The role of the dispersed-phase remnant magnetization on the redispersibility of magnetorheological fluids
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The role of the dispersed-phase remnant magnetization on the redispersibility of magnetorheological fluids Pradeep P. Phule´, Matthew P. Mihalcin, and Seval Genc Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 (Received 7 October 1998; accepted 26 April 1999)
The influence of the remnant magnetization of the soft magnetic particulates, used as a dispersed phase, on the redispersibility of magnetorheological (MR) fluids is discussed. Calculations of the magnetic interaction energy showed that for 33-vol% MR fluids based on particles of iron (∼6 m), manganese zinc ferrite (∼2.3 m), and nickel zinc ferrite (∼2.1 m), the ratios of the magnetic interaction energy to the thermal energy were 161,000, 6400, and 3900, respectively. These calculations showed that even the seemingly small levels of remnant magnetization, associated with particulates employed in MR fluids, introduced significant dipole–dipole interparticle interactions. It is proposed that this interaction causes most MR fluids to show a tendency for “cake formation,” which makes it difficult to redisperse these fluids. Our modeling presented here also suggests practical strategies to enhance the redispersibility of MR fluids.
I. INTRODUCTION
Magnetorheological (MR) fluids represent an exciting family of smart materials. These materials possess the unique ability to undergo rapid (within a few milliseconds), nearly completely reversible, and significant changes in their strength (yield stress change from ∼0 to 100 kPa) upon application of an external magnetic field.1–3 Magnetorheological fluids typically consist of fine (∼1–10 m) particles of a magnetically soft material (e.g., iron, ceramic ferrites, etc.) dispersed in an organic medium such as silicone oil. Magnetorheological fluids have recently received renewed attention because the strength of MR fluids is considerably higher than that of electrorheological (ER) fluids. Furthermore, MR fluids derive their strength primarily from the ferro- or ferrimagnetic polarization and hence, their strength remains fairly constant with temperature. Although quite promising, certain technical challenges remain with most MR fluids reported in the literature. Magnetic particulates used in the most typical MR-fluid compositions reported in the earlier literature4–6 settle out over a period of time (a few hours to a few days). The settling behavior is not surprising given the high density (∼5.2 g/cc for ferrites, ∼7.8 g/cc for iron) of the particulates compared to most organic liquids. Most MR fluid compositions reported in the earlier literature also show poor redispersibility, i.e., once the particles settle out, J. Mater. Res., Vol. 14, No. 7, Jul 1999
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they form a very tightly bound network or “cake,” and it is extremely difficult to “remix” the MR fluids. Thus, the lack of redispersibility has been a serious problem. However, in the prior literature the u
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