Abstract:
An apparatus and method for measuring or comparing rheological properties of fluid samples in parallel is disclosed. The apparatus includes a plurality of sensing elements which are comprised of flow channels and reservoirs in fluid communication. The channels provide flow paths for the fluid samples which are initially contained within external reservoirs. The method includes flowing the fluid samples at variable rates and monitoring simultaneously sample flow rates from the reservoirs through a plurality elements for one or more increments of time. The disclosed method is capable of measuring or comparing rheological properties of at least two fluid samples simultaneously. Useful flow rates monitoring devices include optical array sensors and image analysis systems.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a device and technique for measuring or comparing rheological properties and viscosity of multiple samples simultaneously for rapidly screening, characterizing and comparing a library of material samples. 
         [0003]    2. Discussion 
         [0004]    Combinatorial chemistry generally refers to methods and materials for creating collections of diverse materials or compounds or mixtures commonly known as libraries. Additionally, it refers to techniques and instruments for evaluating or screening libraries for desirable properties. Combinatorial chemistry has revolutionized the process of formulating of mixtures and has enabled researchers to rapidly discover and optimize useful mixtures of materials. 
         [0005]    One useful screening criterion is a liquids flow properties. Viscosity is one such property which is a physical property that characterizes a fluid&#39;s resistance to flow. For laminar flow of Newtonian fluids, including gases and simple liquids, viscosity is proportional to the tangential component of stress divided by the local velocity gradient. Complex fluids such as pastes, slurries, and polymer solutions do not follow a constant relationship between tangential stress and local velocity gradient. For them rate dependent viscosity, its analogs and other rheological measurements can serve as useful screening criteria. 
         [0006]    Rheology is the study of the deformation and flow of fluids under the influence of an applied stress. Stress includes, for example, a shear stress, compressive stress, and extensional stress. The experimental characterization of a material&#39;s Theological behaviour is known as rheometry, although the term rheology is frequently used synonomously with rheometry. Experimentalists often refer to the measurement of rheologic properties as quantification of the movement of flowable materials in response to applied stresses. A stress is a force applied over an area. 
         [0007]    Combinatorial libraries routinely comprise thousands of individual library members. As a result, most viscometers are unsuitable for screening purposes because they were designed to slowly process one sample at a time. Although generally the throughput of serial measurement techniques can benefit from automation, many viscometers have relatively long response times. These instruments often require time-consuming sample preparation making such them impractical for use as screening tools. 
         [0008]    U.S. Pat. No. 6,393,898 teaches simultaneously measuring viscosity of multiple samples by timing the flow of each through a sample tube of known dimensions. While this has utility it does not allow the measurement of time dependent rheological properties. It does not allow the acquisition of rheological data at two or more levels of applied stress or the testing of a property multiple times during withdrawal from a sample reservoir. This teaching does not allow visualization and quantification of complex flow patterns. 
         [0009]    The present invention overcomes these problems noted above. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a method and an apparatus for measuring rheology of fluid samples in parallel using multiple elements. In some embodiments, the element apparatus includes a plurality of flow circuits each in fluid communication with a reservoir. The flow circuits include a tube and a cavity. Each tube has a length and an inner diameter. In addition, the tube provides a flow path for the fluid sample from the reservoir. The apparatus also includes a mechanism for applying a pressure differential to each element to cause the liquid to flow, and a device for monitoring flow of the fluid sample in the element. 
         [0011]    The disclosed apparatus is capable of measuring viscosity or rheology characteristics of at least two fluid samples simultaneously. A rheological characteristic is an observable or measurable response to stress. It includes, but is not limited to a flow pattern, a rate of flow, a rate for filling a cavity, a rate of filling of a circuit, a time to move a sample volume, a rate of change of filling processes, and a rate of change of a flow process. 
         [0012]    The present invention includes an apparatus comprised of an array of flow elements for measuring viscosity or rheological properties of fluid samples in parallel. Each of the flow elements includes a cavity where the filling may be sensed optically or with electromagnetic radiation by a detector. A single detector monitors all cavities simultaneously. The detector and its image grabbing and analysis hardware and software monitors filling in each cavity. The fill and rate of fill of a cavity are two measurements used to determine rheological characteristics of the liquids. 
         [0013]    The method and apparatus optically samples (observes) filling of the cavity flow element at a high sampling rate allowing measurement of rheological properties many times during the short time required to remove a liquid sample from its reservoir. The high rate optical sampling of the flow in the cavity allows the measurement or comparison of time dependent rheological properties or characteristics. 
