Experimental study of vapor bubbles in small-sized channels

This paper investigates the effect of evaporation on the shape of liquid/vapor interfaces in small-scale systems. Vapor bubbles are generated due to localized heating in a small-sized channel (with an inner dimension of 3x3x200 mm) filled with pentane, for which heat fluxes and temperature distributions are simultaneously measured. The length of the resulting vapor bubble is studied as a function of the power input and heater temperature, and is found to be not only repeatable but nonhysteretic. The bubble length depends nearly linearly on the power input, in qualitative agreement with an approximate theory of Ajaev and Homsy [V.S. Ajaev, G.M. Homsy, J. Colloid Interface Sci. 244 (2001) 180]. In addition, it is found that vapor bubbles oscillate very slowly due to the effect of thermal relaxation.     

Dynamic removal of oral biofilms by bubbles

A novel approach to the removal of biofilms from solid surfaces is to pass large numbers of air bubbles over the surfaces. Such a phenomenon occurs when teeth are brushed with some types of powered toothbrushes that accelerate bubbly fluid against or across teeth surfaces. Video recordings of air bubbles propelled against a mature biofilm of Streptococcus mutans showed that the bubbles removed the biofilm at the point of collision. A mathematical model of the removal process was proposed and was able to simulate the kinetics of the biofilm removal process. Removal rate was modeled to be proportional to the bubble footprint area and the number of collisions per time. The fraction of biofilm removed per bubble collision is on the order of 0.4, a value much larger than would have been expected based on previous research employing bubbles that moved slowly along a surface that was partially covered with adherent bacteria. The higher removal efficiency is attributed to fluid dynamic shear forces that occur in conjunction with the thermodynamic forces that pull bacteria from a surface as a bubble contacts the biofilm. Fast bubbly flow is expected to remove bacterial biofilm from hard surfaces such as teeth.     

Excitation of nanoscale vapor bubbles at the surface of gold nanoparticles in water

Intense nonequilibrium femtosecond laser excitation of gold nanoparticles in water leads to a transient heating of the nanoparticles, which decays via heat transfer to the water phase. It is shown that the water temperature rises to near the critical temperature and the water undergoes an explosive evaporation in the subnanosecond range. The formation of vapor bubbles shows a threshold dependence on laser fluence. The nascent nanoscale vapor bubbles change the heat dissipation drastically. The nanoscale structure is resolved directly with a combination of x-ray scattering methods sensitive to the particle lattice expansion and the change in the water structure factor.     

Disjoining pressure and capillarity in the constrained vapor bubble heat transfer system

Using the disjoining pressure concept in a seminal paper, Derjaguin, Nerpin and Churaev demonstrated that isothermal liquid flow in a very thin film on the walls of a capillary tube enhances the rate of evaporation of moisture by several times. The objective of this review is to present the evolution of the use of Churaev's seminal research in the development of the Constrained Vapor Bubble (CVB) heat transfer system. In this non-isothermal "wickless heat pipe", liquid and vapor flow results from gradients in the intermolecular force field, which depend on the disjoining pressure, capillarity and temperature. A Kelvin-Clapeyron model allowed the use of the disjoining pressure to be expanded to describe non-isothermal heat, mass and momentum transport processes. The intermolecular force field described by the convenient disjoining pressure model is the boundary condition for "suction" and stability at the leading edge of the evaporating curved flow field. As demonstrated by the non-isothermal results, applications that depend on the characteristics of the evaporating meniscus are legion.     

Pyridine ligand rotation in self-assembled trigonal prisms. Evidence for intracage solvent vapor bubbles

The rate of interconversion of the two inequivalent edges of the pyridine rings in the trigonal prism 3c, self-assembled from 3 equiv of the star connector, tetrakis[4-(4-pyridylethynyl)phenyl]cyclobutadienecyclopentadienylcobalt, and 6 equiv of a platinum linker, cis-(Me3P)2Pt(2+) 2 TfO(-), was determined by DNMR in nitromethane. It exhibits a highly unusual bilinear Eyring plot. In the low temperature regime, the activation enthalpy DeltaH(double dagger) is approximately 12 kcal/mol and an activation entropy DeltaS(double dagger) ranges from approximately -15 to approximately 0 cal/mol x K as a function of the nature and concentration of the anions present. The reaction is attributed to hindered rotation of the pyridine rings about the Pt-N bond, facilitated by a tight pairing with a counterion. Above a counterion-dependent limiting temperature, DeltaH(double dagger) and DeltaS(double dagger) change abruptly to approximately 35 kcal/mol and approximately 60 cal/mol x K, respectively. The changes largely compensate, such that the reactions have comparable rates in the two regimes, both amenable to DNMR measurement, but their mechanisms clearly differ. Several kinetic models for the involvement of ion pairing equilibria fit the observed data nearly equally well, and they all contain a reaction step with high DeltaH(double dagger) and DeltaS(double dagger) values in the high-temperature regime. Its mechanism is proposed to involve a counterion-assisted reversible dissociation of one or two adjacent Pt-N bonds, followed by nearly free rotation of the terminal pyridine ring or rings and subsequent bond reclosure, which is similar to the last presumed step in the initial prism assembly. An interpretation of the very high DeltaS(double dagger) value is suggested by molecular dynamics calculations: at equilibrium, there is a bubble of gaseous nitromethane solvent inside the prism, and it collapses when the prism opens as the transition state is reached. A simple calculation of the entropy of cavitation provides quantitative support for this tentative proposal. The presence of such voids might be generally important for the formation and properties of self-assembled cages.     

