Question

Rheogram of which type of flow does not start from the origin

• A. Plastic Flow
• B. Dilatant Flow
• C. Pseudoplastic Flow
• D. Newtonian

Hint:

Understanding flow behavior is critical in pharmaceutical formulation, especially for suspensions, emulsions, and ointments to ensure proper processing and patient compliance.

Answer 1

The Correct Option is A

A rheogram is a graph of shear stress versus shear rate used to describe the flow behavior of fluids. - Plastic flow is characterized by the presence of a yield stress that must be overcome before flow begins. This means the rheogram does not start from the origin; instead, it has a positive intercept on the shear stress axis. The fluid behaves like a solid until this yield stress is exceeded. Examples include toothpaste and ketchup.
- Dilatant flow (shear-thickening) fluids show an increase in viscosity with increasing shear rate, but their rheograms start from the origin.
- Pseudoplastic flow (shear-thinning) fluids have decreasing viscosity with increasing shear rate and also start from the origin on the rheogram.
- Newtonian fluids exhibit a linear relationship between shear stress and shear rate with zero intercept, thus their rheogram always starts from the origin.
Therefore, only plastic flow shows a rheogram that does not start from the origin due to the yield value that must be exceeded for flow to commence.

Answer 2

A rheogram plots shear stress against shear rate, and where the curve begins on that plot reveals what kind of fluid is being tested. Checking whether each flow type starts flowing under the smallest possible stress, or needs a push first, sorts out which curve does not begin at the origin.

1. Plastic Flow: A plastic (Bingham) material behaves like a rigid solid until the applied shear stress climbs past a threshold called the yield value. Below that stress, there is no flow at all, so the curve only starts once the shear stress axis is crossed at a positive value, not at zero. This gives the rheogram a positive intercept, meaning it does not start from the origin.
2. Dilatant Flow: A dilatant (shear-thickening) fluid begins to flow the instant any shear stress is applied, and its resistance simply climbs faster as the shear rate rises. Since flow starts at zero stress and zero shear rate together, the curve begins at the origin.
3. Pseudoplastic Flow: A pseudoplastic (shear-thinning) fluid also starts flowing under the smallest stress, with its resistance falling as the shear rate increases. Its curve passes through the origin as well.
4. Newtonian: A Newtonian fluid gives a straight line through the origin, with shear stress directly proportional to shear rate at every point, including zero.

Only a plastic system needs the applied stress to first exceed a yield value before it flows at all, which is why its rheogram alone does not start from the origin.

So the correct answer is Plastic Flow.