Optimal inspection and perfect repair

Models are developed for decision making about monitoring andmaintenance of systems whose performance through time is describedby a general stochastic process. The system is monitored andpreventive and corrective maintenance actions are carried outin response to the observed system state. The decision processis simplified by using the maximum process as a decision variable.The models developed generalize age replacement models and othersimple maintenance strategies. The approach can deal with failuresthat prevent the system functioning further, and also failuresthat are defined by regulation or economic considerations. Attentionis restricted to perfect repair and inspection, but the structureprovides the framework for further developments.     

A theoretical examination of the acid-catalyzed and noncatalyzed ring-opening reaction of an oxirane by nucleophilic addition of acetate. Implications to epoxide hydrolases

Ab initio and density functional calculations have been performed to gain a better understanding of the epoxide ring-opening reaction catalyzed by epoxide hydrolase. The S(N)2 reaction of acetate with 1S,2S-trans-2-methylstyrene oxide to provide the corresponding diol acetate ester was studied with and without general-acid catalysis. MP2 and DFT (B3LYP) calculations predict, for the noncatalyzed reaction, a central barrier of approximately 20-21 kcal/mol separating the reactants from products depending on which carbon center in the epoxide is undergoing attack. From these gas-phase reactions the immediate alkoxide products are not energetically far below their associated transition states such that the reaction is predicted to be endothermic. Inclusion of aqueous solvation effects via a polarizable continuum model predicts the activation barrier to increase by almost 10 kcal/mol due to the solvation of the acetate ion nucleophile. The activation barrier for the epoxide ring-opening reaction is reduced to approximately 10 kcal/mol when phenol, as the general-acid catalyst, is included in the gas-phase calculations. This is due to the immediate product being the neutral ester rather than the corresponding alkoxide. The transition state in the general-acid-catalyzed reaction is earlier than that for the noncatalyzed reaction and the reaction is highly exothermic. Molecular mechanics calculations of 1S,2S-trans-2-methylstyrene oxide in the active site of murine epoxide hydrolase show two possible binding conformations. Both conformers have the epoxide oxygen forming hydrogen bonds with the acidic hydrogens of the catalytic tyrosines (Tyr381 and Tyr465). These two conformations likely lead to different products since the nucleophile (Asp333-CO(2)(-)) is positioned to react with either carbon center in the epoxide.     

Incorporating zosteric acid into silicone coatings to achieve its slow release while reducing fresh water bacterial attachment

Biofouling has posed serious problems in maritime industry including increased fuel consumptions, economic loss from ship-hull maintenances, contamination of drinking water, and serious corrosion for mechanical instruments. Minimizing the attachment of bacteria and formation of biofilm could be advantageous in reducing the early stages of biofouling. Zosteric acid, a natural product present in eelgrass, was found to have ability for preventing the attachment of some bacteria and barnacles. In this study, the antifouling ability of zosteric acid during the early stages of fouling was evaluated using attachment studies of fresh water bacteria. Simultaneously, various methods were sought for incorporating zosteric acid into silicone to prolong the release of the compound. The main results from this study were that zosteric acid exhibited anti-bacterial attachment regardless of whether it dispersed in water or incorporated into a coating. In addition, the release rate of zosteric acid from the incorporated coatings, particularly those where zosteric acid was uniformly dispersed with aggregates size of 4 microm or less, was orders of magnitude slower than those of previous reports. The release results indicate that the service life of our coatings could be far extended even with a small amount of zosteric acid incorporated.     

Initial bacterial attachment in slow flowing systems: effects of cell and substrate surface properties

Bacterial biofilm can have significant effects on the behaviors and/or performance of natural and man-made systems. Understanding the factors governing initial bacterial attachment is critical to biofilm management. In this study, the initial attachment of three bacteria, Pseudomonas aeruginosa, Escherichia coli and Pseudomonas putida, on two substrates, glass and octadecyltrichlorosilane (OTS) modified glass, was examined in flow chambers. The flow chambers were designed and operated to mimic slow moving water bodies and minimize the gravitational settlement of cells. The hydrophobicity of bacterial surface was evaluated by partitioning of cells to the water-hexadecane interface and the liquid contact angles on cell layers collected on filter papers. On the more hydrophilic glass surface, the attachment trend was found to be E. coli>P. putida>P. aeruginosa, while the opposite trend was observed on the hydrophobic, OTS modified surface. The attachment trend on glass could be explained by the magnitude of the negative interaction energy at secondary minima, as predicted by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The much higher attachments of P. aeruginosa and P. putida on the OTS-modified substrate, on the other hand, suggested that these cells could overcome the energy barrier between the primary and secondary minima of interaction energy to become attached to the primary minimum. The extent of primary-minimum attachment appeared to correlate with the scale of the energy barrier, with higher attachments in the bacteria-substrate combinations of lower energy barriers. The study generated important insights into the effects of cell and substrate surface properties on initial bacterial attachment.     