X-ray structures,photophysical characterization,and computational analysis of geometrically constrained copper(I)--phenanthroline complexes

A series of three geometrically constrained C(2)-symmetric Cu(I) mono-phenanthroline complexes were characterized by X-ray structural analysis, and their photophysical properties were investigated by absorption and emission spectroscopy. Visible light excitation yielded metal-to-ligand charge-transfer (MLCT) excited states with luminescence lifetimes up to 155 ns. Ultrafast transient absorption spectroscopy provided further insights into the excited-state dynamics and suggests for all three complexes the formation of a phenanthroline radical anion. In agreement with electrochemical measurements, the data further indicate that coordinative rearrangements are involved in nonradiative deactivation of the excited states. According to time-dependent density functional theory calculations (B3LYP/6-31G), the major MLCT transitions are polarized along the C(2) axis of the complex and originate predominantly from the copper d(xz) orbital. The computational analysis identifies an excited-state manifold with a number of close-lying, potentially emissive triplet states and is in agreement with the multiexponential decay kinetics of the MLCT luminescence. The relationship between structural and photophysical data of the studied Cu(I) mono-phenanthroline complexes agrees well with current models describing the photophysics of the related Cu(I) bis-diimine complexes.     

Dynamic forces between bubbles and surfaces and hydrodynamic boundary conditions

A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.     

Dynamic instability at liquid/vapor interface far from equilibrium

Attempt has been made to elucidate the mechanism of electric potential oscillations at oil-aqueous solution interface involving adsorption at oil-vapor interface on a semi-theoretical basis. The mechanism stipulates adsorption of ammonia, amines and pheromones at the liquid-vapor interface followed by transfer of ions through membrane-aqueous solution interface and subsequent interaction of ammonium (amine) ions and carbocations from pheromones with diffusing halide ions from the bulk. Relationship of the above mechanism with sensing mechanism of smell by olfactory nerves has also been pointed out.     

Dynamic vapor sorption and thermoporometry to probe water in celluloses

Dynamic vapor sorption and thermoporometry probe complementary dimensions of water interaction with cellulose. While sorption is primarily sensitive to the first hydration layers, thermoporometry is primarily sensitive to the nanometric water-filled pores. In this article, these analytical techniques are detailed and applied to model mesoporous materials and to a wide spectrum of celluloses. Correlations between techniques are explored. In dynamic vapor sorption, celluloses present a general characteristic time of desorption. On the other hand, they present highly variable characteristic times of sorption, indicating that material-specific properties may be inferred from sorption kinetics. Regarding thermoporometry, the thermodynamics of ice melting in irregular pore shapes is introduced. Moreover, in our thermoporometry analysis with differential scanning calorimeter, freezing temperature is extended to ?70?°C, allowing pores smaller than a few nanometers to be measured. Nevertheless, several data corrections are required for accurate thermoporometry at this condition. Comparisons between techniques show that sorption hysteresis is positively correlated with wet porosity. The presented developments and results will guide future application of these techniques to probe water in celluloses.     

Process optimization of vapor phase pyridine synthesis using response surface methodology

The vapor phase pyridine synthesis from acetaldehyde, formaldehyde and ammonia over HZSM-5 catalyst was studied. The process parameters like temperature, aldehyde ratio, and Si/Al ratio in HZSM-5 was investigated and the process conditions were optimized using surface response methodology (RSM) based on Box-Behnken design. The influence of process parameters investigated using analysis of variance (ANOVA), to identify the significant parameters. The optimum conditions for high yield of pyridine were identified to be a reaction temperature 400°C, aldehyde ratio 1: 1 and Si/Al ratio 106.7. A maximum of 55% yield of pyridine formed under the optimum experimental conditions. The proposed model equation using RSM has shown good agreement with the experimental data, with a correlation coefficient R ² = 0.99.     

