Calibration Procedures for Orthogonal Superposition Rheology

Orthogonal Superposition Radiology is an advanced technique that allows the measurement of radiological properties of complex fluids under non-linear flow conditions, which offer insights into the micro-structural changes that governs macroscopic properties. The information gained on the structured property processing relationships in complex fluids is important to many industries for the development and manufacturer of reliable products. OSP is now commercially available for the radiology community.

Given the complicated geometry design, a protocol to quantify measurement error is critical to the successful use of this technique. The calibration protocol described in this video is the first standard operating procedure to properly determine the end effect factors, a proper accounting of the end effect factors reduces measurement error. Before installing the geometry, enable the orthogonal superposition feature in the rheometer software.

After lifting the stage to maximum height, install a lower platinum resistance thermometer on the test station for temperature measurement and an environmental control device. Assemble the inner and outer cylinders, to complete the double wall cup configuration. Insert the cup into the environmental control device and align the geometry.

Press the lower geometry cup downward to compress the spring-loaded platinum resistance thermometer, by using a torque screwdriver to tighten the thumbscrew. Disable the motor power and use a finger to spin the geometry to confirm that the cup rotates freely. Install the upper geometry bob onto the transducer shaft, and click the tear transducer button in the transducer control panel of the rheometer software to tear the normal force and torque.

Then click the zero fixture button to zero the gap between the upper and lower geometries. To load the test material, lift the stage to provide enough workspace to load the test material into the cup, and carefully load the test material into the cup. Lower the upper geometry carefully into the fluid to reach the geometry gap set point of eight millimeters.

When the bob end contacts the fluid, reduce the downward velocity of the bob. Lift the bob vertically to a position at which the wetted fluid contact line can be visually inspected. Carefully lift the bob to the previous loading position to allow enough workspace and load an additional amount of test material into the cup as needed.

Lower the upper geometry into the fluid, and use the go-to geometry gap button to set the cup and final geometry gap. Repeat the adjustment until the wetted contact line on the bob is approximately two millimeters above the lower end of the upper bob opening. Then move the bob to the geometry gap set point and allow the test material to relax.

To verify that the correct geometry has been selected, open the orthogonal double wall concentric cylinder geometry. If the orthogonal superposition radiology geometry is not listed, manually create a new geometry with the software, using the dimensions for the geometry as indicated in the table. Then specify the geometry constants, including the inertia and upper tool mass, and enter one for both the end effect factor, and orthogonal end effect factor.

To perform a steady sheer rate sweep test, condition the sample at 25 degrees Celsius for 15 minutes. When the test material has reached thermal equilibrium, under the procedure tab in the rheometer software select flow and sweep. In the environmental control window, set the test temperature to 25 degrees Celsius.

Set the sheer rate range from 0.01 to 100 inverse seconds, with data recording at 10 points per decade, logarithmically, and enable automatic steady state determination. Then start the experiment. To perform an orthogonal frequency sweep test, set the normal force transducer to conditioning and transducer mode.

And condition the sample at 25 degrees Celsius for 15 minutes to ensure thermal equilibration. Select the orthogonal frequency sweep test with the test temperature set to 25 degrees Celsius. Specify the desired normal strain and enter zero inverse seconds for the sheer rate in the rotational direction.

Then specify the angular frequency range from 0.1 to 40 rads per second at 10 points per decade, logarithmically, and start the experiment. To verify that the corrections are valid, using the calibrated end effect factors obtained from the calibration experiments, enter the calibrated values for the end effect factor and orthogonal end effect factor under the geometry constants. The stress constants will be automatically updated.

Then set up an orthogonal frequency sweep test as demonstrated, using one inverse second for the shear rate and start the experiment. Here, representative results from the viscosity calibration measurements on a 12.2 pascal-second silicone viscosity standard are shown. The silicone liquid is a newtonian fluid with an expected constant viscosity independent of the applied shear rate.

The measured torque increases linearly as the sheer rate increases and all of the data are above the low torque limit. The uncorrected viscosity value is higher than the actual viscosity. These orthogonal frequency sweep tests demonstrate different orthogonal strain amplitudes from 0.5 to 9.4%for the 12.2 pascal-second viscosity standard.

Similarly, without correction, the measured orthogonal complex viscosity overestimates the actual viscosity of 12.2 pascal-second. Only the viscosity data with corresponding orthogonal force values above the lower limit of the axial oscillation force for the transducer, are used to calculate the average viscosity for correction. To calculate the end effect factors here, the primary in effect factor is equal to the uncorrected primary viscosity divided by the viscosity of the standard liquid.

The orthogonal end effect factor is equal to the viscosity of the standard liquid divided by the uncorrected orthogonal viscosity. Here, an orthogonal superposition measurement was performed under steady shear for viscosity verification check. Only the data with values greater than the instrument force resolution were plotted.

Since the correct and effect factors were applied, the measured viscosities in both directions matched the accepted oil viscosity value of 12.2 pascal-seconds. Now, you should have a good understanding of how to perform calibration for OSP using newtonian fluids. We recommend that users determine their end effect correction factors for their own instruments and geometries because the actual corrections are material and instrument dependent.

Making accurate measurements are important for scientific research as well as product development. This protocol provides a technical resource for the instrument users and also guidance to manufacturers for optimizing the engineering design of future instruments.