Mohend Chaouche

Rheology of non-Brownian rigid fiber suspensions with adhesive contacts

An experimental investigation is undertaken into the shear-thinning behavior of suspensions of non-Brownian rigid fibers in Newtonian fluids. In particular, the influence of the shear stress and the fiber concentration is investigated. The shear stress is adjusted by varying both the shear rate and the solvent viscosity. In the semidilute concentration regime, where direct mechanical contacts between fibers are rare, the suspension is found to be nearly Newtonian over the stress range investigated. In the concentrated regime, the suspension becomes shear thinning below a certain shear rate. The shear thinning increases with concentration and decreases with solvent viscosity. Although shear-thinning behavior of fiber suspensions has often been reported in the literature, its physical origins are not well understood. Our experiments are interpreted in terms of the formation and breakage of fiber flocs due to the competition between hydrodynamic and colloidal forces. Our interpretation is confirmed by measurements of the adhesive forces between two individual fibers.An experimental investigation is undertaken into the shear-thinning behavior of suspensions of non-Brownian rigid fibers in Newtonian fluids. In particular, the influence of the shear stress and the fiber concentration is investigated. The shear stress is adjusted by varying both the shear rate and the solvent viscosity. In the semidilute concentration regime, where direct mechanical contacts between fibers are rare, the suspension is found to be nearly Newtonian over the stress range investigated. In the concentrated regime, the suspension becomes shear thinning below a certain shear rate. The shear thinning increases with concentration and decreases with solvent viscosity. Although shear-thinning behavior of fiber suspensions has often been reported in the literature, its physical origins are not well understood. Our experiments are interpreted in terms of the formation and breakage of fiber flocs due to the competition between hydrodynamic and colloidal forces. Our interpretation is confirmed by measur...

Squeeze flow of highly concentrated suspensions of spheres

The squeeze flow behaviour of highly concentrated suspensions of spheres in a Newtonian fluid is studied experimentally. Analysing the evolution of the squeeze force as a function of time for different controlled velocities, the suspension is found to present two main flow regimes. The first regime is characterised by the situation in which the force decreases when the velocity decreases, which corresponds to a viscous flow of the suspension. In the second regime, the force increases when the velocity decreases, which is shown to correspond to a filtration of the solvent through the particle skeleton that behaves then as a deformable porous media. By varying the concentration, the sphere diameter and the solvent viscosity, it is found that the transition between the two regimes is a result of a competition between the viscous shear forces due the flow of the suspension and the damping force caused by the filtration of the fluid through the porous media made up by the particles.

WAXD study of induced crystallization and orientation in poly(ethylene terephthalate) during biaxial elongation

This study presents an experimental investigation into the strain-induced crystalline microstructure, under biaxial elongation in poly(ethylene terephthalate) (PET), using wide angle X-ray diffraction (WAXD). We examined how the microstructure of a polymer subjected to a complex strain field evolves in terms of its crystalline ratio, its molecular orientation and the size of its crystallite. PET injection-molded specimens have been subjected to biaxial elongation tests, both equibiaxial and sequential, at different drawing speeds, draw ratios and temperatures above and close to Tg. The strain field was determined using a home-developed image correlation technique that has allowed us to determine all the strain components at each point of the specimen, even with a non-homogeneous strain field. To minimize the effect of quiescent crystallization, specimens are quickly heated with infrared and the temperature was regulated during the test. At the end of the deformation process, the specimens were quenched to room temperature. Their microstructures were later investigated, using both differential densimetry and WAXD with a synchrotron beam. Influences of strain rate, temperature and strain path sequence on the size of the crystallites and their orientation are evaluated. q 2002 Published by Elsevier Science Ltd.

