What is a Viscosity Flow Curve?
A flow curve - also known as a rheogram - is a graphical representation of the behavior of a flowing material (fluid) when subjected to increasing or decreasing shear rates. Calculate the corresponding shear stress and viscosity according to the parameters of the instrument. You can tell the fluid type of the sample (Newtonian/non-Newtonian, thixotropic, dilatant, etc.) by the shape of the flow curve.
Knowing the fluid type and knowing the flow curve is required to ensure your product is right for its application. For example, say you need to pump toothpaste through a pipe. You need to make sure the toothpaste cuts thinner. That is, the force required to pump the toothpaste through the tube will make the fluid less viscous so it will flow through the tube more easily.
Creating a flow curve involves testing a sample at a range of shear rates. It is useful to have test parameters relate to how the sample is handled during manufacturing or how the sample is used by the consumer.
How to create a flow curve:
1. Decide what you want to test.
2. Determine your test parameters.
3. Prepare charts to record test results.
4. Program and run the test, recording the results in a graph.
5. Create a chart based on the results in the chart.
Here is an example:
(1) Suppose you make toothpaste, and you want to make sure that it flows easily when the user squeezes the tube, but doesn't leak when the tube is not squeezed.
(2) You decide to test the product at room temperature, starting at 0 rpm and gradually increasing the speed. At some point, you will gradually reduce your speed to zero.
(3) Create an Excel spreadsheet to record viscosity and shear rate (rpm) results.
(4) Program and run the test, recording the results each time the speed increases, and again when the speed decreases.
(5) Use Excel to create charts from charts.
Common Terms Related to Flow Curves: Yield Stress and Zero Shear Viscosity - What Do They Mean?
Yield stress is the force required to initiate the flow of fluids that would not flow when at rest. This type of fluid is called "plastic". If the viscosity of the product increases as the shear rate is reduced to zero, the yield stress can be seen in the test.
Zero shear viscosity is a term for non-plastic fluids. Using this term, the viscosity of a fluid will stabilize when the shear rate is reduced to zero. This shows that the fluid flows like a liquid when it is at rest.
