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This project focuses on measurements of torque and velocity profiles in suspensions of magnetic nanoparticles (e.g., magnetite and cobalt ferrite) in Newtonian fluids, polymer solutions, and liquid crystals. Suspensions of magnetic nanoparticles are novel magnetically-active complex fluids which exhibit magnetic field dependent viscosities and various types of striking instabilities. In the past, we have measured magnetic field-induced torques using custom magnetorheometers, modeled suspension response, and studied DC field instabilities. Ongoing research focuses on the response of suspensions of spherical and ellipsoidal nanoparticles in Newtonian, non-Newtonian, and liquid crystalline matrices subjected to oscillating magnetic fields. We are following a combined experimental, modeling, and simulation approach. The results of the research will address key issues in development of magnetic nanoparticle based sensors and therapies. This project is supported by the NSF through various grants, including a CAREER award (DMR-0351449, CTS-0457359 and CTS-0547150). Through our collaboration with our external collaborators, we will be able to prepare monodisperse ellipsoidal particles using photolithography, which is not available in Puerto Rico.