Enhancing magnetorheology with unsteady triaxial magnetic fields

Résumé

Magnetorheological (MR) fluids have long been used for industrial applications that require a quick, reversible change in fluid properties such as a spontaneous increase in viscosity in the presence of an external magnetic field. Micron-sized magnetizable particles dispersed in a nonmagnetic fluid carrier structure via their dipolar interactions in line with the field direction — traditionally a uniaxial DC field. For sufficiently large field strengths, particle structuration usually consisting in the formation of chains or thicker columnar structures restricts fluid flow causing the emergence of a yield stress in the MR fluid and making it more robust to deformation under shear. Dampers, shock absorbers, and braking systems are a few industrial applications that take advantage of the unique adaptability of these smart materials. With our homemade triaxial magnetic field generator we have carried out a complete study of the MR response of more complex precession-like magnetic fields, the particle mesostructures formed under these fields, and other timedependent field configurations. For time-varying fields the relation between the hydrodynamic and magnetic forces plays a key role in the aggregation dynamics in a magnetic suspension. A nondimensional parameter known as the Mason number relates these two magnitudes. The triaxial device was designed and constructed with the intention of carrying out both videomicroscopy and rheometry experiments. The MR enhancement was quantified through an analysis of the storage modulus. Columns, spirals, and layered structures are some of the 3D particle structures we can form using the triaxial field generator by means of the particles’ dipolar and time-averaged magnetostatic interactions. Two main aggregation mechanisms are proposed to explain the MR enhancement — lateral coalescence between vicinal columnar structures, and through particle compaction. The former of the two mechanisms is further justified with particle-level simulations and a study of the average cluster size of simulated particle structures under low angle precession fields. Inspired by the emergent dynamics seen in our experimental work, we have integrated particle self-assembly for a hydrogel composed of modified polysaccharide and protein. Directional, or guided, cell growth often involves a fabricated 3D solid scaffolding matrix for which cells can attach to. Although this method is effective, we propose a novel path for tissue engineering for our magnetically-responsive system using both uniaxial and unsteady magnetic fields and the complex structures witnessed in the MR experiments. A 3D anisotropic particle network is first structured under a pre-programmed magnetic field configuration in its fluid-like viscous state. The particle structure, and the suspended cells, are encapsulated in position as the carrier fluid polymerizes via the Schiff reaction. Confocal microscopy shows the cells embedded within the particle network, and multi-day analysis of the hydrogels suggests the particle structure stays intact allowing a lasting scaffold for cell growth. We hope this study establishes a groundwork for the possibilities of less invasive injectable alternatives for cell regeneration remedies.