Ultrahigh yield stress in a general electrorheological fluid under compression

The yield stress induced by external electric field generally of the order of kV mm(-1) is an important parameter for an electrorheological (ER) fluid. An ultrahigh yield stress of over 100 kPa of a general ER fluid in a slow compressive flow under an electric field of less than 0.6 kV mm(-1) has been achieved in this study, while its shear yield stress tested on a shear rheometer is only 0.6 kPa. A lower compressive velocity results in a higher yield stress under the same electric field. The large difference between the yield stresses is ascribed to the chain aggregation caused by the two-phase flow of liquid and particles during the slow compression. The viscous force acting on particles decreases as the compressive velocity increases, while the force acting from the neighboring particles to squeeze particles out increases. The increased particle-particle interaction established more interconnectivities among chain structures and greatly enhanced the yield strength of the ER fluid. The achieved result seems to deviate from the prediction of the Deborah number at a lower test velocity. The chain structure aggregation causing much longer relaxation times under slow compression has been employed to account for this deviation and the proper calculation of the Deborah number.