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Rheological Techniques for Yield Stress Analysis

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Rheological Techniques for Yield Stress Analysis Keywords: yield stress, structured fluids, rheology, viscoelastic, viscosity RH025 ABSTRACT Bingham (Newtonian w/ yield stress) Yield stress analysis is important for all complex structured Bingham Plastic fluids. It is helpful to better understand product performance (Shear-thinning w/ yield stress) in applications such as product shelf life and stability. In σ theory, the yield stress is the minimum shear stress required to , initiate flow. A number of testing techniques are available for ess measuring and analyzing the yield stress using a rheometer. Three of these techniques, including Steady Stress Sweep Shear Str Newtonian (SS), Steady Rate Sweep from High to Low Shear Rates (SR), σ and Dynamic Stress/Strain Sweep (DS) are introduced in this y paper. The SS method was found to be a good approach for yield stress analysis of medium viscosity materials. The SR method was found to be useful for very low viscosity materials as yield stresses can be measured accurately as low as . Shear Rate, γ 4x10-4 Pa. The DS method was found to be useful over a wide Figure 1: Shear stress- shear rate behavior of some rheological viscosity range and was especially practical for testing high models with a yield stress viscosity semi-solid materials such as grease. In this paper, we introduce three different methods for INTRODUCTION analyzing the apparent yield behavior of materials using TA Instruments’ Combined Motor and Transducer and Yield stress analysis has been widely used in the food, Separate Motor and Transducer rheometers. The advantages healthcare, pharmaceutical, and paint industries. Many and disadvantages of each analytical technique are also commercial products, such as emulsions, dispersions, pastes, discussed. grease and foams, etc., are formulated to display yield stresses. The yield stress is an important characteristic of STEADY STRESS SWEEP METHOD FOR YIELD ANALYSIS (SS) structured fluids because it helps stabilizing the material. A higher yield stress prevents the material to undergo phase A common procedure for yield stress analysis is a steady separation, sedimentation or aggregation. It reduces flow state stress sweep on a controlled stress rheometer6-7. In a under shipping vibrations and gravity. Yield stress studies can SS experiment, a stress or torque is incrementally stepped help evaluate the product performance and processability logarithmically from a minimum value up to a value where and predict product’s long-term stability and shelf life1-5. the material starts to flow. Each stress is held on the material for a given amount of time. A software algorithm evaluates the A yield stress in rheology is defined as the applied stress at data and a data point is taken when the shear rate reaches which irreversible plastic deformation is first observed across an equilibrium value. Below the yield point, the change in the sample. It is usually represented as . Figure 1 depicts some σy shear rate with stress is extremely small. The viscosity remains “theoretical ideal” flow models with a yield stress. However constant with increasing stress and is referred to as the zero in reality, the value of yield stress will depend on the testing shear viscosity. At a given stress, the material starts to flow, method. Therefore, a measured yield stress should be referred the material yields as the viscosity decreases many orders of to as an “apparent yield stress”. One of the most extensively magnitude over a narrow range of shear stress (Figures 2-5, used method to determine the yield stress is probably the adapted from Barnes, 2000). continuous shear stress or shear rate ramp, from low to high stress or rate. The yield stress is usually determined by fitting the This approach is widely used for analyzing the yield stress stress/rate curves with Bingham, Casson or Herschel-Bulkley of medium viscosity materials. The yield stress value can be models. Also manual extrapolation of the stress curve to zero obtained either by taking the very last data point before shear rate (shown in figure 1) is frequently used to obtain the viscosity starts to drop significantly, or by taking the a yield stress. In a shear stress (rate) ramp, the yield stress onset value from the viscosity curve. It should be noted can be determined from the transient viscosity maximum in that the accuracy of the yield stress measured will be a double logarithmic plot of viscosity versus stress (Maximum dependent on the number of points per decade chosen, viscosity