Rheological studies in the bulk and at the interface of Pickering oil/water emulsions

The rheological behavior and interfacial properties of olive oil–water emulsions stabilized by surfactant and clay particles (smectite) were studied to evaluate the effect of particles and surfactant distribution both in the bulk phase and at the oil–water interface. The temperature sweep of surfactant solutions and emulsions with and without clay particles showed the critical effect of the solid particles on the viscosity change. The mechanism of adsorption of surfactant molecules onto clay particles has a direct impact on the micellization and gelling temperatures. Indeed, the presence of clay particles caused a slight decrease in the micellization temperature and a total cancellation of the gelling phenomenon. Dynamic interfacial tension values demonstrated that clay particles would not compete with the surfactant for adsorption at the interface. However, the significant increase in the elastic properties of the interface that was observed accounts for their accumulation in the vicinity of the interface, probably at the level of surfactant polar head groups. Thus, the clay particles would form a mechanical barrier, preventing coalescence of emulsion droplets.     

Observation of a γ transition in the Λ4H hypernucleus

The γ spectra induced by stopped K^? mesons in a ⁷Li target were obtained in coincidence with the accompanying charged pions. A thorough analysis of these spectra indicates that the observed 1.09 MeV line can be ascribed to a γ transition in the ⁴_ΛH hypernucleus.     

Comparative yield stress determination for pure and interstratified smectite clays

Different experimental devices and operative procedures were used to obtain the main properties of suspensions of two purified clays, a pure smectite and an interstratified illite-smectite natural clay, at different concentrations. The yield stress values derived from flow and creep tests were found to be very consistent, while those derived from dynamic tests were observed to be much more sensitive to experimental conditions. Qualitatively, the two clays exhibit the same rheological behaviour, which can be modelled using the Herschel-Bulkley model; their yield stress increases with clay concentration and they present a thixotropic character for low concentrations, with an inversion of the curves when the clay concentration increases. However, significant differences were observed when considering numerical values. For the same clay concentration in the suspension, the yield stress of the pure smectite is distinctly higher than that of the interstratified one. The rheological properties of the pure smectite clay can be related to the swelling properties and the organisation of the minerals in water, leading to three-dimensional strong but deformable structures. On the other hand, the presence of a small percentage of illite in the natural clay gives it a brittle behaviour which collapses more easily under stress.     

Meson production inp+U,O+U and S+U interactions at 200 GeV/nucleon

Meson production in proton, oxygen and sulphur interactions with uranium targets at 200 GeV/nucleon is studied. We measure the inclusive meson cross-section d /dP _T ² and its evolution fromp+U to S+U. The cross-section fitted with an exponential gives an inverse slopeP _T0 of the order of 210 MeV/c. As a function of the neutral transverse energy,P _T0 values show a slight rise followed by a plateau. The normalized difference (⁺–^–)/^– between positive and negative meson cross-sections is found to increase withE _T .also at ISEL     

Mixed O/W emulsions stabilized by solid particles: A model system for controlled mass transfer triggered by surfactant addition

This article deals with a model mixed oil-in-water (O/W) emulsion system developed to study the effect of surfactants on mass transfer between dispersed oil droplets of different composition. In this purpose, our goal was to formulate O/W emulsions without any surface active agents as stabilizer, which was achieved by replacing surfactants by a mixture of hydrophilic/hydrophobic silica particles. Then, to study the specific role of surfactants in the oil transfer process, different types and concentrations of surfactants were added to the mixed emulsion after its preparation. In such a way, the same original emulsion can be used for all experiments and the influence of various surface active molecules on the oil transfer mechanism can be directly studied. The model mixed emulsion used consists of a mixture of hexadecane-in-water and tetradecane-in-water emulsions. The transfer between tetradecane and hexadecane droplets was monitored by using differential scanning calorimetry, which allows the detection of freezing and melting signals characteristic of the composition of the dispersed oil droplets. The results obtained showed that it is possible to trigger the transfer of tetradecane towards hexadecane droplets by adding surfactants at concentrations above their critical micellar concentration, measured in presence of solid particles, through micellar transport mechanism.     

