An experimental study on the squeezing flow of electrorheological suspensions

Electrorheological (ER) properties of ER suspensions composed of silica particles and silicone oil under the squeezing flow have been investigated experimentally by changing experimental conditions such as applied electric field, volume concentration of silica particles, medium viscosity of silicone oil, and water content in silica. ER behavior in the squeeze flow is unique in several ways compared to that in the shear flow. Electric field and volume concentration increase as the squeezing proceeds, which increases the strength of chain structures formed between the electrodes. High yield stress and normal stress can be obtained in the case of high voltage, high volume fraction, low medium viscosity, and high water content. As the squeezing proceeds, the normal stress increases, showing some fluctuation due to the deformation of chain structures in the suspension. The fluctuation observed in the stress grows so violent under a certain condition, which seems to be caused partly by macroscopic rearrangement of chainlike structures and by the polydispersity of particles due to aggregation. As the serious fluctuation should be avoided for commercial application, an addition of additive or a surface treatment of particles is strongly recommended to enhance the stability of the dispersed phase. (C) 2000 The Society of Rheology. [S0148-6055(00)00101-2].