method). Since these test methods are performed when programming the test parameters. For example, when under nonsteady state conditions, the measured yield stress choosing five points per decade in a logarithmic sweep, the value is highly dependent on the experimental ramp rate. successive data points will be much further apart than when 1 RH025 108 choosing ten points per decade. Therefore, collecting more x9 107 data points per decade provides a more accurate yield 106 stress, if the material remains stable during the test. η[Pa s] 5 10 The challenge for the steady state stress sweep method are a s] 104 low and high viscosity materials. The yield stress of some low [P η 103 viscosity suspensions/dispersions (e.g., orange juice) can 102 be extremely low and the smallest incremental amount of x1,000,000,000 Viscosity 101 stress added might dramatically alter the measured result. In a stress sweep test, it is possible to step beyond the yield 100 stress of the sample from one data point to the next (Figure 10-1 6). In addition, while testing high viscosity semisolid materials, -2 10 such as grease or paste, wall slippage might occur between 0.1 1 10 100 the geometry and the sample. This greatly affects the Shear Stress σ [Pa] Figure 2: Flow curve of 10% bentonite suspension measurement accuracy and reproducibility (Figure 7), and is difficult to eliminate even if serrated or crosshatched test 108 fixtures are used. Therefore, two other analytical techniques 107 x2 for yield analysis are discussed and compared to the stress 106 sweep method. 105 STEADY RATE SWEEP METHOD FOR YIELD ANALYSIS (SR) a s] 4 [P 10 η Some low viscosity suspensions or dispersions (e.g., orange 3 10 x1,000,000 juice, ink, or milk) have very low yield stress values that 102 Viscosity require the rheometer to operate close to the low-end torque 101 specification. These yield stress values cannot be easily η[Pa s] determined using the stress sweep method, even if small 100 torque increment steps were applied. Furthermore, can 10-1 loading of the sample damage the weak structure before 10 100 the test begins. Shear Stress σ [Pa] Figure 3: Flow curve of mayonaise 101 103 x2 yield? 102 10000 100 101 a s] [1/s] a s] 1000 0 . γ [P 10 η [P -1 e η 10 η[Pa s] -1 x40,000 . 10 100 γ[1/s] Viscosity Shear Rat Viscosity 10-2 10-2 10 10-3 η[Pa s] 1 10-3 10-4 1000 0.01 0.1 1 Shear Stress σ [Pa] Shear Stress σ [Pa] Figure 4: Flow curve of flocculated ink Figure 6: Flow curve of an orange juice 10 108 1E7 107 1 x2 106 1000000 0.1 5 a s] a s] 10 [1/s] . γ [P η[Pa s] 0.01 [P η 100000 e η . 104 γ[1/s] wall slippage? 1E-3 103 x20,000 10000 Viscosity Viscosity η[Pa s] 1E-4 Shear Rat 102 1000 101 1E-5 100 100 1E-6 10 100 100 1000 Shear Stress σ [Pa] Shear Stress σ [Pa] Figure 5: Flow curve of toothpaste Figure 7: Flow curve of grease 2 RH025 To overcome these limitations, a steady state flow shear rate The shear rate sweep test can be easily performed on both sweep test can be used to analyze the yield behavior of CMT and SMT rheometers and shows excellent performance these materials. When performing this test, the sample can when analyzing the yield behavior of low viscosity materials. be loaded either onto a cone-plate, parallel plate or cup/ The benefit of this test method is that by sweeping the shear rate bob geometry. The shear rate is controlled during the test, from high to low, all sample history from loading is eliminated. and is stepped down logarithmically from high (e.g., 10 The measurement results show good reproducibility. Table 1 1/s or 100 1/s) to low (e.g., 10-5 1/s) values. When the yield lists the yield stresses of some low viscosity samples. The yield point has been reached, the stress on the sample reaches a stress can be measured accurately as low as 4x10-4 Pa. plateau and becomes independent of rate. Concomitantly, Table 1: Yield stress of some low viscosity materials the “apparent” viscosity (ηa) of the sample goes to infinity with decreasing shear rate (Figure 8 and Figure 9). SAMPLE YIELD STRESS YIELD TORQUE The viscosity is defined as: [Pa] [μNm] (1) Blue ink 0.3 18.0 Orange juice 0.01 0.6 When = (yield stress), then σ σy Fat free milk 0.00044 0.02 (2) Pectin Solution 0.1% 0.04 1.5 Xanthan gum 0.1% 0.01 0.4 and in the double logarithmic plot (3) the “apparent” viscosity decreases with the rate at a slope DYNAMIC STRESS/STRAIN SWEEP METHOD FOR YIELD of -1. ANALYSIS (DS) The conventional stress sweep test method for determining yield stress is challenged by high viscosity materials such 10 as greases and pastes. With increasing stress wall slippage η is observed, even when crosshatched or serrated plates σ are used.
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