Universidad de Málaga

    Escuela Técnica Superior de Ingenieros Industriales

    Master in Industrial Engineering (2004)

    Master of Advanced Studies (2004-2006)

    PhD Student (2004-2008)

 

 PhD Thesis (2008)

"Rheological characterization of suspensions of pyrogenic silicon dioxide in polypropylene glycol".

Supervisor: Prof. F.J. Rubio-Hernández

The use of pirogenic silicon dioxide as a modifier agent of the rheological properties is a common practice in the seek of new materials with customized flow behaviour. The surface chemistry of the fume silica particles determines their hydrophilicity or hydrophobicity and, consequently, the particle-particle and particle-media interactions. In that way, the knowledge of these interactions lets one predict if a colloidal suspension will be flocculated, non-flocculated or a colloidal gel, and its macroscopic flow properties. In this study three different kind of particles have been considered: one hydrophylic (A200), one hydrophobic (R816) and one partially hydrophobic (R805). These particles have been dispersed in polypropylene glycol of two different molecular weight (400 and 2000g/mol) at a fixed concentration of 5%wt. Thus, the influence of the surface chemistry, as well as the influence of the molecular weight, on the rheological behaviour of the suspension could be studied.  Special attention has been given to the definition of a pre-shear protocol, which allows to erase the shear history of the suspensions without denature them. Transitory behaviours have been analyzed carefully, due to many fumed silica suspensions behave as thixotropic fluids. Considering that transport of materials occurs mostly through a pipe in industrial processes, this work as been complemented with the study of the steady, laminar and fully developed flow in a circular cross-section pipe of the fumed silica suspensions previously characterized rheologically. In order to do that, it has been necessary to define previously the constitutive equations in the steady state. In that way, here they are used the three branched of Rheology: Rheometry, Theoretical Rheology and Computational Rheology.

Advanced Studies Degree (2006)

"Numerical simulation of flow for non-Newtonian fluids with thixotropic behaviour" 

Supervisor: Prof. J. Ortega-Casanova

This work was a continuation of the master thesis and its main objective was further my knowledge in Rheometry and it suposed my very first contact with Computational Fluid Mechanics. The most common geometries used in Rheometry, i.e. parallel plates, cone-plate and concentric cylinders, were drawn and their domains were meshed. Firstly, all cases were solved for a Newtonian liquid and compared with the analytical solutions in order to validate the meshes. Finally, the case for a thixotropic fluid (aqueous suspensions of bentonita) was solved and the hydrodynamics phenomena were analyzed. 

 

MSc Thesis (2004)

"Thixotropy. Methods and influence of the measurement conditions and experimental parameters" 

Supervisor: Prof. F.J. Rubio-Hernández

Bentonite dispersions are widely used in the industry, being used in ceramic products, cement, drilling fluids, moulding sands and paints. These dispersions have a high swelling capacity. The particles of this clay, mainly consisting of the clay mineral montmorillonite, are planar and have two kinds of surface charges. On the faces, a negative surface charge is formed from the substitutions within the interior of the crystal lattice. At the edge of the particles, a pH-dependent positive surface charge is formed. This last charge is due to the protonation/deprotonation of OH groups. Then its sign may change from positive to negative with increasing pH. This special charge distribution gives way to different modes of particle association: face-toface (FF), edge-to-face (EF) and edge-to-edge (EE). Therefore, different pH-dependent particle structures should be formed in bentonite dispersions. Bentonite dispersions are non-Newtonian fluids with a thixotropic behaviour. Thixotropy is the characteristic of a fluid to form a gelled structure over time when it is not sheared and to liquefy when agitated. We will assume that the phenomenon refers to the reversible breakdown of particulate structures under shear. When a shear rate is applied to the system, two kinetic processes appear, breakdown due to applied shear and build-up because of Brownian movements and the increase of the particle encounters favoured by the shear, and this gives place in the thixotropic behaviour. A state of dynamic equilibrium is reached when breakdown and build-up rates are equal. The equilibrium structure is usually a function of the applied shear rate and tends towards greater breakdown at higher shear rates. It is well known that the apparent viscosity of a thixotropic substance depends on the instantaneous shear rate as well as on the past deformation history. Then, for example, the initial or pre-shear state of a system should have a great influence on its thixotropic behaviour. The structural kinetics approach to the thixotropy assumes the existence of a shear- and time-dependent structure. According to this approach, Cheng and Evans (1965) proposed a general mathematical form of the equations of state of a thixotropic material. It is assumed that there is only one type of structure defined by a scalar parameter. The mathematical description of this phenomenon consists of a constitutive equation, which relates the shear stress and the shear rate with the structure parameter, and a kinetics equation that expresses the rate of structural changes as a function of both, the shear rate and the structure parameter. Several expressions for both functions have been proposed. However, there are few systematic experimental studies on the thixotropy of different substances, being necessary to advance in the knowledge of this rheological behaviour to account with feasible and systematic experimental results. In this study, experimental data on the structural breakdown and build-up of bentonite aqueous dispersions have been obtained. The results have been tested with an experimental approach based on the indirect microstructural model.

Last update - 19/10/2014