Abstract:
A scanning rheometer is presented for the rheological property measurement of electrorheological (ER) and magnetorheological (MR) fluids using a non-linear viscoplastic model, based on the fluid height variation with respect to time. The rheometer basically includes a static (e.g., an overhead reservoir) or a dynamic source of fluid, a channel or slit whose sides form electrodes which are in contact with the flowing ER fluid, or a capillary tube exposed to a static/alternating magnetic field for flowing MR fluids, a transfer tube, either one or two riser tubes, and a column level detector for monitoring the column of fluid as it moves in one of the riser tubes. The column level detector is coupled to a processor which analyzes, among other things, column height vs. time data to determine both viscosity and yield stress. The rheometer overcomes one of the major drawbacks of the conventional rheometer: the inability to produce the yield stress of the ER, or MR, fluid in an absolute zero shear rate range. The results with this rheometer are compared with those obtained from a commercially-available rheometer which indicates excellent agreement.

Description:
RELATED APPLICATIONS  
       [0001]    This application is an Application based on a Provisional Application Serial No. 60/227,759 filed Aug. 25, 2000 entitled ELECTRORHEOLOGICAL FLUID SCANNING VISCOMETER, and is also a Continuation-in-Part of application Ser. No. 09/573,267, filed May 18, 2000 entitled DUAL RISER/SINGLE CAPILLARY VISCOMETER, and all of whose entire disclosures are incorporated by reference herein. 
     
    
     
