Monique Piau

Characterization of wheat flour–water doughs. Part I: Rheometry and microstructure

Abstract The results presented in this paper concern doughs prepared from an industrial soft wheat flour mixed with water using the traditional Brabender farinograph. These doughs are characterized using dynamic rheometrical measurements. In parallel, an innovative microscopy study, the Environmental Scanning Electron Microscope (ESEM) is investigated and found to be very well-suited for the observation of such doughs. A change in the slope of the curve giving the maximum of consistency is observed at a typical water content, due to the presence of excess free water. The main rheometrical characteristics |η*| and tan δ are exhibited for this kind of dough. Their adequacy to differentiate between various doughs is emphasized. Parameters such as mixing time, water content and rest time are shown to influence both the rheometrical properties and the microscopic structure of doughs. Changes generated by mixing are interpreted at the molecular level. It is shown that the study of the microstructure is essential to compare the evolution of different doughs.

Characterization of wheat-flour-water doughs: a new method using ultrasound.

In this paper, an original method of evaluating the physical properties of wheat-flour-water systems using high-frequency low-power ultrasound is presented. Most of the experiments were performed with a reflectance technique measuring the acoustic impedance of doughs. The velocity of propagation, attenuation and viscoelastic moduli have been evaluated for both compressional and shear ultrasonic waves in the interval 2-10 MHz for doughs of different hydrations. The 53% water content was found to be critical with respect to the presence of free water. The influence of the mixing and rest times on the longitudinal ultrasonic parameters is also studied.

Dynamic shear rheology of high molecular weight polydimethylsiloxanes : comparison of rheometry and ultrasound

Abstract The viscoelastic properties of three linear polydimethylsiloxanes (PDMS) of high molecular weights are investigated using rheometrical as well as ultrasonic tests over a large range of temperatures. Classical shear rheometrical measurements are carried out in the low frequency range from 10−1 to 102 rad s−1 between −50 and +20°C. The frequency range is enlarged using the time–temperature superposition principle, allowing coverage of about 6–7 decades of pulsation. Ultrasonic tests use an inclined incidence wave reflection technique to measure the complex shear mechanical impedance from 1.5 to 25 MHz, between −10 and +50°C. Rheometrical and ultrasonic experiments are then combined for the three PDMSs at the same reference temperature. They give the reduced shear elastic and loss moduli for reduced frequencies covering 10 decades. A discrete relaxation time spectrum is first deduced from the master curve in each case. More accurate predictions may be obtained using a molecular weight distribution and BSW (Baumgaertel–Schausberger–Winter) model for polydisperse systems.

Analysis of the morphology of polymer blends using ultrasound

Over recent years there has been great efforts towards the understanding of the rheology of immiscible polymer blends. It is now well accepted that their morphology, especially the concentration of the inclusions, their size, and their radii distribution, is a very important factor which controls their mechanical properties. The type of blend considered in this study is a polyamide/polypropylene system, in which the matrix is the polyamide (PA6) with inclusions of polypropylene (PP). Different concentrations have been used, as well as different surfactants. These blends give then rise to different acoustic properties, which have been characterized by measuring velocities of propagation and attenuation of ultrasonic waves during flow through a capillary rheometer. A wave propagation theory for viscoelastic emulsions was used to predict the values of the ultrasonic parameters as functions of the concentration, the radii distribution and the frequency when the thermophysical properties of the blend are known. We can therefore deduce the concentration and average size of inclusions, and then return to the morphology of the blends. Comparison with microscopic photographs seems to correlate well with our predictions. This method appears promising and could be used to differentiate between different blends during flow.

High temperature interfacial tension measurements of PA6/PP interfaces compatibilized with copolymers using a spinning drop tensiometer

Abstract Interfacial tension measurements of polyamide/polypropylene (PA6/PP) interfaces are reported at high temperature, using a spinning drop tensiometer, especially adapted to the study of the effects of copolymers. Copolymers in different amounts are included in the PP drop, and their migration towards the interface is inferred from the evolution of the drop diameter during the experiment. The importance of the compatibilizer is studied; small amounts of copolymer give rise to a significant decrease of the interfacial tension. This is due to effective migration of the copolymer, as deduced from diffusion coefficients. Above a critical concentration, the interfacial tension increases again. This phenomenon has not been observed before for such systems and is attributed to the presence of micelles in the bulk which prevent the copolymer migration through the creation of yield stresses.

