Unsteady rheometry: can we characterize weak gels with a controlled stress rheometer?

This paper is concerned with the influence of apparatus inertia effects in controlled stress rheometry. As evidenced on creep experiments, the coexistence of apparatus inertia and viscoelasticity leads to a coupling frequency. For weak gels, this coupling frequency is typically between 1 and 100 Hz. Therefore, frequency sweeps around and above this coupling frequency also corresponds to an effective shear stress sweep evolution due to a non-trivial resonant effect. In other words, frequency sweep experiments are not made at constant shear stress. The detailed modelling and analysis of this inertia effect on a typical weak gel shows a clear and fundamental limitation for its characterization using a controlled stress rheometer. Also, alternative approaches to standard rheometer software analysis are proposed to take this coupling effect into account. This paper was presented at the 3rd Annual Rheology Conference, AERC 2006, April 27-29, 2006, Crete, Greece.     

Two-dimensional motion of a set of particles in a free surface flow with image processing

A method to analyze bed load with image processing was developed. The motion of coarse spherical particles on a mobile bed entrained by a shallow turbulent flow down a steep channel was filmed with a high-speed camera. The water free surface and the particle positions were detected combining classical image processing algorithms. We developed a particle-tracking algorithm to calculate all particle trajectories and motion regimes, rolling or saltation. At constant slope, the contribution of the rolling particles to the solid discharge only slightly differed when the particle supply was increased. At a slope of 10%, it represented about 40%. In contrast, rolling became the major regime when the slope increased, at a slope of 15% it represented up to 80% of the total solid discharge.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Mortar physical properties evolution in extrusion flow

Extrusion is not a common way to form firm cement-based material. Due to material frictional behaviour, drainage phenomenon appears and leads to process blockage. The properties of the extrusion multiphase flow of such specific materials have to be highlighted to understand the properties of the process flow. An extrudible mortar is formulated according to extrusion criteria provided by literature tests developed to describe the evolution of the paste undergoing extrusion. To study the relationship between flow properties, extrusion force and material consolidation, a technique based on the measurement of paste hardness in the billet zone is developed. Measurements of the local paste density and flow visualisation are also performed. Results show that in the range of used ram velocities, filtration occurs between fluid and solid phases. This creates zones of preferential flow and of non-flowing consolidated material. Paper presented at the 3rd Annual European Rheology Conference held on April 27–29, 2006, Crete, Greece.     

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.     

A convenient synthesis of a 2,7-difunctional tetra(alkoxy)triphenylene involving 4,4′-diacetoxy-3,3′-dialkoxybiphenyl as a key precursor and its conversion to extended hybrid mesogenic compounds

A new rational pathway to 2,7-difunctionalised-β-hexa-substituted triphenylenes is presented, requiring less protection/deprotection and purification steps than more conventional synthetic procedures in the framework of the ‘biphenyl route’. Main improvements are deprotection via alkaline hydrolysis of an ester in ethanol/water medium instead of using toxic and pyrophoric reagents like lithium diphenylphosphide, and the use of easily prepared brominated precursors instead of iodinated reagents for biphenyl synthesis. 4,4′-Diacetoxy-3,3′-bis(hexyloxy)biphenyl has been synthesised under this scheme, and characterised by proton nuclear magnetic resonance (¹H NMR) spectrometry, elemental analysis and single-crystal crystallography. It crystallises in the P-1 space group, and exhibits a layered structure built-up through dipolar, C–H … π and C–H … O=C non-covalent interactions. This compound has been oxidatively coupled with 1,2-bis(hexyloxy)benzene to yield 2,7-dihydroxy-3,6,10,11-tetrakis(hexyloxy)triphenylene, a non-mesogen key precursor for the synthesis of the corresponding liquid-crystalline 2,7-difunctional triphenylenes. Indeed, a reactive 2,7-difunctional mesogen was prepared and used to produce new triphenylene-siloxane hybrid monomeric, trimeric and polymeric mesogens. All of them exhibited columnar hexagonal (Col_h) mesophases.