ACOUSTIC STUDIES OF INTERFACIAL EFFECTS IN AIRLIFT REACTORS

Rational design of airlift reactors for the culture of plant and animal cells is impeded by a lack of understanding of the causes of loss of viability. Some recent speculations in the literature suggest that gas-liquid interfacial phenomena have played prominent roles in observed instances of loss of viability in both insect and animal cell cultures. The important interfacial events may include bubble formation and detachment at the sparger, bubble coalescence and breakage, and bubble disengagement at the free surface-entailing film thinning, film rupture, and film droplet ejection.

We have carried out an experimental investigation of interfacial phenomena using macrovideography and acoustic signals recorded at both the sparger and the free surface. Data have been collected for several liquid media including distilled water, distilled water with electrolyte (NaCl), and aqueous solutions of glycerol (μ = 3 to 9.5 cP). The studies were conducted in a 3-liter, acrylic plastic, split-column airlift reactor, and air was introduced through interchangeable sieve plates (with hole diameters of 1, 2, and 3 mm). Time-series data collected from the microphone were processed for mean and root-mean-square values, and Fourier transforms were computed to identify important signal energies. A sequence of experiments was also conducted that was designed to relate specific interfacial phenomena to the frequencies of the noises produced.     

The jet shape of concentric mixers

In this paper, we present an analytical and experimental study of the mixing region and jet shape in concentric mixers. Based on a simple but effective analytical model, we are able to predict the growth of the mixing region with respect to downstream distance. An experimental study is conducted on the concentric mixing of paper fibre stock and water. By comparing with experimental results, we confirm that the shape of the mixing region is a practical indicator of mixing efficiency with the consideration of jet velocity and geometry including the nozzle wall thickness.     

Chemistry of the Interface Between Aluminum and Polyester Films

It has been observed that the adhesion between vacuum-evaporated aluminum and poly(ethylene isophthalate-co-ethylene sodium sulfoisophthalate) copolymer is approximately five times greater than the adhesion between vacuum-evaporated aluminum and biaxially-oriented poly(ethylene terephthalate) film. To describe the interface between the aluminum and these polymeric substrates, thermoanalytical, spectroscopic and microscopic techniques have been applied. Definite changes in surface elemental composition and chemical functionality occur upon metallization of the polymer films. Aluminized samples contained two new oxygen functionalities; one due to the aluminum oxide and the other due to an organoaluminum species. Thermal degradation, as may occur during vacuum evaporation, would be expected to yield a carboxylic acid endgroup and a vinyl endgroup for each chain scission reaction that occurred. Reaction of aluminum with these carboxylic acid endgroups is thought to be responsible for the organoaluminum oxygen peak that was observed. Based on the XPS data, however, the level of this new functionality was comparable for both types of polyester film. Thus, this new functionality may be involved in promoting aluminum/polyester adhesion, but by itself cannot explain the differences in the level of adhesion that are attained. It appears, based on the transmission electron micrographs, that the aluminum deposit penetrates the copolymer coating to a greater depth than it does the PET. The greater level of penetration could be responsible for the greater adhesion obtained between vacuum-evaporated aluminum and the copolymer film compared with the level of adhesion obtained with the PET film. Based on this work, it appears that the adhesion of the vacuum-evaporated aluminum to both polyesters has a similar chemical component (type and amount) but a different extent of the mechanical component.     

Simplified Model for Oxygen Transport With Reaction in a Polymer‐Electrolyte Fuel Cell

A simplified mathematical model for an ion‐exchange membrane attached to a gas‐fed porous electrode is developed to simulate the oxygen electrode of a solid‐polymer‐electrolyte fuel cell. In particular, the present model is derived from an earlier rigorous one of Bernardi and Verbrugge(1991) by neglecting the Peclet number for the transport of dissolved oxygen within the catalyst. The advantage of this simpler model is that it can be solved analytically, eliminating the need for numerical simulation. Longitudinal profiles for the dissolved oxygen concentration and catalyst current density calculated from the present model are in good agreement with results from the earlier rigorous model.     

Electron Spin Echo Modulation Studies of Dipolar Hyperfine Interactions of Catalytic Reaction Intermediates Involving Transition Metal Ions on Oxide Surfaces

The application of electron spin echo modulation (ESEM) pulsed electron spin resonance methods to detect dipolar hyperfine interactions of catalytic reaction intermediates involving transition metal ions on oxide surfaces is described. This provides a method to determine critical aspects of the geometrical structure of such intermediates. One example involves catalysis of ethylene dimerization by paramagnetic Ni⁺ on silica. ESEM results show direct coordination of two and three ethylene molecules to Ni⁺ as reaction intermediates in ethylene dimerization, depending on the activation temperature of the catalyst pretreatment. A second example involves catalysis of ethylene dimerization by paramagnetic Pd⁺ in X—zeolite. ESEM results show direct coordination of successively one and two molecules of ethylene to Pd⁺ prior to dimerization. Thus ESEM methods provide a powerful tool to develop a molecular picture of the course of catalytic reactions on surfaces.     

