Chemically-adhesive particles form stronger and stiffer magnetorheological fluids

Abstract

Magnetorheological fluids (MRFs) are suspensions of magnetic particles that solidify in the presence of a magnetic field due to the particles forming chains along field lines. The magnetic forces between particles dominate the solidification process and determine the yield stress of the resulting solid. Here, we investigate how reversible chemical links between particles influence MRF behavior in terms of their yield stress and stiffness through rheological testing in flow and oscillation mode. Initially, we functionalize particles with phosphonate groups that are expected to link through hydrogen bonding and find that this MRF exhibits up to 40% higher yield stress and 100% higher stiffness than an MRF composed of unfunctionalized particles. To explain this change, we model the chemical attraction as an adhesion that supplements dipole–dipole interactions between particles. Interestingly, we find that the increase in yield stress is largest for dilute suspensions that are expected to solidify into isolated chains, while the proportional increase in yield stress is less for MRF with higher concentrations. This is explained by the higher concentration MRF forming a body-centered tetragonal lattice in which interparticle adhesion forces are no longer aligned with the applied field. To explore the possibility of dynamically tuning interparticle interactions, we functionalize particles with polystyrene polymers with thymine terminal groups that will only exhibit interparticle hydrogen bonding in the presence of a small linking molecule, namely melamine. We find that MRF formed with these particles also exhibit up to a 40% increase in yield stress and ∼100% increase in stiffness for the polymer grafted particles in the presence of melamine, due to the formation of hydrogen bonding linkages between the thymine and melamine groups. In addition to confirming the role of hydrogen bonding in increasing MRF stiffness and yield stress, these results highlight the possibility of dynamically tuning MRF performance using magnetic fields and chemical modifications.