On the rheology of shear-thickening and magnetorheological fluids under strong confinement

Abstract

Suspension rheology is capturing a great interest in recent years due to the importance of complex suspensions in multitude of industrial applications. Among them, shear-thickening (ST) and magnetorheological (MR) fluids are very valuable materials for their ability of readily tuning their rheological behaviour, well passively by shear or actively in presence of external fields, respectively. Both complex fluids are used in energy dissipating systems: ST fluids are mainly used as impact-resistant materials or shock absorbers in protective applications, while MR fluids are extensively employed in torque transfer applications. The counter-intuitive phenomenon of shear thickening displays a reversible increase in viscosity (continuous or discontinuous) under applied shear rates or stresses. For this non-Newtonian behaviour to occur it is necessary to reach a critical volume fraction and shear rate, in systems where attraction is negligible. These shear-thickening features can be controlled by means of several strategies, such us changing some particle or fluid properties during the formulation of these complex fluids, or introducing net attractive forces. Nowadays scientific community broadly agrees that ST is due to a transition from a hydrodynamically lubricated regime to a friction dominated situation, especially in dense systems. It is in close contact conditions where the fields of rheology and tribology are connected, as the local friction determines the microstructure that give rise to certain macroscopic rheological response. On the other hand, as it happens in the case of ST fluids, the rheological properties of MR fluids can also be varied, but by the action of an external magnetic field. They are suspensions of magnetic micronsized particles suspended in a non-magnetic Newtonian fluid. When subjected to an external magnetic field these particles become polarized and aggregate in chains or columnar structures that orientate along magnetic field lines. As a result of this field-induced assembly, the suspension experiences a reversible liquidto- solid transition, as the viscosity of MR fluids rapidly increases several orders of magnitude, what is known as magnetorheological effect, and it is occasionally accompanied by a yield stress. Magnetorheological applications have to deal with some drawbacks due to particle sedimentation, which is generally improved by the incorporation of additives into the carrier in order to reduce the density mismatch between particles and carrier. The meeting point between ST and MR systems are magnetorheological shear-thickening (MRST) suspensions, i.e., concentrated hybrid systems whose rheological behaviour can be easily tuned, well passively with a given flow deformation or actively through an applied magnetic field strength. These suspensions are still scarcely studied and, apart from controlling the appearance and intensity of the shear thickening behaviour, it has been shown that the partial substitution of magnetic particles by non-magnetic ones in MR fluids produces an increase in yield stress. Besides, the operational mode also affects the MR fluid performance. In this sense, it has been demonstrated a yield stress enhancement when the MR fluid with certain concentration is subjected to slow compression prior to a shear flow mode under the application of an external field, the so-called squeeze strengthening effect. Having said that, the research works presented in this dissertation can be classified in three main topics: rheology of concentrated suspensions that show shear-thickening and/or magnetic response, tribology of non- Newtonian fluids, and squeeze-strengthening effect under constant-volume and constant-area conditions. These three matters were studied experimentally and by simulations. Regarding the first topic, we investigated shearthickening in dense suspensions formulated with one and two types of particles, magnetic and non-magnetic ones, and explore the effect of the type of particle, concentration, carrier fluid and magnetic field. Particle-level dynamic simulations were performed in both monodisperse and polydisperse mixtures of particles in order to reproduce shear-thickening behaviour and the enhancement in yield stress due to partial substitution of magnetic particles in MR fluids. With respect to the second topic we studied tribological behaviour of non-Newtonian fluids, both shear-thinning and shear-thickening fluids, in the elastohydrodynamic regime. Numerical simulations try to reproduce the pressure distribution, film thickness and frictional properties of these fluids within this regime, and a master curved is proposed and evaluated with experimental results. Concerning the last topic, we investigated the slow compression of diluted MR fluids subjected to an external magnetic field, under constant-volume and constant-area conditions. We highlight that higher yield stresses found in constant-area compared to constant-volume conditions, are due to the effect of the densification occurring during the compression of the fluid in the constant-area case. Particle-level simulations mimicked the compression and shear processes and also showed higher yield stresses in constant-area compression.

Los trabajos de investigación presentados en esta tesis pueden clasificarse en torno a tres temas principales: reología de suspensiones concentradas que muestran un espesamiento en flujo de cizalla y/o respuesta magnética, tribología de los fluidos no newtonianos y comportamiento de fluidos MR sometidos a compresión lenta en condiciones de volumen y área constantes. Estas tres materias se han abordado tanto experimentalmente como mediante simulaciones. En cuanto al primer tema, investigamos el espesamiento en suspensiones concentradas formuladas con uno y dos tipos de partículas, tanto magnéticas y como no magnéticas, y exploramos el efecto del tipo de partícula, la concentración, el fluido portador y el campo magnético. Se realizaron simulaciones dinámicas a nivel de partícula en mezclas de partículas monodispersas y polidispersas, con el fin de reproducir el comportamiento espesante y el aumento del esfuerzo umbral provocado por la sustitución parcial de las partículas magnéticas en los fluidos MR. Con respecto al segundo tema, se estudió el comportamiento tribológico de fluidos no newtonianos, tanto fluidos espesantes como fluidificantes, en el régimen de lubricación elastohidrodinámica. Las simulaciones numéricas realizadas pretenden reproducir la distribución de la presión, el espesor de la película y las propiedades de fricción de estos fluidos dentro de este régimen, y se ha propuesto una expresión para la curva maestra, que ha sido validada con los. En cuanto al último tema, investigamos la compresión lenta de los fluidos diluidos de MR sometidos a un campo magnético externo, bajo condiciones de volumen y área constantes. Destacamos la obtención de mayores esfuerzos umbrales en compresiones a área constante con respecto a los experimentos realizados a volumen constante, y que son consecuencia del aumento de la fracción de volumen entre los platos a medida que la compresión avanza. Se han usado nuevamente simulaciones a nivel de partícula que replican los flujos de compresión y cizalla experimentales, y también mostraron mayores esfuerzos umbrales en compresión en área constante.