Periodic Reporting for period 2 - MAMFRHE (Multi-Axial Magnetic Field RHEology)
Reporting period: 2023-09-13 to 2024-09-12
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.
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.
The project has covered almost completely all objectives. On the one hand, it has been shown numerical and experimentally that MR fluid particles under the so-called balanced triaxial unsteady fields (fields that sweep isotropically the three dimensions) and shear flow are driven to form sheets. What is more, these seem to be a minimum energetic state what is macroscopically translated to a viscosity relaxation with shearing time. Besides, using perturbation and precession fields (fields that sweep a plane and a cone, respectively) can enhance the MR fluid yield stress for the same power consumption. On the other hand, a theoretical model has been proposed to explain the sheet formation under uniaxial constant fields and shear flow. Once it was implemented numerically, the model compares satisfactorily with experimental results providing a novel explanation for the sheet formation. Particularly, it predicts that sample size in the field direction and the particle concentration are the control parameters of the sheet pattern morphology (sheet width and separation). At present, those results are the subject of 5 publications in JCR journals (1 published, 2 under review, 2 in preparation) and a possible patent application. They have been communicated in several international conferences and outreach events (e.g. European Researchers’ Night).
Since MR fluids have not been studied under the simultaneous application of shear flow and triaxial unsteady fields before, this project deals with unexplored particle interactions and structures. Thus, it is opening a completely new line of research in the field of magnetorheology with promising outputs. Regarding the sheet formation under uniaxial fields, although it was discovered long time ago, a microscopic description taking into account the changes in the inner structure was absent in the literature.
Results yielded so far show the possibility of tuning the sheet inclination (under triaxial fields) as well as sheet width and separation (under uniaxial fields) in MR fluids just controlling easily accessible parameters (such as flowing time, sample height, field and flow magnitude or particle concentration). This makes MR fluids amenable for several research and engineering fields such as biotechnology and tissue engineering (tunable scaffolds for cell growth), optics (patterns for diffraction purposes), microfluidics (threshold valves, flow controllers and fuses) or electronics (adaptive capacitors and resistances) to cite a few. It is expected that gaining control over the particle structures will allow MR fluids to adapt better to the specific requirements and thus, boosting their performance and presence in daily life applications.
Results yielded so far show the possibility of tuning the sheet inclination (under triaxial fields) as well as sheet width and separation (under uniaxial fields) in MR fluids just controlling easily accessible parameters (such as flowing time, sample height, field and flow magnitude or particle concentration). This makes MR fluids amenable for several research and engineering fields such as biotechnology and tissue engineering (tunable scaffolds for cell growth), optics (patterns for diffraction purposes), microfluidics (threshold valves, flow controllers and fuses) or electronics (adaptive capacitors and resistances) to cite a few. It is expected that gaining control over the particle structures will allow MR fluids to adapt better to the specific requirements and thus, boosting their performance and presence in daily life applications.