High-speed imaging of the pulsed-field flashover of an aluminaceramic in vacuum

An intensified high-speed imaging technique was used to observe the optical events associated with the pulsed-field breakdown of an alumina ceramic tube having two concentric planar electrodes metalized onto its end face. HV pulses, typically of 5 kV amplitude (5 kV μs ^-1 rate of rise), were applied to the radial MIM insulator-electrode regime under ultrahigh vacuum (pressure <5×10^-7 Pa), with video recordings made at 1000 frames per second. Images of the observed breakdown phenomena are presented, viewed both along the center axis and also in the plane of the MIM structure. These breakdown images are discussed in relation to plasma jets associated with vacuum arcs and, in particular, on the nature of the ion species within such jets     

Thermal and nonthermal physiochemical processes in nanoscale films of amorphous solid water

Amorphous solid water (ASW) is a disordered version of ice created by vapor deposition onto a cold substrate (typically less than 130 K). It has a higher free energy than the crystalline phase of ice, and when heated above its glass transition temperature, it transforms into a metastable supercooled liquid. This unusual form of water exists on earth only in laboratories, after preparation with highly specialized equipment. It is thus fair to ask why there is any interest in studying such an esoteric material. Much of the scientific interest results from the ability to use ASW as a model system for exploring the physical and reactive properties of liquid water and aqueous solutions. ASW is also thought to be the predominant form of water in the extremely cold temperatures of many interstellar and planetary environments. In addition, ASW is a convenient model system for studying the stability of amorphous and glassy materials as well as the properties of highly porous materials. A fundamental understanding of such properties is invaluable in a diverse range of applications, including cryobiology, food science, pharmaceuticals, astrophysics, and nuclear waste storage, among others. Over the past 15 years, we have used molecular beams and surface science techniques to probe the thermal and nonthermal properties of nanoscale films of ASW. In this Account, we present a survey of our research on the properties of ASW using this approach. We use molecular beams to precisely control the deposition conditions (flux, incident energy, and incident angle) and create compositionally tailored, nanoscale films of ASW at low temperatures. To study the transport properties (viscosity and diffusivity), we heat the amorphous films above their glass transition temperature, T(g), at which they transform into deeply supercooled liquids prior to crystallization. The advantage of this approach is that at temperatures near T(g), the viscosity is approximately 15 orders of magnitude larger than that of a normal liquid. As a result, the crystallization kinetics are dramatically slowed, increasing the time available for experiments. For example, near T(g), a water molecule moves less than the distance of a single molecule on a typical laboratory time scale (～1000 s). For this reason, nanoscale films help to probe the behavior and reactions of supercooled liquids at these low temperatures. ASW films can also be used for investigating the nonthermal reactions relevant to radiolysis.     

Fluidization of spherical versus elongated particles: Experimental investigation using magnetic particle tracking

In biomass processing fluidized beds are used to process granular materials where particles typically possess elongated shapes. However, for simplicity, in computer simulations particles are often considered spherical, even though elongated particles experience more complex particle– particle interactions as well as different hydrodynamic forces. The exact effect of these more complex interactions in dense fluidized suspensions is still not well understood. In this study we use the magnetic particle tracking technique to compare the fluidization behavior of spherical particles to that of elongated particles. We found a considerable difference between fluidization behavior of spherical versus elongated particles in the time-averaged particle velocity field as well as in the time-averaged particle rotational velocity profile. Moreover, we studied the effect of fluid velocity and the particle's aspect ratio on the particle's preferred orientation in different parts of the bed, which provides new insight in the fluidization behavior of elongated particles.     

Increased Activity of Cell Surface Peptidases in HeLa Cells Undergoing UV-Induced Apoptosis Is Not Mediated by Caspase 3

We have previously shown that in HeLa cells treated with a variety of agents there is an increase in cell surface peptidase (CSP) activity in those cells undergoing apoptosis. The increase in CSP activity observed in UVB-irradiated cells undergoing apoptosis was unaffected when the cultures were treated with the aminopeptidase inhibitor bestatin, and matrix metalloprotease inhibitor BB3103, but greatly enhanced when treated with the caspase 3 inhibitor-DEVD, and reduced in the presence of the poly(ADP-ribose) polymerase (PARP) inhibitor-3-aminobenzamide (3AB). Neither 3AB nor DEVD had an effect on the gross morphology of the apoptotic cells observed under electron microscopy, nor did they have an effect on phosphatidylserine eversion on the cell membrane, or that of PARP cleavage. All the agents except for DEVD had no effect on the level of caspase 3 activity in the cells. The results suggest that other caspases may cleave PARP in these cells. Both 3AB and DEVD treatment reduced the level of actin cleavage seen in the apoptotic cells. The increase in CSP activity observed in cells undergoing UVB-induced apoptosis appears to involve PARP but not caspase 3.     

