The performance of magnetorheological fluid in squeeze mode

In magnetorheological (MR) fluid, the rheological properties can be changed in a controlled way, the changes being reversible and dependent on the strength of the magnetic field. The fluids have potentially beneficial applications when placed in various geometrical arrangements. The squeeze mode is a geometrical arrangement where two flat parallel solid surfaces, facing each other, are pushed towards each other by an external force operating at right angles to the surfaces. The liquid initially in the gap between them is free to move away from this increasingly small gap, and it does so by flowing parallel to the surfaces, and collecting in a region where it is no longer in the gap between them. The performance of an MR fluid in compression ( squeeze) mode has been studied with the magnetic field being generated by a coil carrying different magnitudes of DC electrical current. A test rig was designed to perform this operation with the flat surfaces being horizontal and being pushed together in a vertical direction and the liquid being forced to move in all directions in a horizontal plane. The rig operated by decreasing the size of the gap at a constant rate. For each trial the current in the coil was kept constant and the instantaneous compressive force was recorded. When plotting compressive stress against compressive strain for each trial, the slope of the curve was found to be larger in general when the current was larger. This was an expected result; however, the behaviour is more complicated than this. For a significant range of values of compressive strain, the slope falls to zero, so that the compressive stress shows no increase during this period while the compressive strain continues to increase. The details of this behaviour are strongly dependent on the initial size of the gap.