Crystallization of poly(ethylene terephthalate) under tensile strain: crystalline development versus mechanical behaviour

The crystallization of poly(ethylene terephthalate) (PET) under tensile strain was investigated using wide angle X-ray diffraction. Realtime investigation of the crystallization state, including the crystalline ratio and the crystallite orientation, of the material could be undertaken due to the high brilliance of the synchrotron X-ray source used in our study. Initially amorphous PET specimens were stretched at different strain rates and draw ratios at the same temperature (above and close to Tg). Our experimental set-up was designed to undertake simultaneous recording of the X-ray diffraction patterns and the mechanical parameters. Up to the draw ratio of 500% and draw rate of 0.75 s 21 , the crystalline development dynamics corresponded to three different regimes. (i) For small enough extension rates, there was no measurable crystallinity during the drawing process. The crystallization developed after cessation of deformation. (ii) For intermediate extension rates, the whole crystallization process took place during the deformation. (iii) For the highest extension rate involved in our experiments, the crystallization started during the drawing process and continued after cessation of deformation. The mechanical behaviour of the polymer was simultaneously recorded and correlated with the induced crystalline microstructure. In particular, we were able to discriminate the influence of crystallite orientation and crystallization growth on the mechanical behaviour of the material. q 2002 Published by Elsevier Science Ltd.

Influence of viscosity modifying admixtures on the rheological behavior of cement and mortar pastes

The influence of Viscosity-modifying admixtures (VMA) dosage rate on the steady state rheological properties, including the yield stress, fluid consistency index and flow behaviour index, of cementitious materials is considered experimentally. The investigation is undertaken both at cement paste and mortar scales. It is found that the rheological behaviour of the material is in general dependent upon shear-rate interval considered. At sufficiently low shear-rates the materials exhibit shear-thinning. This behaviour is attributed to flow-induced defloculation of the solid particles and VMA polymer disentanglement and alignment. At relatively high shear-rates the pastes becomes shear-thickening, due to repulsive interactions among the solid particles. There is a qualitative difference between the influence of VMA dosage at cement and mortar scales: at cement scale we obtain a monotonic increase of the yield stress, while at mortar scale there exists an optimum VMA dosage for which the yield stress is a minimum. The flow behaviour index exhibit a maximum in the case of cement pastes and monotonically decreases in the case of mortars. On the other hand, the fluid consistency index presents a minimum for both cement pastes and mortars.

Crystallization of polymers under strain : from molecular properties to macroscopic models

The crystallization of thermo-plastic polymers under strain is considered both theoretically and experimentally. The thermo-mechanical model presented here is performed in the framework of the so-called generalized standard materials. In our model we couple in a very natural way the kinetics of crystallization with the mechanical history experienced by the polymer. The viscoelastic properties of the polymer are described using molecular theories. Therefore, in this model of strain-induced crystallization, the kinetics of crystallization is explicitly linked to the polymer chain conformation. Our model is intended to be valid for both for shearing and elongation, or any other complex strain field. Two different viscoelastic molecular models are considered here, corresponding to Maxwell and Pom-Pom constitutive equations. The model is implemented in a dedicated finite element code and the case of injection molding is considered.To validate our strain-induced crystallization model, which explicitly takes into account the molecular conformation, experiments investigating the material behavior at the molecular scale are required. Several measurement techniques can be used to achieve this task, including infrared spectroscopy, optical polarimetry, X-ray scattering or diffraction, etc. In this paper, the wide angle X-ray diffraction (WAXD) is used to investigate the crystalline texture of the polymer. We consider here the case of poly(ethylene terephthalate) (PET) subjected to a biaxial elongation above its T-g. The strain field is determined using a home-developed image correlation technique that allows us to infer all the strain components at each point of the specimen, even in the case of a non-homogeneous strain field. To minimize the effect of quiescent crystallization, specimens are quickly heated with infrared and the temperature was regulated during the test. At the end of the deformation process, the specimens were quenched to room temperature. Their microstructure was later investigated using the WAXD technique. In order to undertake local and accurate WAXD measurements Synchrotron radiation facilities are used.