ACOUSTIC WAVE PROPAGATION IN TWO-PHASE VISCOELASTIC FLUIDS : THE CASE OF POLYMER EMULSIONS

Wave propagation in two-phase viscoelastic fluid media is studied. A theory based on previous ones for liquid–liquid or liquid–solid mixtures is developed. Acoustical parameters, i.e., velocity of propagation and attenuation, are related to the morphological properties (concentration and size of inclusions) through the thermal and viscoelastic effects. This general theory includes the Newtonian and elastic behaviors as limiting cases. In the case of molten polymer blends, viscoelastic effects may be dominant, unlike most of the Newtonian emulsions studied previously, where thermal effects usually prevail. Moreover, by taking into account the viscoelastic character of the fluids, important changes are obtained not only for the absorption due to the individual components, but also for the excess attenuation in the emulsion.

Maurice Couette, one of the founders of rheology

This article presents a biography of Maurice Couette, whose name is associated with a type of flow, of viscometer, and with a correction method for end effects in capillary flows. His life and work are described, with special mention being made of the cylinder apparatus that he designed. The relevance of his work to present day rheology is stressed.

Easier flow of viscoplastic materials with ultrasonic longitudinal wall motion

Abstract A longitudinal micromovement of ultrasonic frequency, in the same direction as the mean flow velocity, is applied to the walls of Couette and Poiseuille flows. The constitutive equations considered are those for Newtonian or Bingham models in the bulk, adherence or linear slip at the wall. This micromovement is shown to have a strong influence on the flow properties due to the existence of the yield stress in viscoplastic materials. The key parameters of the problem are the Oldroyd (Od) or Bingham (Bn) numbers. The dimensionless velocity of vibrations U0, and the vibrational Reynolds number Ω0 also play an important part. For large Od values, velocity-driven plane (Couette) flows may display significant reductions in stress, even when neglecting vibrational inertia ( Ω 0 ⪡1 ). In contrast, for pressure-driven (Poiseuille) flows in circular tubes, vibrational inertia has to be taken into account so that the movement transmitted to the fluid is different from a rigid block displacement, and a mean flow rate increase is obtained, the range of which depends on the value of Bn. Indications are given on the way slip at the wall, or shear-thinning viscous behavior modify these results. In order to check these effects, at least in part, the experiments were carried out using extrusion flows with high-power ultrasound applied to a circular die and several raw elastomers in two cases: flows with a constant mean flow rate, and the more complex case of flows created by a screw of zero compression ratio rotating at a given speed. In the second case, an instantaneous decrease in pressure is observed, as well as a simultaneous increase in flow rate. Whatever the case, a new stable regime may be reached after a while, and significant decreases in pressure drop are observed when vibrations are applied with rubber compounds, which may be described as yield stress fluids. These variations cannot be explained solely by the increase in temperature of the extrudate, and they compare well with the theory developed.

Note on the ineffective length of a fiber

Abstract A general definition is given of the concept of ineffective length of a fiber, which is of use in the statistical theories of composite strength. The ineffective length, as formulated, may also serve as a measure of the total supporting capability of a fiber. The paper then sets forth models of an in situ fiber, deterministic or stochastic, and shows that the ineffective length may be by orders larger than predicted by the previous formulation. In extreme cases it may become comparable to the fiber length. Particular examples illustrating the above models are also given.

Ultrasonic and microscopic investigation of blends of polydimethylsiloxane and polyisobutylene at all concentrations

Polydimethylsiloxane/polyisobutylene blends are investigated using ultrasonic techniques (megahertz range), in the complete concentration range (0%–100%). The velocities of propagation and attenuations of shear and longitudinal waves are determined experimentally together with simultaneous optical microscopy observations. From the knowledge of the acoustical and physical properties of the separate polymers, the acoustical longitudinal parameters of the blend can be predicted successfully up to 40% volume concentration of the dispersed phase, with a model developed by the authors and especially adapted to wave propagation in such viscoelastic emulsions. For shear waves, two emulsion models (Palierne and Lee–Park) are used for predicting the viscoelastic moduli associated with the acoustical data at ultrasonic frequencies, with a good agreement. The case of composite droplets (40%–60%) has required to consider Friedrich’s extension of Palierne’s model. The ultrasonic method appears to be a very interesting ...