Fractal analysis of atomizing liquid flows

The work reported in this paper questions the relevance of using fractal concept to study liquid primary atomization process by characterizing the shape of the continuous liquid flow from the nozzle exit to the end of the atomization process. First, three fractal methods were tested on synthetic images in order to define the best adapted protocol to the objective of the study. It appeared that the Euclidean distance mapping was the best appropriate method. Second, this technique was applied to analyze series of images of atomizing liquid flows obtained for several working conditions. This application demonstrates that atomizing liquid flows are fractal objects and that primary atomization can be reasonably seen as fractal processes. The appropriateness of fractal concept was also demonstrated by the fact that fractal characteristics such as textural or structural fractal dimension and inner cutoff scale are physically representative of the process investigated here.     

Wetting and nonwetting fluid displacements in porous media

The understanding of simple laminar flow in tubes has often been used to interpret the more complicated flow in porous media. A study of the motion of two immiscible liquids in closed tubes with relatively large diameter (> 0.3 cm i.d), was conducted in order to examine the influence of wetting and nonwetting liquids on the flow behavior. The results indicate that the wetting properties of the fluids with regard to the tube wall have a major efffect on the formation and motion of long bubbles. A physically based model was used to predict the velocity and the conditions for no motion of bubbles and drops in tubes. These results were used to interpret the nature of oil and water flow in porous media. Experiments in which the wetting liquid was displaced by the nonwetting, or vice versa, were conducted by injecting the displacing liquid at a constant flux at the center of a two-dimensional chamber saturated with the displaced liquid. The influence of wetting-nonwetting characteristics on the quantity of liquid displaced, the shape of the interface between the two liquids, and the interpretation of the no motion radius in a closed tube to the case of a porous medium are discussed. It would appear that the no motion radius gives a good indication of the minimum width of a nonwetting penetrating finger and the maximum width of nonwetting ganglia left by drainage.     

GaN nanowires for piezoelectric generators

We demonstrate the high potential of GaN nanowires (NWs) to convert mechanical energy into electric energy. Using an atomic force microscope equipped with a Resiscope module, an average output voltage of –74 mV and a maximum of –443 mV ± 2% per NW were measured. This latter value is the highest reported so far for GaN NWs. By considering these output signals, we have estimated an average and a maximum power density generated by one layer of GaN NWs of the order of 5.9 mW/cm² and 130 mW/cm², respectively. These results offer promising prospects for the use of GaN NWs for high‐efficiency ultracompact piezogenerators. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Adsorption of monovalent and divalent cations on planar water-silica interfaces studied by optical reflectivity and Monte Carlo simulations

Adsorption on planar silica substrates of various monovalent and divalent cations from aqueous solution is studied by optical reflectivity. The adsorbed amount is extracted by means of a thin slab model. The experimental data are compared with grand canonical Monte Carlo titration simulations at the primitive model level. The surface excess of charge due to adsorbed cations is found to increase with pH and salt concentration as a result of the progressive dissociation of silanol groups. The simulations predict, in agreement with experiments, that the surface excess of charge from divalent ions is much larger than from monovalent ions. Ion-ion correlations explain quantitatively the enhancement of surface ionization by multivalent cations. On the other hand, the combination of experimental and simulation results strongly suggests the existence of a second ionizable site in the acidic region. Variation of the distance of closest approach between the ions and surface sites captures ion specificity of water-silica interfaces in an approximate fashion.     

Measurement of the Zeta Potential of Oil Drops with the Spinning Tube Zetameter

The spinning tube zetameter, an apparatus which was utilized up to now to keep a gas bubble from settling while electrophoretic measurement was carried out, can be used to measure the zeta potential of an oil drop as well. This piece of equipment is brought into play for estimating the influence of various formulation variables (pH, ionic strength, surfactant type or mixture) on the zeta potentials of oil drops. The results are compared to those reported in the literature.     

Dispersion and orientation of single-walled carbon nanotubes in a chromonic liquid crystal

A post-synthesis alignment of individual single-walled carbon nanotubes (SWCNTs) is desirable for translating their unique anisotropic properties to a macroscopic scale. Here, we demonstrate excellent dispersion, orientation and concomitant-polarised photoluminescence of SWCNTs in a nematic chromonic liquid crystal. The methods to obtain stable suspension are described, and order parameters of the liquid crystal matrix and of the nanotubes are measured independently.     