Isotactic polypropylene/carbon nanotube composites prepared by latex technology: Electrical conductivity study

Several series of nanocomposites were prepared using a latex-based process, the main step of which consisted of mixing an aqueous suspension of exfoliated carbon nanotubes (CNTs) and a polymer latex. In the present work, a systematic study on the electrical properties of fully amorphous (polystyrene - PS) as well as semi-crystalline (isotactic polypropylene - iPP) nanocomposites containing either single-wall (SWCNTs) or multi-wall carbon nanotubes (MWCNTs) has been conducted. Percolation thresholds as low as 0.05 wt.% or 0.1 wt.% were observed for SWCNT/iPP and MWCNT/iPP nanocomposites, respectively. The formation of a conductive percolating network at such a low CNT concentration is favored by the high intrinsic conductivity and the low viscosity of the polymer matrix. The electrical percolation threshold of the iPP-based system was found to be lower than its rheological percolation threshold. Beyond the percolation threshold, MWCNT-based nanocomposites generally exhibited higher conductivity levels than those based on SWCNTs, most probably due to the higher intrinsic conductivity of the MWCNTs as compared to that of the SWCNTs. These excellent electrical properties, associated with the strong nucleating effect of the CNTs reported earlier [1] and [2], render this type of nanocomposites extremely attractive from a technological point of view.     

Stabilization mechanism of oil-in-water emulsions by β-lactoglobulin and gum arabic

Natural biopolymer stabilized oil-in-water emulsions were formulated using β-lactoglobulin (β-lg), gum arabic (GA), and β-lg:GA solutions as an alternative to synthetic surfactants. Emulsions using these biopolymers and their complexes were formulated varying the biopolymer total concentration, the protein-to-polysaccharide ratio, and the emulsification protocol. This work showed that whereas β-lg enabled the formulation of emulsions at concentration as low as 0.5 (w/w)%, GA allowed to obtain emulsions at concentrations equal to or higher than 2.5 (w/w)%. In order to improve emulsion stability, β-lg and GA were complexed through strong attractive electrostatic interactions. GA solution had to be added to previously prepared β-lg emulsions in order to obtain stable emulsions. Interfacial tension and interfacial rheological measurements allowed a better understanding of the possible stabilizing mechanism. β-lg and GA both induced a very effective decrease in interfacial tension and showed interfacial elastic behaviour. In the mixed system, β-lg adsorbed at the interface and GA electrostatically bound to it, leading to the formation of a bi-layer stabilized emulsion. However, emulsion stability was not improved compared to β-lg stabilized emulsion, probably due to depletion or bridging flocculation.     

Modelling and characterisation of diluted and concentrated water-in-crude oil emulsions: comparison with classical behaviour

Water-in-oil type emulsions can be formed during the crude oil production process. The presence of natural surfactants in oil (asphaltenes, resins) and mechanical stirring (piping/well system) produce emulsions, the stability and rheological behaviour of which depend mainly on the chemical composition of the oil and the internal phase concentration. In this work, water (brine 8 g NaCl/cm³) in oil (crude oil) emulsions were prepared and characterised by varying the internal phase concentration (5–80%). Rheological properties are discussed according to the composition of the oil and the temperature of the system. Relative viscosity was modelled following the classical models of Mooney and Krieger and Dougherty, but the best-fitting model for the experimental results was found with an exponential type equation between relative viscosity and volume fraction, as proposed by Richardson. Moreover, we observed that the plastic behaviour determined through the yield stress determination depended not only on the internal phase concentration but also on the temperature. Quantitative analysis of the emulsions’ viscoelastic parameters (storage and loss modulus) was made. In the case of concentrated emulsions (containing over 70% of internal phase), Princen’s theory of the high internal phase ratio emulsions (HIPRES) was verified.     

Mass 2 and 3 cluster structure study with quasi-free reactions induced by 58 MeV protons on 9Be, 12C and 14N

Cross sections for (p, pd) and (p,p³He) reactions at 58 MeV have been measured for ⁹Be, ¹²C and ¹⁴N targets. An energy resolution of about 400 keV permitted identification of several excited states in the different residual nuclei. The experimental results are presented and analysed in the framework of the distorted-wave impulse approximation. Spectroscopic and mechanism information is discussed. Some additional quasi-free reactions involving t, ³He and α structure of ⁹Be have been observed and are also reported.     

Further investigation of the γ-transitions in Λ4H and Λ4 hypernuclei

The γ-spectra induced by stopped K^? mesons in ⁶Li and ⁷Li targets in coincidence with the accompanying charged pions and π⁰ mesons. The γ-line at (1.04 ± 0.04) MeV observed with charged pions was identified as a γ-transition in _Λ⁴H, whereas the γ-line at (1.15 ± 0.04)_MeV observed with π⁰ mesons was ascribed to a γ-transition in _Λ⁴He. Using these new values for the excitation energies of the 1⁺ levels of _Λ⁴H and _Λ⁴He hypernuclei, the phenomenological ΛN potential for the s-state interaction was recalculated.