant tail groups. These factors make these low-molecular-weight amine esters desirable for tunable CO2-in-water emulsions, as a replacement for more widely used fluorinated surfactants. The facile synthesis of a variety of tertiary amine esters makes this class of surfactants attractive for developing structure-property relationships.
A dynamical estimate of the axial component of a Mach 0.60 axisymmetric jet’s turbulent velocity field is presented here using spectral linear stochastic estimation. The pressure field surrounding the exit of the jet is employed as the unconditional parameter in the estimation technique. A sub-grid interpolation method is used to improve the spatial resolution of the estimate. The model estimate is time-resolved and reconstructed using a purely experimental database. A decomposition of the model estimate using POD and Fourier-azimuthal techniques identifies the turbulent velocity modes that are responsible for driving the near-field pressure when compared with direct measurements of the jet’s modal features. In effect, the signatures left in the near pressure field by the turbulence are a result of the low-order structure, the higher azimuthal modes being inefficient in driving the hydrodynamic pressure. A direct calculation of the source field using a Lighthill approach is performed, from which the low-dimensional features of the sound source mechanisms are illustrated.
Rapid dissolution rates of nanocrystal suspensions of the poorly water-soluble drugs, danazol and itraconazole were measured continuously by in-situ turbidimetry. For pre-wetted suspensions of 300 nm particles, dissolution half-lives as short as a few seconds were determined upon adding surfactant to initiate dissolution. A mass transfer model is presented to determine the particle size distribution and dissolution rate in terms of two steps: interfacial reaction, consisting of micelle uptake and desorption, followed by diffusion of the drug-loaded micelles. The interfacial reaction rate constant, k(S), regressed from turbidity versus time data, in conjunction with the Mie theory of light scattering, was independent of particle size. Therefore, dissolution rate data for micron-sized drug particles, which are widely available, may be used to predict the behavior for submicron particle sizes down to 100 nm. The micellar solubility and k(S) are significantly smaller for itraconazole than danazol, consistent with itraconazole’s larger molecular size. For particles smaller than 1 mu m, the interfacial reaction resistance was dominant. Since this resistance has received little attention in previous studies, further emphasis on the design of drug nanoparticles with more rapid interfacial reaction offers the possibility of improvements in dissolution rates. (c) 2007 Published by Elsevier B.V.
W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 mm. A porouscross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl) triethyoxysilane, to render the particles CO2-philic. The stability of emulsions comprising equal weights of CO2 and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO2 density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO2-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10 6 kT, even at CO2 densities as low as 0.78 g ml(-1). At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.