Question

Thixotropic behavior is associated with

• A. Increase in viscosity
• B. Decrease in viscosity
• C. Solid \& Liquid behavior
• D. Sol-gel transformation

Hint:

Thixotropic gels are ideal for topical and ophthalmic formulations, ensuring ease of application and prolonged residence time.

Answer 1

The Correct Option is B

Thixotropy is a type of time-dependent non-Newtonian flow behavior in which a material’s viscosity decreases under applied shear stress and gradually recovers when the stress is removed. - In a thixotropic system, such as suspensions, gels, or colloidal dispersions, the internal structure is disrupted by shearing, resulting in a temporary reduction in viscosity.
- When the shear force is stopped, the structure rebuilds over time, and viscosity is restored.
- This behavior is useful in pharmaceutical and cosmetic formulations where ease of application (low viscosity during application) and retention at the site of action (high viscosity at rest) are required.
Incorrect options: - (a) Increase in viscosity is not typical of thixotropy; that is more characteristic of negative thixotropy (a rare phenomenon).
- (c) Solid \& Liquid behavior describes viscoelasticity, not specifically thixotropy.
- (d) Sol-gel transformation refers to a phase transition but not the time-dependent shear-thinning of thixotropy.
Hence, thixotropic behavior is best associated with a decrease in viscosity upon shearing.

Answer 2

This question asks what change in viscosity defines thixotropic behavior. Comparing each option against how a thixotropic material actually behaves under and after shear identifies the right answer.

1. Increase in viscosity: A material that gets thicker the longer or harder it is sheared is called rheopectic, the opposite of thixotropic. Thixotropic materials thin out under shear instead, so this does not match.
2. Decrease in viscosity: A thixotropic material's internal structure, often a loose network of particles or polymer chains, breaks down as shear is applied and continued, so its resistance to flow drops. Once shearing stops, that structure slowly rebuilds and viscosity climbs back up. This drop in viscosity during shearing, with recovery at rest, is what defines thixotropy.
3. Solid & Liquid behavior: Showing both solid-like and liquid-like behavior together describes viscoelasticity, a material's ability to store and dissipate energy at once. That is a separate property from the time-dependent thinning that defines thixotropy.
4. Sol-gel transformation: A sol-gel transformation is a change of state, a fluid sol setting into a semisolid gel or the reverse, driven by conditions such as temperature or concentration. Thixotropy is about a temporary, shear-induced drop in viscosity within one state, not a change between two distinct states.

The behavior that defines thixotropy is a temporary loss of viscosity while a material is being sheared, followed by its slow return once shearing stops.

So the correct answer is decrease in viscosity.