       SPECIFICATION  
       FIELD OF THE INVENTION  
         [0002]    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  
         [0003]    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).  
           [0004]    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: 
           τ=τ 0 +μ B {dot over (γ)}  (1) 
           [0005]    where {dot over (γ)} is the shear rate, μ B  is the constant Bingham viscosity and τ 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&#39;Brien &amp; Julien 1988.  
           [0006]    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.  
           [0007]    In simple shear, the constitutive equations for the Herschel-Bulkley fluid are as follows: 
           {dot over (γ)}=0←→τ&lt;τ 0 {dot over (γ)}&gt;0←→τ=τ 0   +K{dot over (γ)}   n   (2) 
           [0008]    where τ 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 τ 0 , i.e., the yield stress increases with the applied electricfield strength (E) as τ 0 ∝ E 60  , where α 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.  
           [0009]    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 τ 0 ∝ B α , 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.  
           [0010]    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/needle 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.  
           [0011]    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.  
           [0012]    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.  
           [0013]    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 in order to cover a range of shear rates, thereby making their use not only cumbersome but expensive.  
           [0014]    Therefore, there remains a need for a rheometer that can measure the viscosity over range of shear rates, as well as the yield stress in an absolute zero shear rate range, of ER and MR fluids and which uses no moving parts, including pressure transducers. Furthermore, this rheometer must be simple to use, exhibit quick operation and be comparatively inexpensive.  
         SUMMARY OF THE INVENTION  
         [0015]    An apparatus for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The apparatus comprises: a fluid source elevated at a first reference position above a horizontal reference position; a flow restrictor (e.g., a slit or capillary tube) having a first end and a second end and wherein the first end is in fluid communication with the fluid source and wherein the flow restrictor has some known dimensions; a lumen (e.g., a transfer tube) having one end in fluid communication with the second end of the flow restrictor and another end that is exposed to atmospheric pressure and wherein the lumen has a portion (e.g., a riser tube) that is positioned at an angle greater than zero degrees with respect to the horizontal reference position, and wherein a pressure differential exists between a column of fluid in the portion and the elevated fluid source; and whereby the column of fluid moves through the flow restrictor and the lumen at a first shear rate caused by the pressure differential; and whereby the movement of fluid causes the pressure differential to decrease from the first shear rate for generating the plural shear rates; a sensor (e.g., a light array/charge coupled device) for detecting the movement of the column of fluid and wherein the sensor generates data relating to the movement of the column of fluid over time; an electric/magnetic field generator for subjecting said flow restrictor to an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field, etc.) when the fluid is flowing therein; and a processor, coupled to the sensor, for calculating the viscosity of the fluid over a range of plural shear rates based on the data relating to the movement of the column of fluid over time and the some known dimensions.  
           [0016]    In accordance with another aspect of the invention, another apparatus is provided for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The apparatus comprises: a fluid source elevated at a first reference position above a horizontal reference position; a valve in fluid communication with the fluid source via a first port for controlling a flow of fluid from the fluid source; a flow restrictor (e.g., a slit or capillary tube) having a first end and a second end wherein the first end is in fluid communication with a second port of the valve and wherein the flow restrictor has some known dimensions and is positioned at the horizontal reference position; a lumen (e.g., a riser tube) having one end in fluid communication with a third port of the valve and another end that is exposed to atmospheric pressure and wherein the lumen is positioned at an angle greater than zero degrees with respect to the horizontal reference position; a processor coupled to the valve for controlling the valve to permit the flow of fluid into the lumen to form a column of fluid therein whereby a pressure differential is formed between a level of the column of fluid and the flow restrictor; the processor is also arranged for operating the valve to isolate the lumen from the fluid source and for coupling the flow restrictor and the lumen together to generate a falling column of fluid in the lumen; and whereby the falling column of fluid moves through the lumen, through the valve and the flow restrictor at a first shear rate caused by the pressure differential and wherein the movement of fluid causes the pressure differential to decrease from the first shear rate for generating the plural shear rates; a sensor (e.g., a light array/charge coupled device) for detecting the movement of the falling column of fluid and wherein the sensor generates data relating to the movement of the falling column of fluid over time; an electric/magnetic field generator for subjecting the flow restrictor to an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field or an alternating magnetic field, etc.) when the fluid is flowing therein; and wherein the processor, also coupled to the sensor, calculates the viscosity of the fluid over a range of plural shear rates based on the data relating to the movement of the falling column of fluid over time and the some known dimensions.  
           [0017]    In accordance with another aspect of this invention, another apparatus is provided for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The apparatus comprises: a fluid source elevated at a first reference position above a horizontal reference position; a valve in fluid communication with the fluid source via a first port for controlling a flow of fluid from the fluid source, wherein the valve comprises a second port having a first lumen (e.g., a transfer tube) coupled thereto, and wherein the first lumen has a portion positioned at the horizontal reference position; a flow restrictor (e.g., a slit or capillary tube) having a first end and a second end, wherein the first end is in fluid communication with a third port of the valve and wherein the flow restrictor has some known dimensions; a second lumen (e.g., a riser tube) having one end in fluid communication with a said second end of the flow restrictor and another end that is exposed to atmospheric pressure, wherein the second lumen is positioned at an angle greater than zero degrees with respect to the horizontal reference position, and wherein the flow restrictor and the valve are located at a position below the horizontal reference position; a processor coupled to the valve for controlling the valve to permit the flow of fluid into the flow restrictor and the second lumen to form a column of fluid therein whereby a pressure differential is formed between a level of the column of fluid and the portion of the first lumen, wherein the processor is also arranged for operating the valve to isolate the flow restrictor and the second lumen from the fluid source and for coupling the flow restrictor and the second lumen to the first lumen to generate a falling column of fluid in the second lumen and the flow restrictor, wherein the falling column of fluid moving through the second lumen, through the flow restrictor, through the valve and through the first lumen at a first shear rate caused by the pressure differential, and wherein the movement of fluid causes the pressure differential to decrease from the first shear rate for generating the plural shear rates; a sensor (e.g., a light array/charge coupled device) for detecting the movement of the falling column of fluid, wherein the sensor generates data relating to the movement of the falling column of fluid over time; an electric/magnetic field generator for subjecting the flow restrictor to an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field) when the fluid is flowing therein; and the processor, also coupled to the sensor, for calculating the viscosity of the fluid over a range of plural shear rates based on the data relating to the movement of the falling column of fluid over time and the some known dimensions.  
           [0018]    In accordance with another aspect of this invention, another apparatus is provided for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The apparatus comprises:a fluid source elevated at a first reference position above a horizontal reference position; a valve in fluid communication with the fluid source via a first port for controlling a flow of fluid from the fluid source and wherein the valve further comprises a second port that is exposed to atmospheric pressure; a flow restrictor (e.g., a slit or capillary tube) having a first end, said flow restrictor having some known dimensions and being positioned at said horizontal reference position; a lumen (e.g., a riser tube) having one end in fluid communication with a third port of the valve and another end that is in fluid communication with the first end of the flow restrictor wherein the lumen is positioned at an angle greater than zero degrees with respect to the horizontal reference position; a processor coupled to the valve for controlling the valve to permit the flow of fluid into the lumen to form a column of fluid therein whereby a pressure differential is formed between a level of the column of fluid and the flow restrictor; the processor is also arranged for operating the valve to isolate the lumen from the fluid source and for coupling the lumen to the third port to generate a falling column of fluid in the lumen, wherein the falling column of fluid moves through the lumen and through the flow restrictor at a first shear rate caused by the pressure differential, and wherein the movement of fluid causes the pressure differential to decrease from the first shear rate for generating the plural shear rates; a sensor (e.g., a light array/charge coupled device) for detecting the movement of the falling column of fluid and wherein the sensor generates data relating to the movement of the falling column of fluid over time; an electric/magnetic field generator for subjecting said flow restrictor to an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field) when the fluid is flowing therein; and the processor, also coupled to the sensor, for calculating the viscosity of the fluid over a range of plural shear rates based on the data relating to the movement of the falling column of fluid over time and the some known dimensions.  
           [0019]    In accordance with another aspect of this invention, a method is set forth for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The method comprises the steps of: (a) elevating a fluid source above a horizontal reference position to establish a pressure differential between the fluid source and the horizontal reference position; (b) placing one end of a flow restrictor (e.g., a slit or capillary) in fluid communication with the fluid source and wherein the flow restrictor comprises some known parameters; (c) placing a second end of the flow restrictor in fluid communication with one end of a lumen (e.g., a riser tube) and wherein a second end of the lumen is exposed to atmospheric pressure; (d) positioning the lumen at angle greater than zero degrees with respect to the horizontal reference position;(e) allowing the fluid to flow from the fluid source through the flow restrictor and the lumen, thereby decreasing the pressure differential which causes the fluid to experience a plurality of shear rates; (f) applying an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field) to the flow restrictor as the fluid flows through the flow restrictor (g) detecting the movement of the fluid through the lumen over time to generate data relating to the movement of the fluid through the lumen; and (h) calculating the viscosity of the fluid over a range of shear rates based on the data and the some known parameters.  
           [0020]    In accordance with another aspect of this invention, another method is set forth for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The method comprises the steps of: (a) elevating a fluid source above a horizontal reference position and disposing the fluid source in fluid communication with a first port of a valve; (b) disposing one end of a lumen (e.g., a riser tube) in fluid communication with a second port of said valve with the other end of said lumen exposed to atmospheric pressure, said lumen being positioned at an angle greater than zero degrees; (c) disposing one end of a flow restrictor (e.g., a slit or capillary tube) in fluid communication with a third port of the valve at the horizontal reference position and wherein the flow restrictor comprises some known parameters; (d) operating the valve to couple the first port with the second port to generate a column of fluid of a predetermined length in the lumen, and wherein the column of fluid of predetermined length establishes a pressure differential between the column of fluid and the horizontal reference position; (e) operating the valve to decouple the second port from the first port and to couple the second port with the third port to generate a falling column of fluid in the lumen, thereby decreasing the pressure differential which causes the fluid to experience a plurality of shear rates; (f) applying an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field) to the flow restrictor as the fluid flows through the flow restrictor; (g) detecting the movement of the fluid through the lumen over time to generate data relating to the movement of the fluid through the lumen; and (h) calculating the viscosity of the fluid over a range of shear rates based on the data and the some known parameters.  
           [0021]    In accordance with another aspect of this invention, another method is set forth for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The method comprises the steps of: (a) elevating a fluid source above a horizontal reference position and disposing the fluid source in fluid communication with a first port of a valve; (b) disposing one end of a flow restrictor (e.g., a slit or capillary tube), having some known parameters, in fluid communication with a second port of the valve with the other end of the flow restrictor being in fluid communication with one end of a first lumen (e.g., a riser tube) and wherein the first lumen has another end exposed to atmospheric pressure, and wherein the first lumen is positioned at an angle greater than zero degrees with respect to the horizontal reference position; (c) positioning the flow restrictor and the valve below the horizontal reference position and coupling a third port of the valve with a second lumen (e.g., transfer tube) and wherein a portion of the second lumen is disposed at the horizontal reference position; (d) operating the valve to couple the first port with the second port to generate a column of fluid of a predetermined length in the flow restrictor and the first lumen and wherein the column of fluid of a predetermined length establishes a pressure differential between the column of fluid and the horizontal reference position; (e) operating the valve to decouple the second port from the first port and to couple the second port with the third port to generate a falling column of fluid in the flow restrictor and the first lumen, thereby decreasing the pressure differential which causes the fluid to experience a plurality of shear rates; (f) applying an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field) to the flow restrictor as the fluid flows through the flow restrictor; (g) detecting the movement of the fluid through the the first lumen over time to generate data relating to the movement of the fluid through said first lumen; and (h) calculating the viscosity of the fluid over a range of shear rates based on the data and the some known parameters.  
