Magnetorheological fluids belong to the class of field-responsive fluids that undergo large, reversible and fast changes in their theological properties when acted upon by an external magnetic field. 'Smart' or controllable composite materials have been obtained by doping polymers, foams, fabrics etc. with these field-responsive fluids. The resulting composite materials have potential applications in numerous fields ranging from adaptive energy absorption, automotive crash protection to microfluidic valves, mixers and separation devices.
A series of stable magnetorheological (MR) fluids have been systematically characterized under steady shearing, creep and large amplitude oscillatory shear (LAOS) flow conditions. A rheometer fixture for applying nearly uniform magnetic fields up to 0.4 T has been fabricated to measure both steady-state and transient changes in the fluid properties under applied fields. Stable MR fluids with a markedly improved dynamic response (yield stress as a function of magnetic field) compared to commercial fluids have been formulated by increasing the constituent particle size and by stabilizing the system against sedimentation. A new "soft-glassy theology" model has been used to model the fluid response time and visco-elasto-plastic response under creep conditions and oscillatory loadings. The experiments and model show that the evolution of chain structure and plastic collapse in these suspensions exhibits a universal scaling with the dimensionless stress s = σ/σ y .
Structure evolution, pattern formation and dynamics of MR fluid flow in microchannel geometries has been analyzed using high-speed digital video microscopy. In order to elucidate the mechanisms that control MR structure formation, experiments have been performed while varying the magnetic field, particle size, channel geometry, concentration and fluid composition. Excellent qualitative agreement has been obtained with Brownian Dynamics simulations and useful scalings based on interplay of magnetostatic & viscous forces have been extracted to understand the field-dependent fluid response on the macro & micro scale.
Novel MR elastomeric materials and microparticles have been synthesized by doping photo-curable or thermo-curable polymers with field-responsive fluids. A high-throughput micromolding technique for synthesis of controllable particles of anisotropic shapes and sizes has been developed. Flexible and permanent chain-like structures have also been synthesized using amidation chemistry. Potential microfluidic applications such as field-responsive valves, mixers and separation devices using these 'smart' materials have also been investigated. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)
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