Unexpected reactivity and coordination in gallium(III) and indium(III) chloride complexes with geometrically constrained thio- and selenoether ligands

Reaction of GaCl(3) with 1 mol equiv of [14]aneS(4) in anhydrous CH(2)Cl(2) gives the exocyclic chain polymer [GaCl(3)([14]aneS(4))] (1) whose structure confirms trigonal bipyramidal coordination at Ga with a planar GaCl(3) unit. In contrast, using [16]aneS(4) and GaCl(3) or [16]aneSe(4) and MCl(3) (M = Ga or In) in either a 1:1 or a 1:2 molar ratio produces the anion-cation complexes [GaCl(2)([16]aneS(4))][GaCl(4)] (2) and [MCl(2)([16]aneSe(4))][MCl(4)] (M = Ga, 3 and M = In, 4) containing trans-octahedral cations with endocyclic macrocycle coordination. The ligand-bridged dimer [(GaCl(3))(2){o-C(6)H(4)(SMe)(2)}] (5) is formed from a 2:1 mol ratio of the constituents and contains distorted tetrahedral Ga(III). This complex is unusually reactive toward CH(2)Cl(2), which is activated toward nucleophilic attack by polarization with GaCl(3), producing the bis-sulfonium species [o-C(6)H(4)(SMeCH(2)Cl)(2)][GaCl(4)](2) (6), confirmed from a crystal structure. In contrast, the xylyl-based dithioether gives the stable [(GaCl(3))(2){o-C(6)H(4)(CH(2)SEt)(2)}] (8). However, replacing GaCl(3) with InCl(3) with o-C(6)H(4)(CH(2)SEt)(2) preferentially forms the 4:3 In:L complex [(InCl(3))(4){o-C(6)H(4)(CH(2)SEt)(2)}(3)] (9) containing discrete tetranuclear moieties in which the central In atom is octahedrally coordinated to six bridging Cl's, while the three In atoms on the edges have two bridging Cl's, two terminal Cl's, and two mutually trans S-donor atoms from different dithioether ligands. GaCl(3) also reacts with the cyclic bidentate [8]aneSe(2) to form a colorless, extremely air-sensitive adduct formulated as [(GaCl(3))(2)([8]aneSe(2))] (10), while InCl(3) gives [InCl(3)([8]aneSe(2))] (14). Very surprisingly, 10 reacts rapidly with O(2) gas to give initially the red [{[8]aneSe(2)}(2)][GaCl(4)](2) (11) and subsequently the yellow [{[8]aneSe(2)}Cl][GaCl(4)] (12). The crystal structure of the former confirms a dimeric [{[8]aneSe(2)}(2)](2+) dication, derived from coupling of two mono-oxidized {[8]aneE(2)}(+?) cation radicals to form an Se-Se bond linking the rings and weaker transannular 1,5-Se···Se interactions across both rings. The latter (yellow) product corresponds to discrete doubly oxidized {[8]aneSe(2)}(2+) cations (with a primary Se-Se bond across the 1,5-positions of the ring) with a Cl(-) bonded to one Se. Tetrahedral [GaCl(4)](-) anions provide charge balance in each case. These oxidation reactions are clearly promoted by the Ga(III) since [8]aneSe(2) itself does not oxidize in air. The new complexes have been characterized in the solid state by IR and Raman spectroscopy, microanalysis, and X-ray crystallography where possible. Where solubility permits, the solution characteristics have been probed by (1)H, (77)Se{(1)H}, and (71)Ga NMR spectroscopic studies.     

Effect of methylation on the coordination of copper by small azacryptands; the role of geometrically constrained hydrogen bonding in stabilizing terminally coordinated oxygen species

Treatment of N-methyl substituted aminocryptand hosts with copper(II) generates monocopper(II) cryptates where copper(II) coordinates an oxygen-centered species, formally H3O+, which is also strongly hydrogen bonded to three aminocryptand N-methyl atoms via bonds which may best be viewed as NH(delta+)...O(delta-) in consequence of charge transfer. The strength of this hydrogen bonding precludes successful competition of another copper ion for the second coordination site thus suppressing formation of any Cu-Cu bonded average-valent system.     

