Design and testing of a new magnetorheometer

A new, commercially available polishing process called magnetorheological f ininshing is used to polish and figure precision optics. To understand and model this process correctly it is important to determine the mechanical pr operties of the fluid under the influence of the magnetic field. Magnetorhe otogical (MR) fluids are commonly modeled as Bingham fluids, so one of the essential properties to measure is the yield stress. Since MR fluids are in herently anisotropic, the yield stress will depend on the mutual orientatio n of the magnetic field and the direction of deformation. The relative orie ntation of the field and deformation in polishing does not coincide with co mmon rheological setups, so a new rheometer has been designed and tested. T his new magnetorheometer design has been shown to give correct stresses dur ing calibration experiments using Newtonian fluids with a known viscosity. The measured stress has also been shown to have a magnitude consistent with published finite element approximations for magnetic fluids. The design of the instrument was complicated because of the requirements imposed upon th e magnetic field, and the difficulty in satisfying the no slip boundary con dition. Our results show the importance of having a homogeneous field in th e test region during measurements. The solutions to these problems and disc ussion of the measurements on nonmagnetic and magnetic fluids are given. (C ) 1999 American Institute of Physics. [S0034-6748(99)01111-9].