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Guerrero A, Santamaria A: Iberian Meeting on Rheology, IBEREO 2008, Appl. Rheol. 19 (2009) 123.Johan Wiklund
The Nordic Rheology Society Conference 2008
Appl. Rheol. 19:2 (2009) 121-122 ►
► Cite this publication as follows:
Wiklund J: The Nordic Rheology Society Conference 2008, Appl. Rheol. 19 (2009) 121.
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Sarraf H: High performance ceramics by advanced colloidal processing (PhD thesis summary), Appl. Rheol. 19 (2009) 119.
The specific properties of bentonite such as hydration, swelling,water absorption, viscosity, yield stress and thixotropy make it a valuable material in the form of mineral powder for a wide range of uses in agronomy, cosmetics and civil engineering. A flow curve is a quick test used to evaluate the rheological basic properties of a viscous fluid. However, many bentonite dispersions exhibit a complex flow curve, with yield stress and thixotropy area, especially at high concentration. In this study, flow curves from raw and activated bentonites dispersed in water were acquired at 6, 8 and 10% mass concentrations. Five stages along the flow curve were identified.To explain each stage, rheograms obtained from a dispersion made with a model material were studied in depth. The model material was a smectite extracted from a raw bentonite then saturated with calcium or sodium. Many homoionic and bi-ionic dispersions were prepared at various concentrations. The analysis and the modelling of some creep-recovery tests by a Zener model showed the relationship between the initial stage in the flow curve, named AB stage, and the viscoelastic properties of the fluid. The AB stage corresponds to the deformation of the material in the solid state. The τB point corresponds to an intermediate yield stress between the solid state and the start of the heterogeneous fracturation. The study of many bi-ionic dispersions allowed drawing the evolution of the yield stress as a function of concentration and saturation. The composition of the raw bentonites was expressed as an equivalent bi-ionic dispersion by calculating an active smectite percentage. A good correlation was obtained at the highest concentrations between τB from the bi-ionic model dispersions and the raw bentonites dispersions► Cite this publication as follows:
Paumier S, Pantet A, Monnet P, Touze-Foltz N: Evaluation of the viscoelastic properties of a clay material using a flow curve, Appl. Rheol. 19 (2009) 23824.
The authors previously introduced an activation model for the onset of shear thickening in electrically stabilized colloidal suspensions. It predicts that shear thickening occurs, when particles arranged along the compression axis in a sheared suspension do overcome the electrostatic repulsion at a critical shear stress, and are captured in the primary minimum of the DLVO interaction potential. A comparison with an experimental investigation on non-aqueous silica suspensions, carried out by Maranzano and Wagner, is performed. For particle systems that fall into the applicability range of the theory, a good coincidence between the experimental data and the model predictions can be found.► Cite this publication as follows:
Kaldasch J, Senge B, Laven J: Shear thickening in electrically stabilized non-aqueous colloidal suspensions, Appl. Rheol. 19 (2009) 23493.
Nanoclay dispersions in organic solvents are widely used in cosmetics for a variety of gels and creams, whose properties depend on the powder content and the processing method. The control of the shear applied during processing is therefore essential for achieving the required properties.This study demonstrates the utility of applying rheological measurements for characterizing cosmetic products based on nanoclays and relating their viscoelastic properties to end-use performances. In particular, a rheological characterization of bentonite dispersions in isododecane at different clay content and shear history is presented. For each inorganic content, both mixed samples and samples subjected to several calendering runs were studied. The effect of shear and clay content on the viscoelastic properties was investigated by a combination of oscillatory shear experiments under small-deformation conditions and by X-Ray diffraction. The tested samples showed a gel-like behaviour with a final structure depending on the applied shear stress. By increasing the inorganic content in the dispersion, a reduction in the gel stability to a further shear application was observed. Two models, developed for colloidal gels,were used to fit the rheological results enabling to evaluate the microstructure and the degree of dispersion of the tested samples and to relate the colloidal structure to the elastic properties.► Cite this publication as follows:
Lionetto F, Maffezzoli A: Rheological characterization of concentrarted nanoclay dispersions in an organic solvent, Appl. Rheol. 19 (2009) 23423.
An optimised method for the detection and analysis of the time dependent pressure associated with the development of melt flow instabilities during extrusion through a capillary die was developed and validated. The magnitude and frequency of the developed quasi-periodic distortions, as well as the pressure profiles along the die length, were found to depend on the MWD, topology, melt elasticity and uniaxial extensional flow properties. Both the onset and magnitude of strain hardening in uniaxial extension appear to be related to the onset for the development of melt flow instabilities under capillary flow. For a better understanding of the role of the extensional properties (namely that of a purely elastic instability) the Hencky strain to failure was also determined and correlated to the observed flow instabilities. Time resolution of the capillary rheometer was improved by a factor of 1000, pressure resolution by a factor of 100 compared to the original set-up.► Cite this publication as follows:
Filipe S, Becker A, Barroso VC, Wilhelm M: Evaluation of melt flow instabilities of high-density polyethylenes via an optimised method for detection and analysis of the pressure fluctuations in capillary rheometry, Appl. Rheol. 19 (2009) 23345.
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