On the rheology of shear-thickening and magnetorheological fluids under strong confinement
Metadatos
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Universidad de Granada
Departamento
Universidad de Granada. Programa de Doctorado en Física y Ciencias del EspacioMateria
Shear-thickening Rheology Magnetorheological fluids Confinement
Fecha
2021Fecha lectura
2020-11-27Referencia bibliográfica
Ortigosa Moya, Elisa María. On the rheology of shear-thickening and magnetorheological fluids under strong confinement. Granada: Universidad de Granada, 2021. [http://hdl.handle.net/10481/65310]
Patrocinador
Tesis Univ. Granada.; Ministerio de Economía y Competitividad (MINECO), mediante los proyectos MAT 2013-44429-R y MAT 2016- 78778-R.; Europeo de Desarrollo Regional (FEDER); Junta de Andalucía mediante el proyecto P11-FQM-7074.; FPI 2014/069341.Resumen
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.