           [0022]    In accordance with another aspect of this invention, another method is set forth for determining the viscosity of a fluid (e.g., an electrorheological fluid, a magnetorheological fluid, etc.) over plural shear rates using a decreasing pressure differential. The method comprises the steps of: (a) elevating a fluid source above a horizontal reference position to establish a pressure differential between the fluid source and the horizontal reference position, and disposing the fluid source in fluid communication with a first port of a valve; (b) disposing one end of a lumen (e.g., a riser tube) in fluid communication with a second port of the valve with the other end of said lumen being in fluid communication with a flow restrictor (e.g., a slit or capillary tube) disposed at the horizontal reference position and wherein the flow resistor comprises some known parameters and wherein the lumen is positioned at an angle greater than zero degrees with respect to the horizontal reference position; (c) positioning a third port of the valve to be exposed to atmospheric pressure; (d) operating the valve to couple the first port with the second port to generate a column of fluid of a predetermined length in the lumen; (e) operating the valve to decouple the second port from the first port and to couple the second port with the third port to generate a falling column of fluid in the lumen, thereby decreasing the pressure differential which causes the fluid to experience a plurality of shear rates; (f) applying an electric/magnetic field (e.g., a static electric field, an alternating electric field, a static magnetic field, or an alternating magnetic field) to the flow restrictor as the fluid flows through the flow restrictor; (g) detecting the movement of the fluid through the lumen over time to generate data relating to the movement of the fluid through the lumen; and (h) calculating the viscosity of the fluid over a range of shear rates based on the data and the some known parameters. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a block diagram of the ER/MR fluid scanning rheometer apparatus.  
         [0024]    [0024]FIG. 2A is an enlarged isometric view of the slit used for applying a static/alternating electric field to an ER fluid passing therethrough and for applying a static/alternating magnetic field to an MR fluid passing therethrough, respectively;  
         [0025]    [0025]FIG. 2B is a Cartesian coordinate system as it applies to the slit;  
         [0026]    [0026]FIG. 2C is an enlarged isometric view of the slit used for passing an MR fluid and which is subjected to a static/alternating magnetic field having north and south poles that form the wall of the slit;  
         [0027]    [0027]FIG. 2D is an enlarged isometric view of the capillary tube for passing an MR fluid and which is subjected to a static/alternating magnetic field from an adjacent magnetic field applicator;  
         [0028]    [0028]FIG. 2E depicts alternative means of generating the magnetic field around the capillary tube that is carrying an MR fluid;  
         [0029]    FIGS.  2 F- 2 G depicts alternative means of generating an alternating electric/magnetic field around the capillary tube.  
         [0030]    [0030]FIG. 3A is a velocity profile of an ER fluid taken along line  3 A- 3 A of FIG. 2A;  
         [0031]    [0031]FIG. 3B is a shear stress profile an ER fluid taken along  3 A- 3 A of FIG. 2A;  
         [0032]    [0032]FIG. 4 is height vs. time plot based on the ER (or MR) fluid column level of the riser tube R 2  in the rheometer apparatus;  
         [0033]    [0033]FIG. 5 is a shear stress vs. shear rate plot of the ER (or MR) fluid column in the riser tube R 2 ;  
         [0034]    [0034]FIG. 6 is viscosity vs. shear rate plot of the ER (or MR) fluid column in the riser tube R 2 ;  
         [0035]    [0035]FIG. 7A is a height vs. time plot for a first test ER fluid (e.g., cornstarch-corn oil mixture) when a static/alternating electric field of 0 kV/mm was applied;  
         [0036]    [0036]FIG. 7B is a height vs. time plot for the first test ER fluid when a static electric field of 0.5 kV/mm was applied;  
         [0037]    [0037]FIG. 8 is a shear stress vs. shear rate curve of the first test ER fluid as determined by the present invention and by a conventional rotating viscometer (e.g., Haake VT-550);  
         [0038]    [0038]FIG. 9 are height vs. time plots for a second test ER fluid (e.g., zeolite-corn oil mixture) using electric fields of 0 kV/mm, 0.5 kV/mm and 1 kV/mm;  
         [0039]    [0039]FIG. 10 are shear stress vs. shear rate curves of the second test ER fluid using static electric fields of 0 kV/mm, 0.5 kV/mm and 1 kV/mm;  
         [0040]    [0040]FIG. 11 is a block diagram of the rheometer apparatus coupled to either a static or dynamic source of ER or MR fluid which utilizes a falling column of fluid for viscosity determination;  
         [0041]    [0041]FIG. 12 is a front view of one embodiment of the ER/MR fluid scanning rheometer apparatus, showing a display and a housing with its door in an opened condition which houses the column level detector;  
         [0042]    [0042]FIG. 12A is a height vs. time plot based on the rheometer of FIG. 12;  
         [0043]    [0043]FIG. 13A is an operational diagram of the first embodiment of the ER/MR fluid scanning rheometer with the flow restrictor forming a part of the transfer tube;  
         [0044]    [0044]FIG. 13B is an operational diagram of the first embodiment of the ER/MR fluid scanning rheometer with the flow restrictor forming a part of the riser tube;  
         [0045]    FIGS.  13 C- 13 D depict the valve mechanism operation during the test run of the first embodiment;  
         [0046]    [0046]FIG. 14A is an operational diagram of the second embodiment of the ER/MR fluid scanning rheometer where the valve mechanism is located at the top of the riser tube; and  
         [0047]    FIGS.  14 B- 14 C depict the valve mechanism operation during the test run of the second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    Referring now in detail to the various figures of the drawing wherein like reference characters refer to like parts, there is shown at  20  an ER/MR fluid scanning rheometer.  
         [0049]    In particular, the rheometer  20  comprises a constant-fluid level, overhead tank or reservoir  22  containing the ER fluid or the MR fluid  24 . This tank  22  is coupled to a riser tube R 1 . An electric/magnetic field generator  26  imposes an electric field, or a magnetic field, or both (depending on what type of fluid  24  is being analyzed) on a flow restrictor  28  of the riser tube R 1  that comprises either a slit or a capillary tube; suffice it to say for now that where the fluid  24  is an ER fluid that is being analyzed using the rheometer  20 , the restrictor  28  comprises a slit whereas if the fluid  24  being analyzed is an MR fluid, either a slit or a capillary tube is used for the restrictor  28 . A transfer tube  29  couples the riser tube R 1  to a valve  30  (e.g., on/off) which is used to control the flow of the fluid  24 . A column level detector  32  is used is to monitor the movement of the fluid  24  as it rises up through a second riser tube R 2 , that is vented to atmosphere. A processor  34  is coupled to the column level detector  32  (e.g., a video camera, a light array/CCD described below, etc.) for analyzing the height vs. time data (h(t)) from the column level detector  34 , along with the slit/capillary data, to determine the fluid viscosity and yield stress. The height of the fluid  24  column in R 2  (h(t)) is determined from a datum level and the constant level of the fluid  24  in the overhead reservoir  22  is known as h R . It should be noted that the riser tube R 2  can be positioned at any angle greater than zero degrees with respect to the horizontal reference position, e.g., datum level; in FIG. 1 this angle is 90°.  
         [0050]    In the preferred embodiment, the riser tube R 1 , flow restrictor  28 , transfer tube  29  and riser tube R 2  form a “U”-tube structure that is in an upright position; except for the generator  26 , this structure is similar to the structure disclosed in A. Ser. No. 09/439,735 and A. Ser. No. 09/573,267, both entitled DUAL RISER/SINGLE CAPILLARY VISCOMETER, both of which are assigned to the same Assignee, namely Visco Technologies, Inc. of the present invention and both of whose entire disclosures are incorporated by reference herein. Using this configuration, the test fluid  24  is subjected to a decreasing pressure differential that moves the test fluid  24  through a plurality of shear rates (i.e., from a high shear rate at the beginning of the test run to a low shear rate at the end of the test run), which is especially important in determining the viscosity of non-Newtonian fluids, as set forth in A. Ser. No. 09/439,735 and A. Ser. No. 09/573,267. In particular, because of the elevated position of the reservoir  22  and with the second riser R 2  exposed to atmospheric pressure, when the valve  30  is opened, the test fluid  24  flows through the riser tube R 1 , flow restrictor  28 , transfer tube  29  and up the riser tube R 2 . A pressure differential exists between the column of fluid in the riser tube R 2  and the elevated reservoir  22 . As the test fluid  24  flows up the riser tube R 2 , the movement of the test fluid  24  causes the pressure differential to decrease, thereby causing the test fluid  24  to slow down. This movement of the test fluid  24 , initially at a high shear rate and diminishing to a slower shear rate, thus covers a plurality of shear rates. However, it should be understood that it is within the broadest scope of this invention to include any other configurations where the test fluid  24  can be subjected to a decreasing pressure differential in order to move the test fluid through a plurality of shear rates.  
         [0051]    It should be understood that the term “electric/magnetic field” as used throughout this Specification implies an electric field, a magnetic field or both an electric field and a magnetic field together. Similarly, the term “electric/magnetic field generator” as used throughout this Specification implies a generator capable of generating an electric field, a magnetic field or both an electric field and a magnetic field together. Furthermore, as will be discussed in detail below, the term “electric/magnetic field” also implies that where an electric field is applied, the electric field may be either static or alternating, or where a magnetic field is applied, the magnetic field may be either static or alternating; where an electric field is applied simultaneously with a magnetic field, alternating electric and magnetic fields are implied.  
         [0052]    As mentioned earlier, the flow restrictor  28  of R 1  may comprise either a slit  28 A or a capillary tube  28 B. FIGS.  2 A- 2 C depict enlarged views of these configurations. In particular, FIGS.  2 A- 2 B depict the slit comprising a pair of walls  36 A and  36 B, whose inner surfaces  38 A/ 38 B are in direct contact with the fluid  24  during flow; in contrast, as shown in FIG. 2C, a capillary tube  28 B is used to confine the flow of MR fluid therein, with the walls  36 A and  36 B being adjacent the capillary tube  28 B. The use of a slit  28 A is necessary for subjecting ER fluids to a static/alternating electric field, i.e., because air is an insulator to electric current, it is necessary to have the ER fluid  24  make contact with the walls  36 A/ 36 B; however, since magnetic fields are capable of passing through air, direct contact with the MR fluid is unnecessary and thus either the slit  28 A (FIG. 2B) or the capillary tube  28 B (FIG. 2C) can be used for restrictor  28  for generating the north (N)-south (S) pole configuration. The electric field generator  26  may comprise any conventional DC voltage supply that can generate electric fields in the 10 kV/mm range; it should be understood that the electric field generator may also comprise any AC voltage supply where both the magnitude and frequency can be varied depending on the ER fluid under test. The magnetic field generator  26  may comprise any conventional magnetic field generators for generating magnetic fields in the range of 100-1000 Gauss range, including any of the configurations shown in FIGS.  2 D- 2 G; these coil configurations may be coupled to a function generator and amplifier that can generate an alternating electric/magnetic field where both the magnitude and frequency can be varied.  
         [0053]    Using the rheometer  20  described above, two exemplary ER fluids were analyzed for viscosity and yield stress and the results were compared with a conventional rotating-type viscometer, i.e., Haake VT-550 (FIGS. 8 and 10). The first ER fluid comprised a zeolite-corn oil mixture (40:60 by weight); in selecting various test suspensions, the zeolite and corn oil were chosen as dispersed particles and suspending medium, respectively. The mean diameters of the zeolite particles ranged from 5 μm to 30 μm. The volume concentration was fixed at 40% for the test. No surfactant was added in the test suspension. The second ER fluid comprised a cornstarch-corn oil mixture (15:85 by weight).  
         [0054]    The rheometer  20  comprised the following during the test run: the slit gap (G S ) and the longitudinal length (L S ) were 1.3 mm and 200 mm, respectively; the slit width (W S ) was 30 mm. The inside diameters of the transfer tube  29  and riser tubes R 1 /R 2  were 6 and 6.5 mm, respectively. The lengths of the transfer tube  29  and riser tubes R 1 /R 2  were 200 mm and 800 mm, respectively. The inside diameter and length of the transfer tube  29  and riser tubes R 1 /R 2  were chosen to ensure that the pressure drops in the tubes were significantly smaller than that in the slit  28 A; for example the diameter of the transfer tube  29  was 6 mm and the diameter of the riser tube R 2  was 6.5 mm. The riser tubes R 1 /R 2  comprised glass tubes, the valve mechanism  30  was an on/off valve, the column level detector  32  comprised a video camera, and the processor  34  was a computer data acquisition system. The value for h R  from the datum was 1525 mm.  
         [0055]    At the beginning of the test, with the on/off valve  30  closed, the ER fluid  24  was filled in the overhead reservoir  22 , the slit  28 A, and the transfer tube  29 . Next, the fluid level in the reservoir  22  was measured. At this time, a DC voltage was applied across the slit  28 A. Then, the on/off valve  30  was opened, and the ER fluid  24  began filling the riser tube R 2 . The walls  36 A/ 36 B of the slit  28 A comprised two copper plates (30 mm×200 mm), to which the DC voltage was applied. The column level detector  32  (e.g., a video camera) was used for measuring the fluid level by recording the fluid level with respect to time, h(t), in the riser tube R 2 . In order to minimize the reading error, the video camera  32  was linearly traversed along a linear guide (not shown) as the fluid level rose. The recorded images were magnified, and the fluid level was read using an image treatment tool (e.g., Paint Shop Pro™) to minimize reading errors. One could determine the actual height change in the rising tube with an accuracy of 0.2 mm.  
         [0056]    As the fluid level in the riser tube R 2  increased, the head difference between the reservoir  22  and the level of the ER fluid in riser tube R 2  continued to decrease. Accordingly, the rising speed of the fluid level in the riser tube R 2  gradually decreased as the fluid level in the riser tube R 2  approached that of the reservoir  22 , and the ER fluid eventually stopped rising. It took approximately five to ten minutes for the fluid level in the riser tube R 2  to reach a plateau value for the ER fluid. The time to complete a run varied depending on types of liquids and the applied electric field strength.  
         [0057]    In order to determine the Theological property of the ER fluid using the present system, it is necessary to know the pressure drop across the slit (ΔP SL ). However, what was measured in the present system was the total pressure drop across the entire system (ΔP total ). In other words, the total head difference between the reservoir  22  and the column level in the riser tube R 2 , h R −h(t), includes not only the pressure drop across the slit  28 A (ΔP SL ), but also the pressure drop across the transfer tube  29  (ΔP transfer ) and the riser tubes R 1 /R 2  (ΔP riser ) If a quasi-steady state is assumed during the test, the pressure drop across each tube (R 1 ,  29  and R 2 ) can be estimated based on the laminar flow theory of an incompressible fluid. Thus, the pressure drops occurring in both the transfer tube  29  and the riser tubes R 1 /R 2  should be subtracted from the total pressure drop. The pressure drop across the slit  28 A (ΔP SL ) can be described as: 
         Δ P   SL   =ΔP   total −(Δ P   transfer   +ΔP   riser )  (3) 
           ΔP   total   =ρg ( h   R   −h ( t ))  (4) 
         [0058]    where h R  is the constant fluid level in the overhead reservoir  22  and h(t) is an instantaneous fluid level in the riser tube R 2 ; ρ is the density of the ER fluid and “g” is the gravitational constant. It should be noted that the contribution from the second term on the right hand side in Eq. (1) is less than 0.5%. Hence the term can be neglected for all practical purpose.  
         [0059]    The fluid level in the riser tube R 2  is the only quantity to be measured. It should be noted that the height vs. time curve (FIG. 4) provides the data not only for the total head pressure (ΔP total ) but also for the fluid velocity (V riser ) in the riser tube R 2 . This fluid velocity, V riser , can be calculated from the gradient of the fluid level curve, h(t), as follows:  
               V   riser     =            h        (   t   )              t               (   5   )                               
 