High-performance electron-transporting polymers derived from a heteroaryl bis(trifluoroborate)

In this communication, we report that dipotassium aryl bis(trifluoroborate)s make stable and easy-to-purify yet reactive monomers under Suzuki polycondensation reactions. A bis(trifluoroborate) of 2-alkylbenzotriazole was prepared successfully and copolymerized with dibromobenzothiadiazole in the presence of a Pd catalyst and LiOH, yielding high molecular weight conjugated polymers. This polymer (P1) composed of all electron-accepting units shows excellent electron-transport properties (μ(e) = 0.02 cm(2) V(-1) s(-1)), which proves the value of the aryl bis(trifluoroborate) monomers and suggests that many other types of semiconducting polymers that could not be accessed previously can be synthesized using this approach.     

Application of solution techniques for rapid growth of organic crystals

Single crystals of a pure hydrocarbon, 1,3,5-triphenylbenzene, with properties suitable for high-energy neutron detection were grown from toluene solutions using slow evaporation and temperature reduction methods with growth rates up to 10 mm/day. Application of the rapid growth technique developed earlier for growth of large water-soluble crystals shows that crystals of aromatic compounds can be successfully grown from solutions in large volumes required for their use as scintillator materials for radiation detection.     

Polymeric microstructure arrays consequence of Marangoni flow-induced water droplets

This paper describes a cost-effective approach to fabricate intricate arrays of polydimethylsiloxane (PDMS) and polymeric microstructures based on porous polystyrene (PS) films generated from arrays of water droplets. To start, a thin layer of ethanol film is exposed to a humid air flow. Upon the evaporation of ethanol and simultaneous condensation of water as the ethanol phase recedes, a Marangoni flow causes the flow of liquid from the ethanol phase into water fingers emerged along the receding contact line, which finally detach to form ordered water droplet arrays behind the receding contact line. The water droplet arrays are subsequently used as templates to generate porous PS films. The porous PS films are then used as sacrificial layers and masters to fabricate various arrays of PDMS dots and PDMS stamps with posts, respectively. The PDMS stamps containing various microstructures are further utilized to create polymer rings, PDMS dots, porous PDMS films, and PDMS aperture rings, and for contact printing of patterns of self-assembled monolayers.     

Marangoni flow-induced self-assembly of hexagonal and stripelike nanoparticle patterns

We have developed a simple Marangoni flow-induced method for self-assembling nanoparticles (NPs) into both hexagonal and stripelike patterns. First, a NPs/ethanol suspension was spread on a slightly nonwettable and a wettable silicon oxide substrate. The Marangoni flow, induced by simultaneous evaporation of ethanol and condensation of water, leads to the formation of the corresponding hexagonal distributed circular NP rings and dotted stripes. The inter-ring spacing and ring size of the hexagonal patterns can be tuned by varying the relative humidity of the N2 stream blown over the slightly nonwettable substrate. Hexagonal patterns of circular NP patches can also be fabricated by lowering the evaporation of the condensed water droplets. On the wettable substrate, complex patterns result when the humidity of the N2 stream changes.     

Photochemical and discharge-driven pathways to aromatic products from 1,3-butadiene

A detailed study of the photochemical and discharge-driven pathways taken by gas-phase 1,3-butadiene has been carried out. Photolysis or discharge excitation was initiated inside a short reaction tube attached to the outlet of a pulsed valve. Bath gas temperatures near 100 K were achieved in the reaction tube by the constrained expansion of the gas mixture into the tube, simulating temperatures of relevance in Titan's atmosphere. Photolysis of 1,3-butadiene was initiated at 218 nm with a laser pulse that counter-propagated the reaction tube. Discharge excitation was carried out using discharge electrodes imbedded in the reaction tube walls, enabling the study of the photochemical and discharge products under similar conditions. Products were detected using either single-photon VUV photoionization (118 nm = 10.5 eV) or resonant two-photon ionization (R(2)PI) spectroscopy in a time-of-flight mass spectrometer. Emphasis was placed on characterization of the aromatic products formed, since these may be of particular relevance to Titan's atmosphere, where benzene has been positively identified and 1,3-butadiene is projected as the principle pathway to its formation. Consistent with previous studies of the photodissociation of 1,3-butadiene, C(3)H(3) + CH(3) is the dominant primary product formed. Under the temperature-pressure conditions present in the reaction tube (T approximately 75-100 K, P = 50 mbar), C(6)H(6) is the dominant secondary photochemical product formed. A 1:1 C(4)H(6):C(4)D(6) mixture was used to prove that the C(6)H(6) product was formed by recombination of two C(3)H(3) radicals; however, a careful search for benzene revealed none, indicating that less than 1% of the C(6)H(6) formed in the reaction tube is benzene. This is consistent with expectations for these temperatures and pressures based on previous modeling of propargyl recombination. Two aromatic products were observed from the photochemistry: ethylbenzene and 3-phenylpropyne. Plausible pathways leading to these products are proposed. In the discharge, C(3)H(3) + CH(3) are also identified as significant primary neutral products and C(6)H(6) as a dominant higher-mass product. In this case, the C(6)H(6) was identified as benzene via its R2PI spectrum, appearing with intensity about 10 times larger than any other aromatic formed in the discharge. R2PI spectra of a total of about 15 aromatic products were recorded from the 1,3-butadiene discharge, among them toluene; styrene; phenylacetylene; o-, m-, and p-xylene; ethylbenzene; indane; indene; beta-methylstyrene; and naphthalene. Previously unidentified spectra in the m/z 142 and 144 mass channels were positively identified as the 1,3- and 1,4-isomers of phenylcyclopentadiene and the analogous 1-phenylcyclopentene.