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates

The absolute magnitude of an "entatic" (constrained) state effect has never been quantitatively demonstrated. In the current study, we have examined the electron-transfer kinetics for five closely related copper(II/I) complexes formed with all possible diastereomers of [14]aneS(4) (1,4,8,11-tetrathiacyclotetradecane) in which both ethylene bridges have been replaced by cis- or trans-1,2-cyclohexane. The crystal structures of all five Cu(II) complexes and a representative Cu(I) complex have been established by X-ray diffraction. For each complex, the cross-reaction rate constants have been determined with six different oxidants and reductants in aqueous solution at 25 degrees C, mu = 0.10 M. The value of the electron self-exchange rate constant (k(11)) has then been calculated from each cross reaction rate constant using the Marcus cross relation. All five Cu(II/I) systems show evidence of a dual-pathway square scheme mechanism for which the two individual k(11) values have been evaluated. In combination with similar values previously determined for the parent complex, Cu(II/I)([14]aneS(4)), and corresponding complexes with the two related monocyclohexanediyl derivatives, we now have evaluated a total of 16 self-exchange rate constants which span nearly 6 orders of magnitude for these 8 closely related Cu(II/I) systems. Application of the stability constants for the formation of the corresponding 16 metastable intermediates--as previously determined by rapid-scan cyclic voltammetry--makes it possible to calculate the specific electron self-exchange rate constants representing the reaction of each of the strained intermediate species exchanging electrons with their stable redox partners--the first time that calculations of this type have been possible. All but three of these 16 specific self-exchange rate constants fall within--or very close to--the range of 10(5)-10(6) M(-1) s(-1), values which are characteristic of the most labile Cu(II/I) systems previously reported, including the blue copper proteins. The results of the current investigation provide the first unequivocal demonstration of the efficacy of the entatic state concept as applied to Cu(II/I) systems.     

Dynamic response of the fluoride ion-selective electrode

The dynamic response of the fluoride-selective electrode is shown to result from four distinct processes: ion diffusion, an undefined reaction, LaF3 dissolution and a calibration drift. Empirical equations are derived which describe the time—e.m.f. relationship over times of the order of days. Dissolution of LaF₃ is shown to be a minor factor in determining the lower limit for measurement of fluoride concentration. The calibration drift process is the main obstacle. The time to thermodynamic equilibrium when the fluoride concentration is reduced becomes very long. A model is given for the calibration drift process. This model explains many of the anomalies reported in the literature for the behaviour of the fluoride-selective electrode. A much more detailed understanding of the calibration drift process is a prerequisite to general application of the fluoride electrode to the measurement of very low concentrations of fluoride.     

Dynamic response of super-twisted nematic liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays was studied by detailed computer simulation. The time evolution of director configuration and velocity of flow as obtained by solving Ericksen-Leslie hydrodynamic equations. The influence of d/p value and pretilt angle on the dynamic response was also studied. A comparison was also made between twisted nematic and STN liquid crystal displays.     

Dynamic response of super-twisted nematic liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays was studied by detailed computer simulation. The time evolution of director configuration and velocity of flow as obtained by solving Ericksen-Leslie hydrodynamic equations. The influence of d/p value and pretilt angle on the dynamic response was also studied. A comparison was also made between twisted nematic and STN liquid crystal displays.     

Stabilization of bubbles and foams

This review summarizes the principal new developments reported in the research literature from 2000 (inclusive) to the present date, on the topic of foam and bubble stabilization, in the context of foods. The main areas covered are novel foamed products, processes and foaming agents; new methods of study; protein adsorption and competitive adsorption; surface rheology; particle-stabilized systems; modelling, simulation and theory.     

Stretching liquid bridges with bubbles: the effect of air bubbles on liquid transfer

Liquid bridges containing bubbles are relevant to industrial printing and are also a topic of fundamental scientific interest. We use flow visualization to study the stretching of liquid bridges, both with and without bubbles, at low capillary numbers. We find that whereas the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force air bubbles selectively toward the least hydrophilic plate confining the liquid bridge.     

Simultaneous measurement of vapor diffusion and solubility coefficients in polymers by frequency response techniques

An experimental technique for the simultaneous measurement of solubility and diffusion coefficients in polymers by frequency response techniques has been developed. A sample of polymer suspended from an electrobalance is exposed to a permeating gas whose pressure is being varied sinusoidally at ultra-low frequencies. The phase angle and amplitude of the weight changes are measured as a function of the frequency of the pressure wave. It is shown that in the linear range the diffusivity coefficient for polyethylene–ethane calculated from the phase angle lag and the Henry's law solubility and the diffusivity obtained from the amplitude ratios are in excellent internal agreement and also agree with values obtained from transient measurements.     

Dynamic range and response speed of heat-flux differential calorimeters

A technique for the linearization of calorimeter cell (CC) thermal feedback in differential calorimeters was investigated. The technique was shown to ensure the linearity of the tract of rapid compensation measurements of thermokinetics (W _in(t)) in a dynamic range of heat-fluxes limited in principle only by the linearity of the CCs themselves, while their original identity is not required. The technique was employed in prototype models of updated DAK calorimeters, in which W _in(t)_max reached 0.5 W while the duration of the transition process associated with the insertion of the test specimen was reduced by a factor of 2.3. This was shown to reduce calorimeter inertia, extend the possibilities of thermokinetic measurements, and record earlier stages of the initial thermokinetics.