Bake oven induced variation of surface chemistry on electrocoat paint: Effect on primer-electrocoat intercoat adhesion

The interplay of bake oven processing environment and surface/interfacial chemistry, and their effect on intercoat adhesion has been investigated for a model primer/electrocoat paint system. X-ray photoelectron spectroscopy (XPS) surface analysis established that a prominent ether component in the C 1s core level spectrum correlated with a polyether-based crater-control additive in the electrocoat that surface-segregates during cure. Laboratory-simulated bake oven experiments confirmed that better adhesion characteristics were realized by removal of this electrocoat overlayer through reaction with nitrogen oxides and high levels of moisture present in the cure environment of production direct-fired ovens. Details of the effect of bake oven atmosphere on cure chemistry for direct- versus indirect-fired ovens are presented.     

Stresses Due to Temperature Gradients in Ceramic-Matrix-Composite Aerospace Components

Ceramic-matrix composites (CMCs) are being considered as replacement materials for divergent flaps and seals in advanced aerospace turbine engines. During service, these components are subjected to severe temperature gradients across the width of the flaps. This paper discusses an analytical procedure to estimate the stresses generated in the CMC flaps due to the temperature gradients. The analytical procedure can be used for a material with nonlinear temperature-dependent stress–strain behavior. This procedure was used to predict the thermal stresses in four candidate CMC systems due to temperature gradients. The thermal stresses along the edges typically exceed the proportional limit, and sometimes the fatigue limit of the CMC.     

Influence of gluten on the viscoelastic properties of starch pastes and gels

Viscoelastic properties of dispersions (60–300 g kg⁻¹) of gluten (G) and wheat starch (S) blends (0 < G/S < 0·20) and wheat flour have been studied during heating and cooling. In both cases, the moduli followed power law relationships with concentration. The temperature at which the transient network development began, caused by granule–granule interactions, decreased as the concentration increased and increased with an increase in the proportion of gluten. Moreover, gluten weakened the strength of both starch pastes and gels, as shown by the lower values of the moduli. The viscoelastic behaviour of flour samples reflected the role played by internal lipids. A structural model is proposed in order to explain the influence of gluten on the rheological behaviour of starch pastes and gels. © 1998 Society of Chemical Industry.     

The efficacy of linalool, a major component of freshly-milled Ocimum canum Sims (Lamiaceae), for protection against postharvest damage by certain stored product Coleoptera

Linalool was present at 8.6 ± 0.9 mg/g in the dried leaves of Ocimum canum Sims, an annual mint used in Rwanda to protect against postharvest insect damage. Direct exposure of adults of Zabrotes subfasciatus (Bohem.) to milled, dried O. canum leaves resulted in 100% mortality of males and 50% mortality of females after 48 hr. Dose-response curves for linalool were completed with adult Z. subfasciatus, Acanthoscelides obtectus (Say), Rhyzopertha dominica (F.), and Sitophilus oryzae (L.) using a filter paper bioassay. The LC₅₀ values were: 428 μg/cm² for Z. subfasciatus; 405 μg/cm² for A. obtectus; 428 μg/cm² for R. dominica; 427 μg/cm² for S. oryzae. Knockdown was occasionally followed by recovery at doses less than the LC₅₀ for all species. There are significant differences in the LC₅₀ and LT₅₀ values for male and female Z. subfasciatus. At the lower dosages hyperactivity rarely preceded moribundity and mortality where these occurred, while at higher dosages hyperactivity occurred soon after initial exposure and preceded imminent death. A concentration increase from 250 to 750 μg/cm², representing a tripling of dosage, spanned th 10–100% response mortality for all species at 24 hr. Air-exposure of linalool-treated papers (500 μg/cm²) for up to 24 hr significantly decreased toxicity to both sexes of Z. subfasciatus. Quantitative analysis showed the only significant decrease in the amount of linalool to occur after 0.25 hr, and this did not fully correlate with the resulting decrease in efficacy against both sexes of Z. subfasciatus. The results are discussed in terms of the efficacy of using O. canum for the protection against loss due to insects in the traditional food storage systems of Rwanda.     

Effect of inoculum size, relative humidity, storage temperature, and ripening stage on the attachment of Salmonella Montevideo to tomatoes and tomatillos

The influence of inoculum populations and environmental factors on attachment of Salmonella Montevideo to the surface of tomatoes and tomatillos was evaluated. To study the effect of inoculum size, red, ripe tomatoes were spot-inoculated with bacterial suspensions (10(5) and 10(8) CFU/fruit) and stored at 22 degrees C under 100% relative humidity. The effects of temperature (12, 22, and 30 degrees C) and relative humidity (75, 85, and 97%) on attachment of the pathogen (10(7) CFU/fruit) to tomatoes (red and green) and ripe tomatillos were also evaluated. Inoculated fruits were stored for 90 min at all combinations of temperature and relative humidity, and after rinsing with water, the number of cells attached to the surface was determined. Salmonella Montevideo attached to the surface of tomatoes within 90 min. A direct correlation between the number of attached cells and the population in the inoculum was observed. The percentage of cells that attached immediately after inoculation was approximately 0.3% for the three test products. After storage for 90 min at various temperature and relative humidity conditions, the number of adhering cells ranged from 4.0 to 5.4 log CFU/fruit (1.2% of inoculum). Both the type of product and the temperature/relative humidity combination had a significant (P < 0.05) effect on attachment of Salmonella Montevideo to the surfaces of tomatoes and tomatillos. Scanning electron micrographs of the cuticles of inoculated washed tomatoes and tomatillos revealed typical skin cell patterns, and only a few randomly dispersed Salmonella Montevideo were observed. Deposition of Salmonella Montevideo on the surface of tomatoes and tomatillos could result in attachment and subsequent colonization under suitable conditions.