Impact of Nanowires on the Properties of Magnetorheological Fluids and Elastomer Composites

Magnetorheological (MR) fluids are a type of smart material whose effective viscosity may be rapidly changed (~few ms) in a nearly reversible manner by the application and removal of an externally-applied magnetic field. Conventional MR fluids are composed of micron-scale, ferromagnetic spherical particles (typically 30 to 40 volume percent) suspended in a hydrocarbon, silicone, or aqueous carrier fluid (Klingenberg, 2001). The viscosity and apparent shear strength of these suspensions can be controlled by varying the strength of an applied magnetic field. Without an applied magnetic field (off-state), MR fluids are a viscous liquid/particle suspension with a viscosity in the range of 0.1 – 3 Pa· s. Upon application of a magnetic field (on-state), the particles acquire a magnetic polarization and attract one another forming chain-like structures that join to form columnar structures parallel to the applied field (schematic, Fig. 1). The newly formed columns span the surfaces of the device parallel to the field lines resulting in a material that behaves as a Bingham plastic fluid, with increased viscosity and apparent yield stress under shear. The viscosity and yield stress of the fluid is scalable with the magnitude of the applied magnetic field until magnetic saturation of the particles is reached (Jones & Saha, 1990). At high fields, the fluid is converted to a semi-solid with a five to six orders-of-magnitude change in apparent viscosity (Genç & Phulé, 2002). As the particle loading approaches 40 vol. %, the field-induced yield stress can reach 100 kPa. Exceeding the yield stress of the fluid causes the fibril chains and columnar structures to continuously break and re-form, resulting in a post-yield viscosity.