         [0014]    When the applied pressure force is varied during the duration measurement time period, the high rate optical sampling of the flow in the cavity allows the rheological measurement or comparison at differing stress levels. 
         [0015]    The present invention includes a method of screening fluid samples for differences in rheology. The method comprises (1) providing fluid samples to a plurality of reservoirs; (2) allowing the fluid samples to flow from the reservoirs through a plurality flow elements; and (3) detecting differences in the volumetric flow rates of at least two fluids simultaneously. The fill and rate of fill of a cavity are used to compare rheological characteristics of the fluids. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic of a two element parallel viscometer apparatus that can measure or compare viscosity of two samples simultaneously. 
           [0017]      FIG. 2  shows a top view of a two element flow assembly. 
           [0018]      FIG. 3  shows a cross sectional view of a two element flow assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Overview of a Parallel Element Viscometer 
       [0019]    A parallel multi-element viscometer made in accordance with the present invention generally includes two or more flow circuit elements. The flow circuits can be constructed of any material or combinations of materials including, but not limited to, metal, glass and plastic. Each of the flow circuits has a resistance to flow over its length, a rheology or viscosity measurement region, and an optical sensing region. Typically, these circuits are identical in element. They commonly include a tube and a cavity. 
         [0020]    One can vary the inner dimensions of individual circuit components to tune the element set design for a particular range of viscosities or rheological characteristics. In addition, the inner dimensions of the circuits may assume any value necessary to product the desired flow response. The flow may be laminar or turbulent flow if a relative measurement of a flow property is desired. From a practical standpoint, wetted volumes are minimized to allow measurements using a small sample volume. This is often the case when screening combinatorial libraries because the amount of any particular sample or library member can be as small as about 0.1 milliliters. 
         [0021]    The multi-element viscometer includes reservoirs for holding the liquid samples prior to their introduction into the flow circuits. The reservoirs should be chemically inert with respect to the fluid samples. As noted below, it is often desirable to monitor the volumetric flow rate through the tubes by detecting changes in sample volume flowing from the reservoir. Since optical techniques are well suited for this task, flow circuits include a cavity with see-through wall which allows a portion of the circuits to be monitored optically. This is referred to as the detection cavity or cavity. 
         [0022]    The parallel viscometer also includes an optical imaging capturing device for monitoring the degree and rate of fill of the detection cavity. The volumetric flow rate of the samples flowing through the circuits may be calculated from the images. Once the volumetric flow rate is known, one may calculate the viscosity of the samples as the fluid viscosity is related to the volumetric flow rate and the pressure drop across the viscosity measurement region of an individual element. The parallel element viscometer includes a mechanism for applying and monitoring a pressure differential that drives the liquid samples through the elements. Typically, the parallel viscometer employs a pressure reservoir to supply the pressure differential driving the liquid through the elements 
         [0023]    Useful devices for monitoring the volumetric flow rate include in general electromagnetic radiation imaging sensors that monitor the flow elements simultaneously. The sensor may comprise a light source and an image grabber device, which generates a digital image of the degree of fill of a cavity in each element as a function of time. If a simple tube is connected to the sensing cavity, the time interval for a known volume of sample to pass through the viscosity measurement region (the tube) allows calculation of viscosity. The parallel viscometer typically uses an image data acquisition device in tandem with a computer and necessary software to record the sensor output. The data acquired is used to determine flow rate as a function of time. In its very simplest form the frame grabber device is a digital camera which is used to take multiple photos of the flow progressing through the cavity portion of the element with a see through wall. If the flow channels in each element have the same flow resistance, comparisons of the images of the first and the second elements allows determination of a difference in apparent viscosity between the two fluids. 
         [0024]    When a simple tube is connected to the optical cavity, the apparent viscosity is calculated from the tube diameter, the flow rate, and the pressure differential from the inlet to the outlet of the tube. 
         [0025]    When complex or non-Newtonian fluids are tested, rheological similarities or differences may be determined by comparing the degree of fill of the cavities, rate of fill of cavities or flow patterns as a function of time. 
       Details of a Two Element Parallel Rheometer 
       [0026]      FIG. 1  shows a schematic of a two element parallel rheometer system that can measure viscosity or rheology of two samples simultaneously. The rheometer includes an optical image grabbing device  102  connected to an electronic image processing device  104 . The image grabber  102  looks at a compound viscosity measuring plate assemble  106  consisting of two individual measurement flow channel elements  108  and  110 . 
