Aziz Omari

Gelation of polymer solutions under shear flow

Abstract Gelling a polymer solution under constant shear rate was recently found to be an attractive procedure for preparing size-controlled microgels. In the theoretical approach developed in this paper, the microgels formed during gelling are modelled as non-covalent ‘star’ polymers. This approach provides scaling laws to predict the effects of the shear rate applied during gelation on: (1) the gelling kinetics in three flow regimes; (2) the microgel size dependence in weak and strong flows; and (3) the rheological and elastic properties of the microgel solutions. The reported experiments were carried out by gelling semi-dilute high molecular weight polymer solutions by a crosslinker small enough to diffuse inside macromolecules and thus able to form both intra and intermolecular cross-links. All available experimental results are in agreement with our theoretical predictions, but further experiments are needed for a complete validation of the proposed theory.

Rheology of Water in Crude Oil Emulsions

Abstract The present paper deals with the rheological study of simplified sea water in crude oil emulsions. This includes viscosity dependence vs. shear rate, creep experiments and oscillatory shear measurements. Our results show the existence of a critical water volume fraction φc in addition to the maximum packing fraction φm. This critical fraction marks the onset of physical contact between water droplets. The determined thickness of the hydrodynamic layer due to surfactant molecules is shown to decrease as the water volume fraction increases. On the other hand, when emulsions contain an additional surfactant, the loss modulus G″ measured at f=1xa0Hz shows a maximum for a given shear strain γmax. Such shear strain, however, decreases as the water volume fraction increases and remains constant above φ∼φc. Finally, we attempt to scale shear rate values with relaxation time determined from creep experiments using the analogy with concentrated macromolecular solutions.

Zirconium speciation in lactate solutions and polyacrylate gels

Controlling gelation kinetics is an important objective for several applications (ceramic and thin film syntheses, reduction in permeability of porous rock, etc). There is a growing interest in studying the gelation of polymers by zirconium, a crosslinker of lower toxicity than the chromium which is still commonly used. XAS at the Zr K-edge was performed at the European Synchrotron Radiation Facility (ESRF) on the BM32 beamline. The fluorescence detection was used to carry out successful in situ speciation at concentrations as low as 36 ppm. The Zr speciation was determined both in ZrLa (where La stands for lactate) aqueous solutions and in gels of a terpolymer of acrylamide having 2% of zirconium reactive acrylate side groups and 2% of sulfonate groups introduced to prevent syneresis. XANES results show that Zr is always in a dodecahedral geometry. In ZrLa solutions. EXAFS results indicate that Zr species grow from a dimer Zr2(La)6 to a tetramer (Zr4(La)x) and then to larger polymers resulting from tetramer associations, as the Zr concentration decreases from 51840 ppm to 36ppm. In polymer gels, Zr species appear to be dimers at pH 6 while tetramers are found when gelation occurred at pH 7. Calculations taking into account multiple scattering effects as well as dynamic molecular calculation confirmed conclusions derived from conventional EXAFS analysis.

Colloidal Particle Deposition in Porous Media Under Flow: A Numerical Approach

The objective of this study is to simulate the transport and deposition of colloidal particles at the pore scale by means of computational fluid dynamics simulations (CFD). This consists in the three-dimensional numerical modeling of the process of transport and deposition of colloidal particles in a porous medium idealized as a bundle of capillaries of circular cross section. The velocity field obtained by solving the Stokes and continuity equations is superimposed to particles diffusion and particles are let to adsorb when they closely approach the solid wall. Once a particle is adsorbed the flow velocity field is updated before a new particle is injected. Our results show that both adsorption probability and surface coverage are decreasing functions of the particle’s Peclet number. At low Peclet number values when diffusion is dominant the surface coverage is shown to approach the Random Sequential Adsorption value while it drops noticeably for high Peclet number values. Obtained data were also used to calculate the loss of porosity and permeability.

Displacement of Colloidal Dispersions in Porous Media: Experimental & Numerical Approaches

The main objective of this paper is to give more insight on colloids deposition and re-entrainment in presence of a rough surface. Experiments on retention and release of colloids in a porous medium are first presented. The influence of physicochemical and hydrodynamic conditions is investigated. The experimental results cannot be qualitatively interpreted using the DLVO theory and knowledges at pore scale are then needed. A 3D numerical simulation approach at the pore scale is therefore proposed where the motion of colloids is solved in presence of collector surfaces bearing various kinds of asperities and by taking into account physico-chemical interactions calculated at each time step during colloid movement. It is obviously observed that both deposition and mobilization of particles are dependent on solution chemistry and hydrodynamic conditions and are significantly affected by the form and size of the local roughness of the pore surface. Therefore, depending on solution ionic strength and surface topography, colloids may be adsorbed or not and when a particle is retained an increase of flow strength is then needed to remove it and such an increase is specific to the location of occurrence of the adsorption step. In general, simulation results allow us to explain our experimental results that show that by steeply increasing the flow strength, more and more fractions of particles retained inside the porous medium are released until all particles are removed.