         [0060]    From the fluid velocity in the riser tube R 2 , the velocity at the slit  28 A (V SL ) and volume flow rate are determined as shown below:  
               V   SL     =              h        (   t   )              t              A   riser       A   SL                 (   6   )               Q   =         A   SL          V   SL       =              h        (   t   )              t            A   riser                 (   7   )                               
 
         [0061]    where “A” represents the area of the indicated components.  
         [0062]    In determining the Herschel-Bulkley parameters, certain assumptions were made; FIG. 2B depicts the Cartesian coordinates referenced below; FIG. 3A depicts the flow velocity profile; and FIG. 3B depicts the shear stress profile of the flow. The assumptions made during the test were:  
         [0063]    1) a fully-developed, steady, isothermal, laminar flow;  
         [0064]    2) no velocity in the x and y directions (see FIG. 2A);  
         [0065]    3) no slip at the walls  36 A/ 36 B, V z =0 at y=±H, and;  
         [0066]    4) the fluid is incompressible with viscosity being independent of pressure.  
         [0067]    In order to determine the shear stress of the ER fluid, the pressure drop across the entire system is necessary. In the test, the wall shear stress, τ w , can be expressed with the pressure drop as follows:  
                 τ   w          (   t   )       =       Δ                   P   SL        H       L        (     1   +       2      H     W       )                 (   8   )                               
 
