Electrorheological and magnetorheological fluid scanning...

Details Electrorheological and magnetorheological fluid scanning... Electrorheological and magnetorheological fluid scanning...

2002-07-01

2003-07-29

Lefkowitz, Edward (Department: 2855)

Measuring and testing

Liquid analysis or analysis of the suspension of solids in a...

Viscosity

C324S071100, C073S054010, C073S054140

Reexamination Certificate

active

06598465

ABSTRACT:

SPECIFICATION
FIELD OF THE INVENTION
The invention pertains to electrorheological (ER) and magnetorheologocial (MR) fluid devices, and more particularly, to a device for changing the viscosity and yield stress of ER and MR fluids and for measuring the change in those parameters.
BACKGROUND OF INVENTION
An electrorheological (ER) fluid is typically a suspension of solid particles in dielectric carrier liquids that undergo a rapid and reversible viscosity transition upon the application of electric fields. This dramatic transition of viscosity is often referred to as the ER effect or sometimes the “Winslow effect,” after Willis Winslow (1949) who first reported the phenomenon. The ER effect has not been fully understood but can be described as follows: the external electric field induces electric polarization within each particle relative to the carrier liquid (an electric dipole), and the resulting electrostatic interaction forces between the particles lead to the formation of aggregates aligned in the direction of the field. The presence of these particles aggregates in the flow field causes an increase in the fluid viscosity and a decrease in flow rate. During the past two decades, the ER-related investigations have increased due to the potential applications of the special properties of the ER fluids for the performance improvement of devices such as engine mounts, clutches, brakes, and shock absorbers; for examples, see U.S. Pat. No. 5,088,703 (Takano et al.); U.S. Pat. No. 6,082,715 (Vandermolen); U.S. Pat. No. 5,988,336 (Wendt et al.); U.S. Pat. No. 5,358,084 (Schramm); and U.S. Pat. No. 5,322,484 (Reuter).
Numerous experiments show that ER-fluids are generally visco-plastic fluids. Various rheological models have been proposed (e.g., R. B. Bird, R. C. Amstrong and O. Hassager, “Dynamics of Polymeric Liquids”, Vol. 1,
Fluid Mechanics
, Wiley 1987), and the most often used model under shearing deformation is the Bingham plastic model (also referred to as “linear viscoplastic model”), where the shear stress is given by:
&tgr;=&tgr;
0
+&mgr;
B
{dot over (&ggr;)}  (1)
where {dot over (&ggr;)} is the shear rate, &mgr;
B
is the constant Bingham viscosity and &tgr;
0
is the yield stress induced by the electric field. However, it has been found that yielded ER-fluids may experience shear thinning, i.e., its viscosity decreases gradually with the increase of shear rate. This is probably because the destruction of the internal structure responsible for the yield behavior is a gradual process, during which the resistance to deformation becomes weaker, and is not completed until a high shear stress level is reached. Therefore, the Bingham plastic model may overestimate the true yield stress significantly due to the shear thinning at low shear rates. Wan 1982; O'Brien & Julien 1988.
A Herschel-Bulkley rheological model (also referred to as a non-linear viscoplastic model) seems to be more appropriate in depicting the ER-fluid behavior. This rheological model is empirical, nonetheless the results predicted using this model are often accurate over a wide range of shear rates and are reproducible. The key feature of this rheological model is that when the applied stress is smaller than the yield stress, there is no flow; the material supports a finite stress elastically without flow. For the Herschel-Bulkley model, the elastic strains are taken to be small such that the material is considered to be rigid. Once the applied stress exceeds the yield stress of the material, there is a transition from elastic to plastic behavior and the material behaves like a power-law fluid. This behavior can be interpreted to the microstructure of the fluid; for instance, in some ER fluids under a static/alternating electric field, it is found that electrostatic interactions between the dispersed particles lead to a chain-like structure, indicating a yield stress of the ER fluid. Substantial stresses may be required to break down this structure; the ER fluid will then flow. When the stresses are removed, the chain-like structure reforms.
In simple shear, the constitutive equations for the Herschel-Bulkley fluid are as follows:
{dot over (&ggr;)}=0←→&tgr;<&tgr;
0
{dot over (&ggr;)}>0←→&tgr;=&tgr;
0
+K{dot over (&ggr;)}
n
  (2)
where &tgr;
0
is a yield stress, K is a flow consistency, and n is a flow index ranging from 0 to 1 for shear thinning fluid. The upper limit where n=1 corresponds to a Bingham plastic fluid, and K becomes the regular dynamic constant viscosity. It has been shown that &tgr;
0
, i.e., the yield stress increases with the applied electricfield strength (E) as &tgr;
0
∝ E
60
, where &agr; assumes values close to 2 for low to moderate field strengths, but often appears to fall below 2 for higher E fields. In this rheological model, the yield stress, the fluid consistency, the flow index which are often referred to as the Herschel-Bulkley parameters should be determined from the measurement.
Much of the same discussion also applies to magnetorheological (MR) fluids except that magnetic fields (B) are applied to the MR field rather than a static/alternating electric fields. An MR fluid is typically a suspension of solid particles in diamagnetic liquids that undergo a rapid and reversible viscosity transition upon the application of magnetic fields. This dramatic transition of viscosity is often referred to as the MR effect. In addition, although it has been shown that yield stress increases with the applied magnetic field strength (B) as &tgr;
0
∝ B
&agr;
, the range for a is not necessarily close to 2 for low to moderate field strengths, or below 2 for higher field strengths, as is the case for ER fluids, as mentioned previously.
There exist several flow-measuring devices (i.e., rheometers) to measure the ER or MR properties. Those rheometers can be classified into three types: 1) capillary tube type, 2) rotating cylinder type, and 3) falling ball
eedle type.1-2 These rheometers produce ER/MR-property data (shear stress etc.,) at a shear rate at a time. Thus, in order to measure the ER/MR property over a range of shear rates, it is necessary to repeat the measurement by varying shear rates. In order to cover a range of shear rates, it is necessary to vary pressure, rotating speed, or the density of the falling object. Such operations make an ER/MR-property measurement system complicated and labor intensive. Therefore, there is a need to develop a new rheometer for ER and MR fluids that is simple and accurate.
In U.S. Pat. No. 6,019,735 (Kensey et al.), which is assigned to the same Assignee, namely Visco Technologies, Inc., of the present invention, there is disclosed a scanning-capillary-tube viscometer for measuring the viscosity of a fluid, e.g., circulating blood of a living being. One of the important features of the scanning-capillary viscometer is that both flow rate and pressure drop at a capillary tube can be determined by fluid level variation with time in a U-type tube system, with a only single fluid level variation measurement required for Newtonian fluids, and a range of fluid level variation measurements required for fluids. In particular, using the U-type tube structure, the fluid is exposed to a pressure differential that causes the fluid to move through the U-tube at a first shear rate. This movement of fluid causes the pressure differential to decrease, thereby subjecting the movement of the fluid to a plurality of shear rates, i.e., decreasing shear rates from the first shear rate.
However, the governing equation and apparatus for the ER/MR-property measurement system are quite different from the scanning-capillary-tube viscometer. Thus, the present invention is a combination of the scanning-capillary-tube viscometer with an ER/MR-property measurement system.
Conventional rheometers utilize moving parts that must be calibrated, tend to wear and eventually must be replaced (e.g., pressure transducers). In addition, many of these rheometers must have test runs repeated

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