Recent Developments in ER Fluids

Abstract

Electrorheological (ER) or smart fluids are materials which have the ability to reduce their flow when strong electric fields are present, in the manner illustrated in figure 1. There is an extra shear stress (Δσ) needed to maintain the flow of such a liquid against the action of the field or the flow is reduced and in some circumstances, may cease altogether. In the latter event the material has converted from liquid to solid. Such dual behaviour is reminiscent of a Bingham plastic and the ER phenomenon can be considered as invoking electrically induced Bingham body behaviour. Since removal of the field causes the viscoelastic moduli of such a fluid to revert to their original state an ER ‘fluid’ still under stress will recommence flowing when that field is removed. Thus, ER fluids are amenable to switching by electric field and this, together with the rapidity (≈ ms) of at least parts of these interconversions for many ER fluids, renders them of practical importance. Electrically enhanced stresses in the kPa range for electric fields of ≈ kVmm−1 can be achieved with a wide variety of fluids. The phenomenon occurs with ac as well as dc fields provided that the field frequency is not too high.