Question

The flow of viscosity increases when the substance is sheared. This is known as

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

Hint:

Plastic flow behavior is important in semisolid dosage forms like creams and pastes, where controlled application and spreadability are desired after a certain pressure is applied.

Answer 1

The Correct Option is B

Plastic flow behavior is a type of non-Newtonian flow where the material behaves as a solid under low shear stress but begins to flow like a viscous liquid once a certain yield stress is exceeded. - Plastic substances resist flow until a critical stress (yield value) is applied. Once this stress is exceeded, the substance flows, and its viscosity may appear to increase under further shear due to internal structural resistance.
- A typical example is Bingham plastic flow, where the flow does not start until the yield value is reached. Examples include toothpaste, ointments, and some suspensions.
- In contrast: - Dilatant systems (option a) show an increase in viscosity with increasing shear rate (opposite of shear-thinning). - Pseudoplastic systems (option c) show a decrease in viscosity with increasing shear rate (shear-thinning behavior). - Newtonian fluids (option d) have constant viscosity regardless of the shear rate.
Therefore, an increase in viscosity upon shearing after surpassing a threshold is best described by plastic flow behavior.