         [0027]    The assembly  106  samples fluid from reservoirs  112  and  114 . The physical flow path connections to the reservoirs are provided by tubes  116  and  118 . 
         [0028]    The movement of fluid through the measurement assembly  106  is powered by an air pressure tank  120 . The pressure in this tank may be positive or negative relative to the air above the reservoirs  112  and  114 . It may be controlled and changed during the duration of the rheology test. The assembly  106  is connected to the pressure tank  120  by process pipe  122  and three way valve  124 . The valve  124  is electronically control by a data acquisition and process controller  126  through signal communication line  128 . The pressure in the tank  120  is supplied and controlled by a system not shown. This will commonly be an air compressor or a vacuum pump. Pressure in the tank is measured by a sensor not shown which sends a signal to the process controller  126  by way of signal communication line  130 . 
         [0029]    Device  132  measures the air pressure into the element assembly  106  and sends a signal to the data acquisition and process controller  126  by way of line  134 . 
         [0030]    Imaging processing device  104  converts the image signal grabbed by device  102  and converts it into an array of digital data for further processing by the data acquisition and process control system  126 . The imaging processing device  104  controls the image grabber and the timing of the acquisition of multiple images of the assembly  106 . The grabber  102  may be a two dimensional array device such as exemplified by a digital camera. 
         [0031]    The data acquisition and process control system  126  is capable of receiving and sending digital and analog signals. It is capable of processing signals and sending digitalized data to computer  136 . It is capable of responding to data calculated by the computer  136 . Communication is through communication line  138 . 
         [0032]    Referring to both  FIGS. 2 and 3  simultaneously,  FIG. 2  shows a detail top view of a two element flow assembly  106 , and  FIG. 3  shows a cross sectional view of this assembly  106 . The assembly contains the flow circuits. 
         [0033]    It consists of a main block  200  and an optical cover  201 . Block  200  includes two elements  202  and  204  which are positioned to the left and right of the dashed line  205  respectively. These two elements are both machined into the same base block  200 . The two elements include a cavities  206  and  208 , flow tubes  210  and  212 , and fluid reservoirs  214  and  216 . Flow tubes  210  and  212  connect fluid reservoirs  214  and  216  to cavities  206  and  208  respectively. 
         [0034]    In main base block  200  are two air passageways  218  and  220  which extend horizontally through the block. Passageway  218  is blocked at both ends by plugs  222  and  224  and may be used if the assembly is coupled to other assemblies (not shown) to construct a compound assemble of more than two viscometers. 
         [0035]    Passageway  220  is blocked at one end by plug  226 , and connects at the other end to flow pipe  122 . Vertical holes  230  and  232  connect passageway  220  to cavities  206  and  208  respectively. 
         [0036]    To operate the viscometer the reservoirs  214  and  216  are placed under block  200  so that the tubes  210  and  212  are submerged in the two fluids to be tested. Then a suction pressure is applied through pipe  122 . Threeway valve  124  is activated to allow a negative gage pressure to be applied to passageways  218  and  220 . Negative pressure in passageway  220  is transmitted through vertical holes  230  and  232 , cavities  206  and  208 , and tubes  210  and  212 . This pressure forces fluid from the reservoirs and into the tubes  210  and  212 . 
         [0037]    The measurement of the flow resistance in the first element  202  takes place in its flow channels  210  and  206 . More specifically, flow is observed in cavity  206 . As fluid enters the cavity a liquid-air interface is created. The flow radiates outward as indicated by the arrows  240  and the position of the interface changes with time. 
         [0038]    The interface is normally visible to the naked eye, a camera or an image grabbing optical device. One such device may be a digital camera commonly used for photography. Industrial frame grabbing inspection devices are also widely available, and these interface with digital image processors and high speed image storage devices all of which can down load data to a computer for analysis. 
         [0039]    Usually, the differing optical properties of the liquid versus the air it is displacing in cavity  206  allows the differentiation of the liquid and air filled areas in cavity  206 . This is especially true if the liquid is opaque. Therefore, one is able to monitor the degree of fill of the cavity  206  as time proceeds. The degree of fill is a prime response variable and will vary with the liquid rheology, the geometry of the internal channels  206  and  210 , and the air pressure applied through pipe  122 . 
