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.

Answer 2

The question describes a substance whose resistance to flow builds up as shear is applied to it. Working through how each flow type actually behaves as shear stress is applied shows which one fits that description.

1. Dilatant: A dilatant material flows as soon as any stress is applied and its resistance climbs steadily as the shear rate rises, a behavior seen in concentrated starch suspensions that stiffen the harder they are stirred. It flows from the very start, though, rather than resisting flow altogether until a threshold is crossed.
2. Plastic: A plastic material, such as toothpaste or an ointment base, holds its shape and does not flow at all under light shear, behaving like a soft solid. Only once the applied shear stress is pushed past a certain threshold, the yield value, does the material start to flow. Right up to that point, the resistance the material puts up keeps building with the applied stress, and it takes a genuine push to get it moving at all.
3. Pseudoplastic: A pseudoplastic material flows readily under the lightest shear and actually becomes easier to shear, not harder, as the shear rate goes up. Ketchup thinning out once you shake the bottle is a familiar example.
4. Newtonian: A Newtonian liquid such as water offers the same resistance to flow no matter how hard or gently it is sheared, so its behavior does not change with shear at all.

Among these, a plastic material is the one that visibly resists being sheared, building up resistance until its yield value is overcome, before it will flow at all.

So the correct answer is Plastic.