         [0068]    where H is the half of the slit gap (G S ), L is the slit length (L S ), W is the slit width (W S )  
         [0069]    It should be noted that ER fluids under the influence of a static/alternating electric field apparently exhibit yield stresses. In this test, the yield stress at a low (e.g., zero) shear rate can be determined from Equation (8) at the final hydrostatic equilibrium state. In other words, the yield stress of an ER fluid causes a fluid level difference between the level of the overhead reservoir  22  and the column level in the riser tube R 2  even at t=∞.  
               τ   0     =       Δ                     P   SL          (     t   =   ∞     )          H       L        (     1   +       2      H     W       )                 (   9   )                               
 
         [0070]    where ΔP SL  (t=∞) represents the pressure difference across the slit  28 A at the final equilibrium state.  
         [0071]    Meanwhile, the shear rate information for the Herschel-Bulkley fluid flowing in the slit  28 A is obtained from experimental data with a suitable mathematical treatment. For Bingham plastic or power-law fluids, the shear rates were determined from the flow rate and pressure drop quantities and can be obtained from any standard handbook. (e.g., R. B. Bird, R. C. Amstrong and O. Hassager, “Dynamics of Polymeric Liquids”, Vol. 1 , Fluid Mechanics,  Wiley 1987). For a Herschel-Bulkley fluid, it is necessary to derive equations with a similar procedure for Bingham plastic and power-law fluids.  
         [0072]    In the test, only a longitudinal shear flow was considered, hence there is only one non-zero velocity component. Also, the aspect ratio of the slit  28 A is 1:23 so that the flow can be assumed as one-dimensional flow; this velocity component is taken to be V Z (y) in the z-direction. Hence, the momentum flux distribution for flow of any kind of fluid through the slit  28 A is given by the following equation:  
               τ   yz     =         (       Δ                 P     L     )        y     =       τ   0     +     K                     γ   .     n                   (   10   )                               
 
         [0073]    Both distribution of velocity and shear stress for a Herschel-Bulkley fluid are shown in FIGS. 3A and 3B, respectively. Substituting the Herschel-Bulkley model, Equation 2 into Equation 10, then gives the following differential equation for the velocity:  
                      V   Z            y       =     -       (           Δ                 P     KL        y     -       τ   0     K       )       1   n                 (   11   )                               
 
         [0074]    The volume flow rate of the Herschel-Bulkley fluid flow in the slit  28 A may be calculated from:  
             Q   =     2      W          ∫   0   H              V   Z          (   y   )               y                   (   12   )                               
 
         [0075]    Integrating Equation 14 by parts and using the non-slip condition, the following is obtained:  
                   Q   =                  -   2        W        ∫       (            V   Z            y       )        y           y                       =                  -   2          W        [         ∫   0     h   p              (            V   Z            y       )        y           y         +       ∫     h   p     H            (            V   Z            y       )        y           y           ]                       (   13   )                               
 
         [0076]    where h p  represents the distance from the centerline of the plug flow region, as shown in FIGS.  3 A/ 3 B; the plug flow region is defined as that region where velocity is constant, (see PFR, FIG. 3A) and is due to the presence of yield stress. The first integral in Equation 13 becomes zero because dV Z /dy=0 for y≦h p , as shown in FIG. 3A. Hence, the volume flow rate of the flow is:  
             Q   =       -   2          W        [       ∫     h   p     H            (            V   Z            y       )        y           y         ]                 (   14   )                               
 
         [0077]    Now, the shear rate dV Z /dy is related with the volume flow rate, but it is in the integral. In order to obtain the shear rate, a mathematical treatment is required to replace the y-variable with τ. As shown in FIG. 3B, the shear stress profile can be described by the y-variable:  
             τ   =         τ   w     H        y             (   15   )                               
 
         [0078]    From the above Equation 15 gives dy=(H/τ w )dτ. Replacing the y-variable with τ using Equation 14 gives:  
                 Q                   τ   w   2         2      W                   H   2         =     -       ∫     τ   0       τ   w                     V   Z            y          τ           τ                   (   16   )                               
 
         [0079]    Now, substituting Equation 11 of the Herschel-Bulkley model into Equation 18, integrating and then re-arranging, yields the following flow rate of the Herschel-Bulkley fluid:  
             Q   =         2                 W                   H   2           2      n     +   1              (       τ   w     K     )       1   n              (     1   -       τ   0       τ   w         )       1   +     1   n              (     1   +       n     n   +   1              τ   0       τ   w           )               (   17   )                               
 
         [0080]    From the above Equation 17, the flow consistency, K, can be determined. Also, re-arranging Equation 17 provides the shear rate, {dot over (γ)}, as follows:  
                       γ   .     w     =         (         τ   w     -     τ   0       K     )       1   n       =                  Q     2      W                   H   2                  (     2   +     1   n       )          [       (     1   -   c     )          (     1   +       n     n   +   1          c       )       ]         -   1                       =                  1   3                  γ   .     aw          (     2   +     1   n       )            [       (     1   -   c     )          (     1   +       n     n   +   1          c       )       ]         -   1                       (   18   )                               
 
         [0081]    where {dot over (γ)} aw  is the apparent or Newtonian shear rate at the wall,  
                 γ   .     aw     =         3      Q       2        WH   2         =         3        A   riser         2        WH   2                     h        (   t   )              t                   (   19   )                               
 
         [0082]    and c is the distance ratio of the plug flow region to the wall from the centerline which can be defined as follows:  
             c   =         h   p     H     =         τ   0       τ   w       =       Δ                   h        (     t   =   ∞     )           Δ                   h        (   t   )                       (   20   )                               
 
         [0083]    where Δh(t) and Δh (t=∞), which is also referred to as Δh ∞ , are defined as shown in FIG. 4 and n is the power-law exponent which can be defined and determined as:  
             n   =              ln                   Δ                 P            ln                   Q       =              ln        (         H                 Δ                   h        (   t   )         -     Δ                   h   ∞           L        (     1   +       2      H     W       )         )                ln                     (              h        (   t   )              t            A   r       )         .               (   21   )                               
 
         [0084]    When c is zero, the shear rate for the Herschel-Bulkley model in Equation 18 reduces to that of the power-law model,  
           γ   .     w     =       1   3                γ   .     aw          (     2   +     1   n       )       .                             
 
         [0085]    Meanwhile, when n becomes 1, the shear rate for the Herschel-Bulkley model reduces to that of the Bingham plastic model,  
           γ   .     w     =           γ   .     aw       (     1   -     c   2       )       .                           
 
