What is MR fluid?
MR fluids are a special type of rheological fluids whose yield stress can be varied by an applied magnetic field. A typical MR fluid consists of micron sized magnetizable iron/ferrous particles suspended in a base fluid like silicone oil or water. The term yield stress refers to a threshold value of shear stress and the fluid starts to flow only when the applied shear stress crosses that particular threshold value. Since this yield stress is variable in a MR fluid, we can use this property to design devices for different applications. The advantages of MR fluids are easy controllability (as magnetic fields can be precisely controlled by current-driven electromagnets), direct interface with electronics, higher magnitude of yield stress (compared to ER Fluids), simple construction of devices, low power requirements for control, fast response, etc.
What is meant by a Rheological Fluid?
Water is a simple but popular example of a Newtonian fluid. These fluids flow as soon a shear stress is applied and the shear rate produced is directly proportional to the applied shear stress. Non-Newtonian fluids on the other hand do not follow the simple Newtonian equation. Rheological fluids are one class of non-Newtonian fluid. The significant property of rheological fluids is that they do not flow until the applied shear stress crosses a threshold value called the "yield stress". Paint can be cited as an example of a rheological fluid. When paint is thrown against a wall it does not flow easily although gravity acts on it. However, when water is splashed on a wall it flows down quickly. This is because of the fact that paint is a rheological fluid and the shear stress due to gravity is not sufficient to overcome its yield stress whereas water is a Newtonian fluid and hence flows freely.
The MR fluid behavior is described by the Bingham model, which relates the applied shear stress to the shear rate and the dynamic yield stress.
Research in the ISSL centers on MR Fluid device design with an emphasis on control system development. A list of past and present projects include: