Electro-rheological and magneto-rheological properties of polymers
Electrorheological fluids are suspensions of fine particles in an electrically insulating fluid, usually mineral or silicone oil. When exposed to an electric field their apparent viscosity can increase by several orders, thus shifting their substance from a liquid to almost a solid-like behavior. Hence, their structural and rheological properties can be controlled by electric field strengths.
Magnetorheological (MR) suspensions belong to a class of smart materials that typically consist of magnetizable micronsized particles dispersed in non-magnetic liquids which in the presence of a magnetic field exhibit a rapid change in their rheological properties. When an external magnetic field is applied to these systems with randomly distributed particles, the magnetic dipoles of the particles align parallel to the external field, resulting in a particle chain-like structure. In the flow field, the shear stress increases and even a yield stress may appear. When the magnetic field is switched off, the particle chains are destroyed due to shear forces and the shear stress rapidly decreases. A fast change in the rheological properties of MR suspensions in response to a magnetic field offers the possibility to use this phenomenon in technical applications such as clutches, control valves, shock absorbers, torque transducers, dampers, polishing fluids and various medical applications.