Proton Catalysis in the Redox Responsivity of a Mini‐Sized Photochromic Diarylethene

A thermally irreversible dithienylethene (DTE) photochrom can be turned into a thermally reversible one in presence of Cu^II triflate. A ring opening (DTEC closed→DTEO open) occurs through the formation of a copper‐containing fast transient intermediate. Stopped‐flow experiments monitored at 410 and 780 nm have allowed to show that the stoichiometry of this intermediate is DTE/Cu=1:1. At longer monitoring times (i.e., several seconds after mixing), the intermediate undergoes a slow decay while the residual DTEC closed form opens. A joint detailed kinetic and electrochemical analysis has unveiled a proton catalysis scenario in which electron transfer between DTEC and Cu^II, ligand exchange, protonation‐deprotonation equilibria of the cation radicals and ring opening are embedded into two main reaction cycles. At the beginning of the reaction, Cu^II is reduced into Cu^I and DTE is degraded without ring opening. Then, as the reaction progresses, the triflic acid released from the Cu^II reduction switches‐on a propagation cycle during which ring opens without any more Cu^II consumption. Cyclic voltammetry, spectro‐electrochemical measurements, delayed photocoloration experiments in presence of Cu^II and acid–base additions have confirmed the main features of the proton catalysis.     

Structural Observations of Heterometallic Uranyl Copper(II) Carboxylates and Their Solid‐State Topotactic Transformation upon Dehydration

The hydrothermal reactions of uranyl nitrate and metallic copper with aromatic polycarboxylic acids gave rise to the formation of five heterometallic UO₂²⁺? Cu²⁺ coordination polymers: (UO₂)Cu(H₂O)₂(1,2‐bdc)₂ ( 1 ; 1,2‐bdc=phthalate), (UO₂)Cu(H₂O)₂(btec) ? 4 H₂O ( 2 ) and (UO₂)Cu(btec) ( 2′ ; btec=pyromellitate), (UO₂)₂Cu(H₂O)₄(mel) ( 3 ; mel=mellitate), and (UO₂)₂O(OH)₂Cu(H₂O)₂(1,3‐bdc) ? H₂O ( 4 ; 1,3‐bdc=isophthlalate). Single‐crystal X‐ray diffraction (XRD) analysis of compound 1 revealed 2D layers of chains of UO₈ and CuO₄(H₂O)₂ units that were connected through the phthalate ligands. In compound 2 , these sheets were connected to each other through the two additional carboxylate arms of the pyromellitate, thus resulting in a 3D open‐framework with 1D channels that trapped water molecules. Upon heating, free and bonded water species (from Cu? OH₂) were evacuated from the structure. This thermal transition was followed by in situ XRD and IR spectroscopy. Heating induced a solid‐state topotactic transformation with the formation of a new set of Cu? O interactions in the crystalline anhydrous structure ( 2′ ), in order to keep the square‐planar environment around the copper centers. The structure of compound 3 was built up from trinuclear motifs, in which one copper center, CuO₄(OH₂)₂, was linked to two uranium units, UO₅(H₂O)₂. The assembly of this trimer, “U₂Cu”, with the mellitate generated a 3D network. Complex 4 contained a tetranuclear uranyl core of UO₅(OH)₂ and UO₆(OH) units that were linked to two copper centers, CuO(OH)₂(H₂O)₂, which were then connected to each other through isophthalate ligands and U?O? Cu interactions to create a 3D structure. The common structural feature of these different compounds is a bridging oxo group of U?O? Cu type, which is reflected by apical Cu? O distances in the range 2.350(3)–2.745(5) Å. In the case of a shorter Cu? O distance, a slight lengthening of the uranyl bond (U?O) is observed (e.g., 1.805(3) Å in complex 4 ).     

Gas-Phase Structure of Amyloid-β (12 – 28) Peptide Investigated by Infrared Spectroscopy, Electron Capture Dissociation and Ion Mobility Mass Spectrometry

The gas-phase structures of doubly and triply protonated Amyloid-β_12-28 peptides have been investigated through the combination of ion mobility (IM), electron capture dissociation (ECD) mass spectrometry, and infrared multi-photon dissociation (IRMPD) spectroscopy together with theoretical modeling. Replica-exchange molecular dynamics simulations were conducted to explore the conformational space of these protonated peptides, from which several classes of structures were found. Among the low-lying conformers, those with predicted diffusion cross-sections consistent with the ion mobility experiment were further selected and their IR spectra simulated using a hybrid quantum mechanical/semiempirical method at the ONIOM DFT/B3LYP/6-31 g(d)/AM1 level. In ECD mass spectrometry, the c/z product ion abundance (PIA) has been analyzed for the two charge states and revealed drastic differences. For the doubly protonated species, N – C_α bond cleavage occurs only on the N and C terminal parts, while a periodic distribution of PIA is clearly observed for the triply charged peptides. These PIA distributions have been rationalized by comparison with the inverse of the distances from the protonated sites to the carbonyl oxygens for the conformations suggested from IR and IM experiments. Structural assignment for the amyloid peptide is then made possible by the combination of these three experimental techniques that provide complementary information on the possible secondary structure adopted by peptides. Although globular conformations are favored for the doubly protonated peptide, incrementing the charge state leads to a conformational transition towards extended structures with 3₁₀- and α-helix motifs.