         [0086]    Thus, the shear rate can be determined from the shear stress at the same point (i.e., at the wall) in Equation 7. Therefore, the Herschel-Bulkley viscosity, η HB =K{dot over (γ)} w   n , can be directly related with the volume flow rate and pressure drop as follows:  
               η   HB     =           τ   w     -     τ   0           γ   .     w       =           2      ρ                 g2                   WH   3           L        (     1   +       2      H     W       )            A   riser                  (     1   -   c     )          (     1   +       n     n   +   1          c       )         (     2   +     1   n       )          Δ                   h        (   t   )         -       Δ                   h   ∞                h        (   t   )              t                     (   22   )                               
 
         [0087]    Meanwhile, the generalized Newtonian viscosity (η) of the Herschel-Bulkley fluid corresponding to the wall shear rate can also be determined from the measured quantity, h(t) as:  
             η   =         τ   w         γ   .     w       =         2      ρ                   g2WH                3             L        (     1   +       2      H     W       )            A   riser                  (     1   -   c     )          (     1   +       n     n   +   1          c       )         (     2   +     1   n       )                                      Δ                   h        (   t   )                    h        (   t   )              t                     (   23   )                               
 
         [0088]    Furthermore, based on Equation 9, the yield stress τ 0  is given by:  
               τ   0     =       ρ                 g                 Δ                   h   ∞        H       L        (     1   +       2      H     W       )                 (23A)                               
 
         [0089]    The above analysis is the same where a magnetorheological (MR fluid) is used along with the slit  28 A. In contrast, where a magnetorheological (MR) fluid is used, and instead of the slit  28 A, a capillary tube  28 B is used, the equations for the viscosity and shear stress are slightly modified.  
         [0090]    In particular, using the capillary tube  28 B having a radius (R) and utilizing a cylindrical coordinate system, assumptions no. 2 and no. 3 (mentioned previously) are:  
         [0091]    2) no velocity in the radial direction (r) and angular direction (θ); and  
         [0092]    3) no slip at the walls  36 A/ 36 B, V Z =0 at r=R.  
         [0093]    Furthermore, as with the ER fluids, since the fluid level in the riser tube R 2  is the only quantity that needs to be measured, the fluid velocity in the riser tube R 2  is also given by Equation (5). As a result, both the velocity and volume flow rate for the flow in the capillary tube  28 B is given by Equations (6) and (7), respectively, but with V SL  replaced with V C  and A SL  replaced with A C . Thus, the expression for wall shear stress, τ w , is expressed as:  
                 τ   w          (   t   )       =       Δ                   P   C        R       2      L               (   24   )                               
 
         [0094]    where L is the length of the capillary tube  28 B and ΔP C  represents the pressure drop across the capillary tube  28 B. Similarly, the yield stress at low (zero) shear rate can be determined from Equation (7) at the final hydrostatic equilibrium state where the yield stress of an MR fluid causes a fluid level difference between the reservoir  22  fluid level and the riser tube R 2  column level even at time t=∞ as:  
               τ   0     =       Δ                     P   C          (     t   =   ∞     )          R       2      L               (   25   )                               
 
         [0095]    where ΔP C (t=∞) represents the pressure difference across the capillary tube  28 B at the final equilibrium state.  
         [0096]    Meanwhile, the shear rate information for the Herschel-Bulkley fluid flowing in the capillary tube  28 B is obtained from experimental data with a suitable mathematical treatment. For Bingham plastic or power-law fluids, the shear rates were determined from the flow rate and pressure drop quantities and can be obtained from any standard handbook. (e.g., R. B. Bird, R. C. Amstrong and O. Hassager, “Dynamics of Polymeric Liquids”, Vol. 1,  Fluid Mechanics,  Wiley 1987). For a Herschel-Bulkley fluid, it is necessary to derive equations with a similar procedure for Bingham plastic and power-law fluids.  
         [0097]    In the test, only a longitudinal shear flow was considered, hence there is only one non-zero velocity component. Adopting cylindrical coordinates, the velocity, V Z (r) is defined in the z-direction. Hence, the momentum flux distribution for flow of any kind of fluid through the capillary tube  28 B is given by the following equation:  
               τ   z     =         (       Δ                   P   C         2      L       )        r     =       τ   0     +     K          γ   .     n                   (   26   )                               
 
         [0098]    Both distribution of velocity and shear stress for a Herschel-Bulkley fluid are shown in FIGS. 3A and 3B, respectively, but with the term “y” replaced with the “r”. Substituting the Herschel-Bulkley model, Equation 2 into Equation 10, then gives the following differential equation for the velocity:  
                      V   Z            r       =     -       (           Δ                 P       2      KL          r     -       τ   0     K       )       1   n                 (   27   )                               
 
         [0099]    The volume flow rate of the Herschel-Bulkley fluid flow in the capillary tube  28 B may be calculated from:  
             Q   =     2      π          ∫   0   R            V   Z            γ   .          (   r   )                     (   28   )                               
 
         [0100]    Integrating Equation 28 by parts and using the non-slip condition, the following is obtained:  
             Q   =     2        π        [         ∫   0     r   0              V   z        r                      r         +       ∫     r   0     R            V   z        r                      r           ]                 (   29   )                               
 
         [0101]    The first integral in Equation (29) becomes zero when dV Z /dr=0 for r≦r 0  where r 0  replaces h p  in FIGS.  3 A- 3 B which represents the radial distance from the centerline of the plug flow region. Hence, the volume flow rate of the flow is:  
             Q   =     -     π        [       ∫     r   0     R            (            V   Z            r       )          r   2                        r         ]                 (   30   )                               
 
         [0102]    Now, the shear rate dV Z /dr is related with the volume flow rate, but it is in the integral. In order to obtain the shear rate, a mathematical treatment is required to replace the r-variable with τ. As shown in FIG. 3B, the shear stress profile can be described by the r-variable:  
             τ   =         τ   w     R        r             (   31   )                               
 
         [0103]    From the above Equation 31 gives dr=(R/τ w )dτ. Replacing the r-variable with τ using Equation 27 gives:  
                 Q                   τ   w   3         π                   R   3         =     -       ∫     τ   0       τ   w                     V   Z            r            τ   2             τ                   (   32   )                               
 
         [0104]    Now, substituting Equation 27 of the Herschel-Bulkley model into Equation 32, integrating and then re-arranging, yields the following flow rate of the Herschel-Bulkley fluid:  
               Q   =         2      n                 π                   R   3         n   +   1              (       τ   w     K     )       1   n              (     1   -   c     )       1   +     1   n              (       1   2     -       n       2      n     +   1            (     1   -   c     )       +           n   2          (     1   -   c     )       2         (       2      n     +   1     )          (       3      n     +   1     )           )              
              where                   c     =         τ   0       τ   w       =         Δ                   h   ∞         Δ                   h        (   t   )                           and  where                           
          β   =         (     1   -   c     )       1   +     1   n              (       1   2     -       n       2      n     +   1            (     1   -   c     )       +           n   2          (     1   -   c     )       2         (       2      n     +   1     )          (       3      n     +   1     )           )                 (   33   )                               
 
         [0105]    From the above Equation 33, the flow consistency, K, can be determined. Also, re-arranging Equation 33 provides the shear rate, {dot over (γ)}, as follows:  
                       γ   .     w     =                    (         τ   w     -     τ   0       K     )       1   n       =       1   4          (       4      Q       π                   R   3         )          (     1   +     1   n       )          1   β                     =                  1   4              γ   .     aw          (     1   +     1   n       )            1   β                     (   34   )                               
 
         [0106]    where {dot over (γ)} aw  is the apparent or Newtonian shear rate at the wall,  
                 γ   .     w     =         4      Q       π                   R   3         =         4        A   riser         π                   R   3                     h        (   t   )              t                   (   35   )                               
 
         [0107]    and c is the distance ratio of the plug flow region to the wall from the centerline which can be defined as follows:  
             c   =         r   0     R     =         τ   0       τ   w       =       Δ                   h        (     t   =   ∞     )           Δ                   h        (   t   )                       (   36   )                               
 
         [0108]    and n is the power-law exponent which can be defined and determined as:  
             n   =              ln                   Δ                 P            ln                   Q       =            ln                     (       ρ                 g                   R        (       Δ                   h        (   t   )         -     Δ                   h   ∞         )           2      L       )              ln        (              h        (   t   )              t            A   riser       )                     (   37   )                               
 
         [0109]    When c is zero, the shear rate for the Herschel-Bulkley model in Equation 34 reduces to that of the power-law model,  
           γ   .     w     =       1   4                γ   .     aw          (     3   +     1   n       )       .                             
 
