The Performance of an Electrorheological Fluid in Dynamic Squeeze Flow: The Inﬂuence of Solid Phase Size Ali K. El Wahed,* John L. Sproston,* ,1 and Roger Stanway† *Department of Engineering, University of Liverpool, Liverpool L69 3GH, United Kingdom; and †Department of Mechanical Engineering, University of Shefﬁeld, Mappin Street, Shefﬁeld S1 3JD, United Kingdom Received May 13, 1998; accepted November 9, 1998 This paper is concerned with an experimental investigation of a comparison of the rheological performance of an ER ﬂuid, con- sisting of a carrierwith a range of solid phase sizes, in oscillatory squeeze-ﬂow under DC conditions. The ﬂuid is sandwiched be- tween two parallel plane electrodes, the upper one stationary and the lower one oscillating normal to its plane. It is seen that the dynamic performance of the ﬂuid, in terms of the capacity for the transmission of imposed forces across the ﬂuid, is highly depen- dent on the size of the dispersed solid phase and has an optimum value which is dependent on the mean value of the interelectrode gap. In addition the paper includes some measurements of the current through the ﬂuid which may help to shed some further light on the mechanism of the ER effect. Finally the implications of the results to vibration control are discussed. © 1999 Academic Press Key Words: ER ﬂuids; suspensions; vibration control. INTRODUCTION The majority of vibration damping devices utilizing an elec- trorheological (ER) ﬂuid has employed the ﬂuid in simple shear mode where the ﬂuid is deformed in a direction orthog- onal to the arrays of particulate chains and where the gap between the shearing electrodes remains constant. Stanway et al. (1) and Stevens et al. (2) have already demonstrated the feasibility of constructing a simple vibration damper using this mode of operation. An alternative arrangement, called squeeze mode, in which the ﬂuid is subjected to compressive and tensile stresses in a direction parallel to the chains (resulting in a variable ﬂuid layer thickness) has been identiﬁed and inves- tigated by Stanway et al. (3) and Monkman (4) where it was shown that the yield stress in this mode is greater than that available in shear typically by a factor of ten. These results are in accordance with those, for example, of Lukharinen and Kaski (5) and Gong and Lim (6). Papers by El Wahed et al. (7), Stanway and Sproston (8), and Stanway et al. (9) provide substantial overviews of the classiﬁcation of the modes of operation of ER ﬂuids and of their potential applications in vibration control, respectively. In parallel with device investigations, during the last decade numerous studies have been carried out aimed at improving the mechanical and electrical characteristics of ER ﬂuids, but the inﬂuence of particulate size on their rheological performance remains unclear. Jordan and Shaw (10) referred to particulate sizes in the range 0.04 to 50 m while Block (11) mentioned the range 5 to 50 m and Conrad (12) 0.1 to 100 m. Recently the inﬂuence of the shape of the particulates was assessed by Yatsuzuka et al. (13) and Asano et al. (14) and also their electrical properties by Barry et al. (15) on the magnitude of the ER effect. However the limits beyond which no ER activity will exist or the optimum size of the particulate for a particular application have not been clearly deﬁned. It is this which prompted this particular investigation. This paper is concerned with the assessment of an ER ﬂuid in dynamic squeeze-ﬂow under DC excitation and the inﬂuence of the solid phase size on its rheological performance. The relevance of this work in the application to vibration control in a short-stroke damper is discussed. EXPERIMENTAL ARRANGEMENT Experimental Facility The experimental rig shown in Fig. 1 consisted of a Ling Dynamic Systems electromagnetic shaker (Model No. 406) which was capable of providing vertical oscillatory motion with a minimum and maximum amplitude of 0.1 mm and 7 mm, respectively, over a frequency in the range DC to 9 kHz. The shaker head was attached rigidly to a Kistler (Model No. 9311A) piezoelectric force link and an earthed brass electrode having a recessed cylindrical cavity of diameter 95 mm which provided the reservoir for the ER ﬂuid. The high voltage upper electrode was a circular brass disk of diameter 56 mm, its circumferential edge and rear face surrounded and supported by a PTFE collar. This was rigidly attached to a second identical force link and positioning assembly to the supporting frame. The instantaneous displacement of the lower electrode was determined using an RDP (type GTX 2500) LVDT and the acceleration using an Endevco (type 7254-100) accelerometer 1 To whom correspondence should be addressed. Journal of Colloid and Interface Science 211, 264 –280 (1999) Article ID jcis.1998.5972, available online at http://www.idealibrary.com on 264 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.