DHR/AR Rheometer: Setting Up a Flow (Continuous) Ramp Test

In this topic
Overview
Test Setup

Overview

In a rate or stress ramp experiment, the rate or stress is varied linearly up and down, and the stress, i.e., rate (strain), is recorded over time. The material function of interest is the instantaneous viscosity. The rate ramp was originally designed for simple viscometers to evaluate the time-dependence with shear rate for structured fluid, such as a paint (thixotropy). The ramping up, combined with the ramping down, of the shear rate is also referred to as a thixotropic loop test. When the rate or stress is increased quickly, in reference to the average relaxation time of the material, the measured stress or shear rate and viscosity are non-equilibrium values.

The main application of the stress ramp experiment is to evaluate the yield stress of structured fluids. A structured fluid, such as a cosmetic cream, behaves like a solid at rest, and flows when a shear stress above a critical stress (the yield stress) is applied. Therefore, the strain increases only marginally when a low stress is applied, but flows rapidly past the yield stress.

Two methods are used to evaluate the yield stress. The shear strain tangent method determines the yield stress from the intercept of two lines extrapolated from the low rate and high rate strain. The instantaneous viscosity at low stress initially grows very quickly. With increasing stress, the material becomes liquid-like and, consequently, the viscosity decreases and a unique viscosity maximum is developed. The maximum viscosity method determines the yield stress from the stress at the viscosity maximum.

The critical parameter in a ramp test is the ramp rate. If the rate and stress are increased slow enough, the measured viscosity is independent of the ramp rate. If not, up and down ramps generate different results. The difference is used to evaluate a thixotropy index.

Test Setup

To select an Flow Ramp test, see Using Experimental Procedures for detailed instructions.

When you perform a Flow Ramp test, the following parameters need to be chosen.

Environmental Control

Select the following environmental control parameters:

Temperature: The temperature selection to maintain throughout the test. Enter the desired test temperature, or select Inherit set point to maintain the previously specified temperature at the start of this step. The temperature range is dependent on the configuration of your instrument and the installed environment control system.
Wait for temperature: Select this option to wait until the entered temperature is reached before beginning data collection. If you wish to begin data collection, while achieving the temperature, disable this option.
Soak time: The amount of time to delay data acquisition at the start of the step, typically to allow for temperature equilibration. This time is measured from the start of the step if Wait for temperature is not selected, or from the point at which the measured temperature becomes stabilized at the specified Temperature, if Wait for temperature is selected. Because of the mass of the sample, test fixtures, and environmental control systems a "soak time" (i.e., time to equilibrate at temperature) is recommended, particularly when starting steps at different temperatures. A five-minute soak time is sufficient for most samples in cases where the change in temperature is not too large. This time is also used at each increment temperature.

Test Parameters

Set up the following test parameters:

Enter the Duration of the test, in seconds. Alternatively, you can select Inherit duration to base the test time on the selected ramp parameters.

Select the ramping Mode for the ramp.

Logarithmic: The shear rate, velocity, stress, or torque are ramped logarithmically with time.
Linear: The shear rate, velocity, stress, or torque are ramped linearly with tie.

Select the desired control variable. Choose between Torque, Stress, Velocity, or Shear rate. This test can be run using either torque/stress or velocity/shear rate as the controlling variable.

Torque: The torque is defined as the torque to be applied by the motor at each measurement. This value is used to extract the torque M_s applied to the sample during the measurement. The torque should be selected to be within the linear viscoelastic range of the sample, and still provide a large enough signal to ensure good data.
Stress: The stress is defined as the stress to be applied to the sample at each measurement. This value is determined from the sample torque M_s applied to the sample during the measurement and the sample geometry and dimensions. The stress can be selected to simulate real-life end-use conditions, or it can be a value selected to be within the linear viscoelastic range of the sample, and still provide a large enough signal to ensure good data.
Velocity: The velocity is defined as the velocity to be applied to the sample by the motor at each measurement. The angular velocity should be selected to be within the linear viscoelastic range of the sample, and still provide a large enough signal to ensure good data.
Shear rate: The shear rate is defined as the shear rate applied to the sample at each measurement. This value is used, along with the sample geometry and dimensions, to calculate the peak angular deflection to be applied to the sample during the measurement. The shear rate can be selected to simulate real-life end-use conditions, or it can be a value selected to be within the linear viscoelastic range of the sample, and still provide a large enough signal to ensure good data.

Enter the Initial and Final values, or select Inherit initial value to maintain the current shear rate/stress conditions at the start of this step.
Select Inherit duration to automatically set the test duration based on the ramp parameters.

Enter the data collection parameters:

Points per decade: The number of points determines the total number of values collected between the specified range. This includes both the initial and final value. This is the only option available when Log mode selected.
Sampling interval: The value entered in this field defines the time interval between data points. This option is only available during linear sampling.

Controlled Rate/Stress Advanced

If you selected Velocity or Shear rate as the control variable:

Choose between Auto, Soft, Medium, and Stiff, depending on the sample stiffness. As a rule of thumb, leave this option set to Auto. Matching the mode to the stiffness of your sample may increase the quality of your data.
Select the Oversampling option to smooth the test results. In this mode, the analog data are sampled at a faster rate than required and an averaged (smoothed) value is recorded.
NOTE: Flow inertia correction is not performed if oversampling is selected.

If you selected Torque or Stress as the control variable, select the Oversampling option to smooth the test results. In this mode, the analog data are sampled at a faster rate than required and an averaged (smoothed) value is recorded.

Data Acquisition

There are additional data collection options that can be adjusted to control how data is obtained and what additional information is collected during the measurement.

End of step: The default behavior is set to zero the torque. However, in certain situations, for example a ramp up followed by a ramp down, select maintain final torque/velocity. Note that the final ramp value will be maintained through any environmental control block in the subsequent step.

There is also an option to zero the velocity, which might be useful when measuring low viscosity fluids where just zeroing the torque would not slow the velocity quickly enough prior to the next step starting.

Save images: Select to store images of the test within the data file for future recall when using the Camera accessory.

Save image every: Specify the collection rate for time-based experiments. Select the minimum time in seconds between images. An image is stored with the next test point when the time between images has elapsed.

Step Termination

TRIOS Software allows you to define conditions in which a step is halted ahead of its normal termination conditions (Limit checking). You can use this to ensure that, for instance, the instrument does not over speed or apply excessive strains.

Rather than running a step for a certain amount of time, you may wish to run it until stable data is obtained. You can set an Equilibrium limit (such as the viscosity value becoming constant when running a single shear with time) that will stop the currently active test.