         [0110]    Meanwhile, when n becomes 1, the shear rate for the Herschel-Bulkley model reduces to that of the Bingham plastic model,  
                 γ   .     w     =         γ   .     aw          3       (     1   -   c     )          [         (     1   -   c     )     2     +   2     ]                   (   38   )                               
 
         [0111]    Thus, the shear rate can be determined from the shear stress at the same point (i.e., at the wall) in Equation 7. Therefore, the Herschel-Bulkley viscosity, η HB =K{dot over (γ)} w   n , can be directly related with the volume flow rate and pressure drop as follows:  
               η   HB     =           τ   w     -     τ   0           γ   .     W       =         2      ρ                 g                   R   4         2                   LA   riser                n                 β       (     n   +   1     )            (         Δ                   h        (   t   )         -     Δ                   h   ∞                  h        (   t   )              t         )                 (   39   )                               
 
         [0112]    Meanwhile, the generalized Newtonian viscosity (η) of the Herschel-Bulkley fluid corresponding to the wall shear rate can also be determined from the measured quantity, h(t) as:  
             η   =         τ   w         γ   .     w       =         π                 ρ                 g                   R   4         2      L                   A   riser                  n                 β       (     n   +   1     )       [       Δ                   h        (   t   )                  h        (   t   )              t         ]                 (   40   )                               
 
         [0113]    Furthermore, based on Equation 25, the yield stress τ 0  is given by:  
               τ   0     =       ρ                 g                 Δ                   h   ∞        R       2      L               (   41   )                               
 
