Generally speaking, magnetorheological (MR) fluids are suspensions of magnetizable microparticles in a liquid. Under the action of a magnetic field, the particles acquire a magnetic moment and self-assemble providing the MR fluid with an inner structure. The magnetic interactions between the particles in that structure can increase the viscosity of the system so much that the MR fluid eventually can develop solid-like properties. Thus, MR fluids are smart materials whose mechanical properties are controlled at will by tuning the magnetic field they are exposed to. This has made them very useful systems for torque-transfer applications in several engineering branches (civil, automotive, robotic, etc.).
Traditionally, the viscosity enhancement has been triggered using uniaxial constant magnetic fields that give rise to strings of particles in the field direction. In this project, it is addressed for the first time the mechanical behavior of MR fluids (how they are strained by a shear flow) when other inner structures are induced. In particular, it is studied the effects of unsteady triaxial magnetic fields. Those fields change their magnitude and direction periodically in time, inducing magnetic interactions between the particles very different from the traditional uniaxial constant case. As a consequence, a new family of unexplored structures (foams, percolating networks, tubes of particles, etc.) is accessible.
The project faces the mechanical characterization of those structures from numerical and experimental points of view. In a first objective, it is proposed to correlate the structure evolution under shear with the viscosity evolution of the MR fluid through Molecular and Stokesian Dynamics simulations. In a second objective, those results will be validated through experiments with a novel instrument. Namely, it will be used a rheometer (to measure the sample viscosity) coupled to a triaxial magnetic field generator (to create magnetic fields in any direction with periodic time dependency in any of the components) and a confocal microscope (to have access to the structure while the MR fluid is sheared).
Experiments and simulations show that unsteady triaxial fields constitute a valuable tool to control till a higher extent the rheological behavior of MR fluids. By choosing the adequate field configuration it is possible to arrange the particles in structures compatible (or not) with the shear flow and thus to increase (decrease) the sample yield stress above (below) the value obtained in a classical uniaxial constant configuration.
Finally, the physics behind sheets formation in the flow/field plane under the application of shear flow and uniaxial constant field are investigated. It is shown that the ability to tune the inner microstructure (e.g. separation between sheets) arises as a very attractive opportunity to make precise and ad hoc micro-patterns with more applications other than the traditional ones in magnetorheology.