         [0040]    Viscoelastic liquids and non-Newtonian fluids commonly exhibit flow path patterns distinctly different from Newtonian fluids. It is a teaching of this invention to capture and quantify these with the imaging system. 
         [0000]    A. Comparison Flow Testing with a Single Element 
         [0041]    The liquid flow paths in element  202  have a liquid flow resistance which is a function of the flowing fluid rheology. Because of this the degree of fill of cavity  206  with time will be a function of rheology of the fluid being tested. Different rheology of fluids tested in element  202  will result in different fill versus time data if the applied pressure through pipe  122  is the same. Therefore, sequential testing of liquid samples through element  202  can determine if they have different rheologies. 
         [0000]    B. Comparison Flow Testing with Two or More Elements 
         [0042]    If the elements  202  and  204  resist liquid flow equally for identical liquids then differences in rheology of two different liquids may be detected by observing the filling characteristics such as the amount of fill as a function of time of cavities  206  and  208 . Likewise, if the flow paths are unequal but calibrated relative to each other using a standard fluid, then differences in rheology of two liquids may be detected by observing the filling characteristics such as the amount of fill as a function of time of cavities  206  and  208 . In a similar manner the simultaneous testing of more than two liquid samples may be carried out simultaneously using a number of elements simultaneously as long as they have identical or calibrated resistances to the flow of liquids through them. 
       C. Design of the Liquid Flow Channels 
       [0043]    Design of the liquid flow channels offers multiple possibilities with respect to the steady flow rheological characteristics one may measure or compare. The process time scale, the accuracy desired, the cost of fabrication on an element, and many other factors may be adjusted. The geometry of the channels  210  and  206  offers many possibilities. 
         [0044]    As shown in the  FIGS. 2 and 3  and described above, channel  210  is a tube, and pressure driven tube flow regime dominates the fluid dynamics if the entrance and exit region effects are small in comparison to the flow in the tube. However if the tube length flow resistance is small relative to the entrance and exit resistances the converging flow into the tube entrance or the diverging flow out of the tube into the cavity may dominate the flow resistance. If this is the case, rheological properties other than shear viscosity may be investigated for liquids especially non-Newtonian liquids. Converging and diverging flows react to the elongational flow rheology of fluids. Examples of such fluids where this may be important are solutions of long chain polymers. Many commercially used solution thickening agents produce quite pronounced elongational flow resistance. 
         [0045]    As shown in the  FIGS. 2 and 3 , the liquid flows from tube  210  into cavity  206  and proceeds radially outward in a slot. This axisymetric radial flow can be used for non-Newtonian liquids to investigate rheological properties other than simple shear viscosity. 
       D. Design for Time Dependent Flow Testing 
       [0046]    The design of the viscometer allows comparative testing of time dependent flow properties of liquids. The pressure applied by to the elements by pipe  122  may be varied with time. Since the optical sensing of the filling of cavities can be recorded as a function of time, their filling rates can be compared under transient conditions. Transient filling rates in response to step changes in applied pressure can be used to compare time dependent rheology of liquid samples. 
       E. Design for Shear Dependent Flow Testing 
       [0047]    The design of the viscometer allows comparative testing of shear rate dependent flow properties of liquids. The pressure applied to the elements by pipe  122  may be varied with time. The pressure differential driving the flow through tube  210  and its companions may be ramped up with time producing higher and higher tube flow shear rates. Since the optical sensing of the filling of cavities can be recorded as a function of time, their filling rates or degree of fill can be compared at different times during the filling process and related to different tube flow shear rates. 
       F. General Extensions of the Method 
       [0048]    Two or more cavities may be connected in series or parallel may be provided in a single viscometer element. This may be used to provide redundant measurements. This may be also used to provide measurements at two or more flow rates. 
         [0049]    Those skilled in the art of fluid flow will recognize that the flow resistances of liquid contacting flow channels and their entrances and exits may be changed by altering their dimensions or geometry. This can make a particular resistance large and dominant relative to the others. 
         [0050]    The cavity geometry may be changed to create a cavity where the cross section geometry is that of a constant width and height slot. The cavity may be made with a varying height. The cavity may have a height that varies with the distance from the entrance so that the observed surface area does not vary linearly with the volume of fill. The cavity may be a tortuous narrow channel which follows a path that wraps back on itself. The cavity channel path may be a spiral. 
         [0051]    Those skilled in the art of fluid flow or rheometry will recognize many other geometry variations of the element flow circuits are possible all of these are within the scope of the invention.