An objective classification of condensation regimes in direct contact condensation

Intensified heat treatment, using direct contact condensation (DCC), is applied in the production of dairy products to ensure a high level of food safety. The key challenge with DCC is the fouling due to the protein reactions that limits operational efficiency and sustainability. Using a condensation regime map can improve operational decision-making. Pilot plant scale experiments were conducted for a wide range of steam mass fluxes and inlet temperatures at high and low channel pressures. High-speed images were recorded and analyzed to obtain penetration lengths and plume area. The experimental data and image analysis supplemented with temperature and pressure measurement, were processed using machine learning (ML) to develop a data driven model to predict the regime maps. The linear discriminant analysis (LDA) was found to be the most suitable model. From the ML models it was also found that the best parameters to make a condensation regime map are the steam pressure, channel pressure, subcooling temperature, water Prandtl number, and the relative velocity ratio between gas and liquid. The condensation outcomes were presented with various two-dimensional regime maps. New regime maps are proposed using the Prandtl number and velocity ratio as dimensionless parameters.     

Single amino acid substitutions can further increase the stability of a thermophilic L-lactate dehydrogenase

Lactate dehydrogenases are of considerable interest as stereospecificcatalysts in the chemical preparation of enantiomerically pure-hydroxyacid synthons. For such applications in synthetic organicchemistry it would be desirable to have enzymes which tolerateelevated temperatures for prolonged reaction times, to increaseproductivity and to extend then applicability to poor substrates.Here, two examples are reported of significant thermostabilizations,induced by sitedirected mutagenesis, of an already thermostableprotein, the L-lactate dehydrogenase (EC 1.1.1.27 [EC] , 35 kDa permonomer subunit) from Bacillus stearothermophilus. Thermal inactivationof this enzyme is accompanied by irreversible unfolding of thenative protein structure. The replacement of Argl71 by Tyr stabilizesthe enzyme against thermal inactivation and unfolding. Thisstabilizing effect appears to be based on improved interactionsbetween the subunits in the core of the active dimeric or tetramericforms of the enzyme. The thermal stability of L-lactate dehydrogenasevariants with an active site Arg residue, either in the 171(wild-type) or in the 102 position, is further increased bysulfate ions. The two stabilizing effects are additive, as foundfor the Argl71Tyr/ Gln1O2Arg double mutant, for which the stabilityof the protein in 100 mM sulfate solution reaches that of L-lactatedehydrogenases from extreme thermophiles. All mutant proteinsretain significant catalytic activity, both in the presenceand absence of stnhilfoing salts, and are viable catalysts inpreparative scale reactions.     

Polyionic fusion peptides function as specific dimerization motifs

The de novo design of a molecular adapter for directed associationand covalent linkage of two polypeptides is presented. Usingpeptides containing charged amino acid residues and an additionalcysteine residue (AlaCysLys₈ and AlaCysGlu₈) we demonstratethat the electrostatic interaction promotes the associationof two synthetic peptides and, subsequently, disulfide bondformation. The reaction depends on both the redox potentialand on the ionic strength of the buffer. Varying the redox potential,the interaction of the peptides was quantified by a G^0' of 6.6± 0.2 kcal/mol. Heterodimerization of the peptides ishighly specific, a competition of association by other cysteinecontaining compounds could not be observed. Two proteins comprisingcysteine-containing polyionic fusion peptides, a modified Fabfragment and an -glucosidase fusion, could be specifically conjugatedby directed association and subsequent disulfide bond formation.Both proteins retain their functional characteristics withinthe bifunctional conjugate: enzymatic activity of the glucosidaseand antigen-binding capacity of the Fab fragment are equivalentto the non-conjugated components.     

Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol-gel conversion coating (Boegel-EPII), derived from an acid-catalyzed aqueous solution of organofunctional silane and zirconium alkoxide precursors, is being used as an adhesion promoter for adhesive bonding and painting applications in the aerospace industry. A unique advantage of the sol-gel process is that strong and durable bonds are produced without the hazardous chemical usage and rinse-water requirements of conventional anodizing or etching processes. In this study, a fracture mechanics method was used to investigate the adhesion properties of sol-gel-reinforced epoxy/aluminum joints. The Hugh Brown asymmetric double cantilever beam (ADCB) wedge test was employed, which allowed the measurements of the critical energy-release rate, subcritical crack-growth kinetics, and threshold energy-release rate on a single sample in a reasonably short period of time. These experiments were carried out with aluminum substrates on which the surface morphology was systematically varied by polishing, sanding, grit-blasting, and chemical etching. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to identify the locus of failure. The surface morphology of the substrates was characterized with SEM, optical profilometry, and spreading kinetics. The macrorough structures drive the crack to within a thin epoxy layer close to the polymer/metal interface, which enhances the initial strength of the sol-gel-reinforced interface. The microroughness of the substrate is, however, more effective than the macroroughness in enhancing the durability. Lastly, an attempt has been made to correlate the energy-release rate with the fractal dimension for sol-gel-reinforced joints with macrorough substrates.     

Water sorption of poly(vinyl alcohol)/ poly(diallyldimethylammonium chloride) interpenetrating polymer network hydrogels

Temperature‐responsive interpenetrating polymer network (IPN) hydrogels constructed with poly(vinyl alcohol) and poly(diallyldimethylammonium chloride) using the sequential IPN method were studied. The characteristics of IPN hydrogels were investigated using the dynamic vapor sorption system. IPN hydrogels exhibited a relatively high sorption ratio, 180–360% at room temperature. The sorption ratio of hydrogels depended on temperature. Diffusion coefficients were calculated according to the Fickian Law at several temperatures. The apparent activation energy was 5.43 kJ mol^?1, which corresponds to typical diffusion processes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1389–1392, 2003