Dynamics of magnetorheological fluids at the microscale
Metadatos
Mostrar el registro completo del ítemAutor
Shahrivar, KeshvadEditorial
Universidad de Granada
Departamento
Universidad de Granada. Departamento de Física AplicadaMateria
Materiales inteligentes Fluidos magnéticos Polímeros Propiedades térmicas Reología Deformaciones (Mecánica)
Materia UDC
532.1 2204
Fecha
2017Fecha lectura
2017-05-19Referencia bibliográfica
Shahrivar, K. Dynamics of magnetorheological fluids at the microscale. Granada: Universidad de Granada, 2017. [http://hdl.handle.net/10481/47125]
Patrocinador
Tesis Univ. Granada. Programa Oficial de Doctorado en: Física y Ciencias del EspacioResumen
In this dissertation, we investigated a new class of MRFs that bridges the gap between
conventional MRFs and MR elastomers. In a novel approach, a thermoresponsive
polymer-based suspending medium, whose rheological properties can be externally
controlled through changes in the temperature, was used in the formulation of the
MRFs. We used, particularly, Poly (ethylene oxide)–poly (propylene oxide)–poly
(ethylene oxide) triblock copolymers and Poly(N-isopropylacrylamide) microgels. Thus,
we found a feasible way to prevent particle sedimentation in MRFs but at the same time
retaining a very large MR effect in the excited state.
The study of the creep flow behavior of MRFs is of valuable help in understanding the
yielding behavior of these materials. A direct comparative study on the creep-recovery
behavior of conventional MR fluids was carried out using magnetorheometry and
particle-level simulations. We show that the recovery behavior strongly depends on the
stress level. For low stress levels, below the bifurcation value, the MRF is capable to
recover part of the strain. For stresses larger than the bifurcation value, the recovery is
negligible as a result of irreversible structural rearrangements.
From a practical point of view, it is interesting to study the thin-film rheological and
tribological properties of FFs. Recently, it has been shown that by using FFs in
mechanical contacts it is possible to actively control the frictional behavior. In this
dissertation, we explored a new route to control friction in the isoviscous-elastic lubrication regime between compliant point contacts. By superposition of nonhomogeneous
magnetic fields in FFs lubricated contacts, a friction reduction was
achieved. Also, we compared the tribological performance of FFs and MRFs using the
same tribological conditions and tribopairs. In the case of FFs lubrication the sliding
wear occurs mainly by two-body abrasion and in the case of MRFs lubrication the
sliding wear occurs by two-body and three-body abrasion.
Finally, the study of the growing rate of the field-driven structure formation is also
important, in particular, for the prediction of the response time of MRFs since their
practical applications depend on the rate of change in their properties. The irreversible
two-dimensional aggregation kinetics of dilute non-Brownian magnetic suspensions was
investigated in rectangular microchannels using video-microscopy, image analysis and
particle-level dynamic simulations. Especial emphasis was given to carbonyl iron
suspensions that are of interest in the formulation of MRFs; carrier fluid viscosity,
particle/wall interactions, and confinement effect was investigated. On the one hand, the
carrier fluid viscosity determines the time scale for aggregation. On the other hand,
particle/wall interactions strongly determine the aggregation rate and therefore the
kinetic exponent. It was found that aggregation kinetics follow a deterministic
aggregation process. Furthermore, experimental and simulation aggregation curves can
be collapsed in a master curve when using the appropriate scaling time. The effect of
channel width is found to be crucial in the dynamic exponent and in the saturation of
cluster formation at long times. On the contrary, it has no effect in the onset of the tipto-
tip aggregation process.