         [0114]    [0114]FIG. 4 depicts the fluid level variation, h(t), in the riser tube R 2 . FIG. 5 depicts the shear stress vs. shear rate characteristic and FIG. 6 depicts the fluid viscosity vs. shear rate characteristic.  
         [0115]    As mentioned earlier, two ER fluids were analyzed: a cornstarch-corn oil mixture (15:85 by weight) and a zeolite-corn oil mixture (40:60 by weight).  
         [0116]    [0116]FIG. 7A depicts the fluid level variation, h(t), in the riser tube R 2  obtained with the cornstarch-corn oil mixture at room temperature with E=0 kV/mm and 0.5 kV/mm. As time passed beyond 200 seconds, the fluid level in the riser tube asymptotically reached a plateau value (h ∞ ) which is the same as the fluid level, h R , in the reservoir  22 . In particular example, the height at t=176 seconds was 605 mm in FIG. 7A, whereas the height at t=216 seconds was 609 mm (h ∞ ). This corresponds to h R  which was also 609 mm. Next, applying a static/alternating electric field of 0.5 kV/mm, as shown in FIG. 7B, the plateau value, h ∞ , was determined to be much smaller than h R , a phenomenon which can be attributed to the yield stresses (τ 0 ) of the ER fluid. This phenomenon can be explained as follows: when an E field is applied to the ER fluid, it causes a transition of the ER fluid from a liquid state to a solid state at low shear rates, resulting in the yield stress of the ER fluid, and also resulting in a hydrostatic equilibrium even for non-zero pressure head difference between the reservoir  22  level and the riser tube R 2  column. FIG. 8 shows the flow curve for the cornstarch-corn oil mixture without the influence of a static/alternating electric field. The ER fluid exhibited a Newtonian behavior as shown in FIG. 8. Moreover, the results obtained from the rheometer  20  showed good agreement with the conventional rotating viscometer&#39;s (e.g., Haake VT-550) result in the shear rate range. It should be noted that the rheometer  20  provides viscosity data in the low shear rate range as compared to the rotating viscometer.  
         [0117]    [0117]FIG. 9 depicts the fluid level variation, h(t), in the riser tube R 2  obtained with the zeolite-corn oil mixture at room temperature with varying electric field (E) magnitudes. For E=0 kV/mm, as time passed beyond 800 seconds, the fluid level reached a plateau level, h ∞ , asymptotically. In particular, the height at t=800 seconds, was 1475 mm in FIG. 9, whereas the height at t=8000 seconds (not shown in FIG. 9 ) was 1480 mm. As shown in FIG. 9, at the end of the test run (t=∞), there remained a significant difference (Δh ∞ ) between the initial fluid level, h R , in the reservoir  22  and the final level of the column in the riser tube R 2 . As mentioned earlier with the cornstarch-corn oil mixture, this difference can be attributed to the yield stress (τ 0 ) of the zeolite-corn mixture. Moreover, the rheometer&#39;s 20 test results demonstrated excellent agreement with those from the conventional rotating viscometer (Haake VT-550) over a range of shear rates (e.g., 10 1 ˜10 3  s −1 ), including low shear rates.  
         [0118]    It should be understood that the position of the flow restrictor  28  is not limited to the riser R 1  but could be located as part of the transfer tube  29 , or even located in the riser tube R 2 .  
         [0119]    In light of the above, the rheometer  20  can be used to determine the viscosity over a range of shear rates as well used to determine the yield stress of a variety of different fluids in an absolute zero rate range. FIG. 11 is a block diagram of the rheometer  120  of the present invention that can be coupled to either a static fluid source (e.g., the reservoir  22  of the rheometer  20  having a test fluid deposited therein) or a dynamic fluid source (e.g., the vascular system of a living being). For example, the yield stress of the circulating blood of a living being can be analyzed using the rheometer  120 . Furthermore, unlike the rheometer  20 , the rheometer  120  uses a falling column of fluid for the viscosity determination. However, the operation of the decreasing pressure differential is the same.  
         [0120]    In particular, the rheometer  120  comprises a fluid receptor  122  and an analyzer  124 . An output section  126  can be coupled to the analyzer  124  for providing the results to other peripheral devices (e.g., computers, plotters, printers, etc.) whether they are local or remote. Furthermore, where the fluid source  10  is dynamic (e.g., the vascular system of a living being), a fluid conveyor  127  (e.g., a needle, a catheter, etc.) couples the fluid source  10  to the fluid receptor  122 .  
         [0121]    The fluid receptor  122  comprises a valve mechanism  128 , a riser tube R, the electric field generator, or magnetic field generator,  26 , the flow restrictor  28  (comprising either the slit  28 A or the capillary tube  28 B), the transfer tube  29  and a fluid collector  130 . In this configuration, it should be noted that the flow restrictor  28  forms a portion of the transfer tube  29  rather than forming a portion of the riser tube R as shown earlier. Furthermore, it should be noted that the riser tube R can be positioned at any angle greater than zero degrees with respect to the horizontal reference position, e.g., h ∞ ; in FIG. 13A, this angle is 90°.  
         [0122]    As mentioned earlier, the electric field generator  26  may comprise any power supply capable of generating E fields in the 10 kV/mm range and that the magnetic field generator  26  may comprise any conventional magnetic field generators for generating magnetic fields in the range of 100-1000 Gauss, including any of the configurations shown in FIGS.  2 D- 2 G;  
         [0123]    these coil configurations may be coupled to a function generator and amplifier that can generate an alternating electric/magnetic field where both the magnitude and frequency can be varied.  
         [0124]    The fluid collector  130  comprises any receptacle for collecting that fluid that exits the riser tube R after the test run. This collector  130  may be disposable, as is the valve mechanism  128 , the flow restrictor  28 , the transfer tube  29  and the riser tube R where the fluid under test is a bio-fluid (e.g., blood).  
         [0125]    The fluid collector  130  as shown most clearly in FIG. 13A comprises an inner circular wall  135  that divides the collector  130  into a central portion  131  and an annular portion  139 . The central portion  131  receives the far end, or outlet,  133  of the transfer tube  29 , which, during the test run, remains submerged under the fluid level to minimize any surface tension effects. As the fluid fills the collector  130 , the fluid  24  can spill over the top of the inner circular wall  135  while maintaining the outlet  133  of the transfer tube  29  submerged.  
         [0126]    The analyzer  124  comprises the processor  34 , a column level detector  132 , a display  134 , a bar code reader  136 , an environmental control unit  138 , and a first battery B 1  and a second back-up battery B 2 . The column level detector  132  monitors the level of blood in the riser tube R. The processor  34  (e.g., a “386” microprocessor or greater, or any equivalent) is arranged to analyze the data from the detector  132  and calculate the viscosity and yield stress therefrom. Furthermore, the processor  34  also controls the display  134  for providing the viscosity/yield stress information and the other information to the operator as well as to the output section  126 . The processor  34  also controls the valve mechanism  128  based on the data from the detector  132 , as will be discussed later. Battery B 1  provides all of the requisite power to the analyzer  124 , with battery B 2  serving as a back-up power supply. It should be understood that power for the electric field generator, or the magnetic field generator,  26  is not supplied from the batteries B 1 /B 2 , but requires an external source. The bar code reader  136  provides an automated manner in which the details of the flow restrictor  28 /riser tube R can be automatically fed to the processor  34  for viscosity/yield stress analysis. The environmental control unit  138  (e.g., a heater, fan and/or thermostat) can be used where the fluid under test is a temperature-dependent fluid (e.g., circulating blood of a living being) and the fluid needs to be maintained at the living being&#39;s body temperature throughout the test run.  
         [0127]    As shown more clearly in FIG. 12, a first embodiment of the rheometer  120  comprises a fluid receptor housing  140  having a door  142 . The housing  140  contains the riser tube R, the detector  132 , the valve mechanism  128 , the flow restrictor  28 , the collector  130 , the electric (or magnetic) field generator  26 , the bar code reader  136  and the environmental control unit  138 . The door  142  permits the operator to gain access to the fluid receptor components, especially in those scenarios where the components are disposable. For example a bracket  147  may be used to releasably secure the upper portion of the riser tube R. The column level detector  132  is preferably not removable from the housing  140 . Once the components are inserted, the rheometer  120  is ready for testing, and the door  142  is closed to provide a dark environment for the detector  132 . The detector  132  may comprise any conventional level detector, e.g., an LED (light emitting diode) array  64  and a CCD (charge coupled device)  66  located on opposite sides of the riser tube R, as discussed in A Ser. No. 09/439,795, which is incorporated by reference herein and therefore will not be repeated here.  
         [0128]    It should be understood that, although not shown, an electric/magnetic (EMF) shield surrounds the generator  26 / flow restrictor  28  to shield the detector  132 , as well as the analyzer  124 , from the effects of the electric or magnetic field during activation.  
         [0129]    The display  134  may comprise any suitable conventional device, e.g., an ELD (electroluminescent display) or LCD (liquid crystal display) that permits the visualization of both text and graphics. The resolution of this display  28  is preferably 800×600 VGA or above. Furthermore, while the preferred embodiment utilizes a touch screen display which incorporates, among other things:  
         [0130]    graphical display  146   
         [0131]    instruction, and/or data, display  148  (which also includes the command line display shown as “RUN TEST”; e.g., “TESTING”, “TEST IN PROGRESS,” etc.)  
         [0132]    alphanumeric keypad  150   
         [0133]    emergency stop button  152   
         [0134]    battery status indicators,  154 A and  154 B  
         [0135]    function buttons  156 .  
         [0136]    It should be understood that any equivalent display device is within the broadest scope of the invention. Thus, any number of user interfaces and buttons may be available through the display  134 . Therefore, the rheometer  120  is not limited to the embodiment that is shown in FIG. 12. Moreover, the display  134  can be operated to minimize or maximize, or overlay any particular graphic or text screen, as is available in any conventional object-oriented operating system, such as Microsoft® WINDOWS. Furthermore, the processor  34  may be located in the same housing as the display  134 . A wire harness  137  electrically couples the display  134 /processor  34  to the detector  132  and valve mechanism  128 .  
         [0137]    [0137]FIGS. 13A and 13B provide enlarged views of the rheometer  120  operation but with the flow restrictor  28  located in different fluid receptor  122  components. In particular, in FIG. 13A, the flow restrictor  28  forms a portion of the transfer tube  29  whereas in FIG. 13B, the flow restrictor  28  forms a portion of the riser tube R. In either case, operation of the rheometer  120  is similar.  
         [0138]    FIGS.  13 C- 13 D provide the sequence of the valve mechanism  128  operation as controlled by the processor  34 . In particular, the valve mechanism  128  may comprise a stop cock valve  158  and a valve driver  160  (e.g., 500 mA solenoid, or step motor, etc.) such as that disclosed in A Ser. No. 09/439,795, which is incorporated by reference herein. The fluid conveyor  26  is coupled to the valve mechanism  128  at a port  153 ; the flow restrictor  28  is coupled to the valve mechanism  128  at a port  155 ; and the riser tube R is coupled to the valve mechanism  128  at a port  157 . When the rheometer  120  is coupled to the fluid source  10  via the fluid conveyor  26 , the processor  34  commands the valve driver  160  to rotate the valve  158  such that fluid flow is upward from the fluid conveyor  26  into the riser tube R (FIG. 13A). The detector  132  monitors the rise of the column level in the riser tube R. When a predetermined column level is detected, the detector  132  informs the processor  34  which commands the valve driver  160  to rotate the valve  158  to the position shown in FIG. 13B. As the fluid flows down the riser tube R, the processor  34  then energizes the electric field, or magnetic field, generator  26  to alter the fluid viscosity. The detector  132  monitors the falling column of fluid as it flows downward and through the flow restrictor  28 .  
         [0139]    [0139]FIG. 14A depicts an enlarged view of the rheometer  120  operation but with the valve mechanism  128  located at the top of riser tube R. The advantage of this valve mechanism  128  position is that there is no need to first fill the riser tube R to a predetermined level before proceeding with the test run; instead, in accordance with the valve mechanism  128  operation as shown in FIGS.  14 B- 14 C, the test run proceeds with the processor  34  commanding the valve driver  160  to rotate the valve  158  to the position shown in FIG. 14B and then the processor  34  stops any more input flow from the fluid conveyor  26  as shown in FIG. 14C. In particular, as used in this embodiment, the fluid conveyor  26  is coupled to the valve mechanism  128  at a port  163 ; the top end of the riser tube R is coupled to the valve mechanism  128  at a port  165 . The valve mechanism  128  also includes a vent coupler  162  that couples the top of the riser R to third port  164  that is exposed to atmospheric pressure; thus when the valve  158  is rotated into the position shown in FIG. 14C, the fluid in the riser tube R will flow downwards.  
         [0140]    The viscosity determination and the yield stress determination using the rheometer  120  utilize the same mathematical principles discussed earlier for the rheometer  20  and therefore will not be repeated here. Thus, the viscosity and yield stress profiles (FIGS.  5 - 6 ) would be the same for the rheometer  120 . The only difference is that instead of using a rising column of fluid as does the rheometer  20 , the rheometer  120  uses a falling column. Therefore, it is within the broadest scope of this invention to include the use of either a rising or a falling column of fluid.  
         [0141]    As a result of using a falling column, the definition of Δh(t) and Δh ∞  are defined as shown in FIG. 12A, where h ∞  is defined as the centerline of the flow restrictor  28 , or as the top level of the central portion  131  in the fluid collector  130 .  
         [0142]    Without further elaboration, the foregoing will so fully illustrate our invention and others may, by applying current or future knowledge, readily adapt the same for use under various conditions of service.