Question

Fluids which show an apparent increase in viscosity with time are called

• A. rheopectic
• B. thixotropic
• C. ideal fluid
• D. dilatant fluid

Hint:

Time-dependent fluid reference guide: - Viscosity increases with time \(\implies\) Rheopectic (e.g., print pastes, gypsum paste). - Viscosity decreases with time \(\implies\) Thixotropic (e.g., ketchup, modern wall paint).

Answer 1

The Correct Option is A

Concept: Fluids whose absolute apparent viscosity depends on the total duration of applied shear strain are classified as time-dependent non-Newtonian fluids. This sets them apart from time-independent fluids (like Bingham plastics, pseudo-plastics, or dilatants), whose viscosities change only with the applied shear rate, not with time. Time-dependent fluids are broadly split into two distinct categories based on how their apparent viscosity changes over time under steady shearing action:

• Thixotropic Fluids: Apparent viscosity decreases over time under constant shear stress (e.g., paints, printing inks).

• Rheopectic Fluids: Apparent viscosity increases over time under constant shear stress (e.g., gypsum suspensions, certain lubricants).

Step 1: Analyzing the specific condition given in the question.
The problem statement asks for fluids that exhibit an "apparent increase in viscosity with time." This means that as a constant shear stress or mechanical agitation is steadily sustained over a period of time, the internal microstructure of the fluid gradually builds up resistance, causing its apparent viscosity (\(\eta\)) to rise: \[ \frac{\partial \eta}{\partial t} \gt 0 \quad \text{(at a constant shear rate } \dot{\gamma}\text{)} \]

Step 2: Identifying the corresponding fluid class.
This behavior is the defining characteristic of rheopectic fluids (also sometimes referred to as anti-thixotropic fluids). Mechanical agitation or sustained shearing causes these fluids to solidify or thicken over time, increasing their resistance to flow.

Step 3: Differentiating from alternative choices.
Let us review why the other options do not fit this definition:

• Thixotropic: Viscosity decreases with time (opposite of the question's criteria).

• Ideal Fluid: Has zero viscosity, which never changes.

• Dilatant Fluid: Viscosity increases with an increase in the *shear rate* (shear-thickening), but this behavior is independent of time.
Thus, Option (A) is the correct choice.