Oscillatory shear alignment of a non-Brownian fiber in a weakly elastic fluid

Observations of the orientation of single fibers suspended in a polybutene-based Boger fluid under oscillatory shear were made to test a recent theory [O.G. Harlen, D.L. Koch, J. Non-Newtonian Fluid Mech. 73 (1997) 81] that predicts a preferred alignment in the flow direction at strain amplitudes much less than unity. The steady state fiber orientation was found to be dependent on the amplitude of the fiber rotation during a cycle of the oscillation. For large fiber rotations, due to large strain amplitude (e ≥ 1.0) or large fiber projections in the velocity gradient direction, the majority of the fibers drifted slowly towards the vorticity axis, as would be expected for steady shear. A slow rotation towards the flow direction was observed for small strains (e = 0.05), as expected, and for moderate strains with smaller fiber projections in the velocity gradient direction. The theory predicted the correct order of magnitude of this orientational drift. The observations at moderate strains suggest the existence of multiple steady states with both the flow and vorticity directions being stable fixed points.

Tackiness and cohesive failure of granular pastes: Mechanistic aspects

Granular pastes are dense dispersions of non-colloidal grains in a simple or a complex fluid. Typical examples are the coating, gluing or sealing mortars used in building applications. We study the rupture of a thick layer of mortar paste in a simple pulling test where the paste is confined between two flat surfaces. It is shown that, depending on the rheological properties of the paste and the plate separation velocity, two main failure modes are obtained. The first mode is the inwards shear flow of the paste with viscous fingering instabilities, similarly to what has been observed with Newtonian fluids and with non-Newtonian colloidal suspensions or polymer solutions. The second failure mode is stemming from the expansion of bubbles, similarly to what has been observed in soft adhesive polymer layers and, more recently, in highly viscous fluids. It is shown that the crossover between the two failure modes is determined by the conditions required to generate a pressure drop able to trigger the growth of pre-existing micro-bubbles smaller than the inter-granular distance.

Sedimentation of a sphere in a suspension of neutrally buoyant fibers

The sedimentation of a dense sphere in a suspension of neutrally buoyant non-Brownian fibers is investigated experimentally. We consider in particular the effect of the ratio of the sphere diameter D to the fiber length L on the extra drag force experienced by the sphere in a broader range than in previous studies reported in the literature. For a given fiber concentration, the drag coefficient is found to be a strong function of the sphere diameter to the fiber length ratio, particularly when the ball diameter is on the order of the fiber length. When the ball diameter is increased, the drag coefficient rises, passes through a maximum for D=O(L), and then decreases to a steady state value for large spheres. Our experimental results are in qualitative agreement with the numerical simulations of Harlen et al. (O. G. Harlen et al., J. Fluid. Mech. 388, 355 (1999)].

Phase Evolution of Oil Well Cements with Nano-Additive at Elevated Temperature/Pressure

Phase characterization of Class A oil well cement slurries was performed through synchrotron X-ray diffraction technique. This allowed for real-time, in-place measurements of X-ray diffraction patterns to be obtained and, subsequently, the continuous formation and decomposition of select phases over time (up to 8 hours). Phases of interest included alite, ferrite, portlandite, ettringite, monosulfate, and jaffeite (crystalline form of calcium silicate hydrate). The effects of elevated temperatures (140, 185, and 300°F [60, 85, and 149°C]) at elevated pressure (up to approximately 15 ksi [100 MPa]), as well as the effect of nanomaterial addition were investigated. Rate of conversion of ettringite to monosulfate increased with increasing temperature, and monosulfate became unstable when temperatures reached 185°F (85°C). The results of synchrotron X-ray diffraction provided evidence of a seeding effect introduced by nano-sized attapulgite clays at 0.5% addition by mass of cement, where acceleration in the rate of formation of portlandite and jaffeite was observed. This was supported by isothermal calorimetry results.