When solid gets wet completely, the contact angle is:
First, the core concept here is surface tension and wettability. Contact angle is a measure of how a liquid interacts with a solid surface. If the contact angle is less than 90 degrees, the liquid wets the surface. If it's more than 90, it doesn't. When a solid is completely wet, the liquid spreads out completely, so the contact angle should be zero. That makes sense because there's no bead formation.
Now, the correct answer is probably zero degrees. Let me check the options. Since the options aren't given, I have to assume typical choices. Common options for contact angles might be 0°, 90°, 180°, or maybe 45°. If the solid is completely wet, the contact angle is 0°, so the correct answer would be the option stating 0°.
For the wrong options: 90° is the threshold between wetting and not wetting. 180° would mean the liquid forms a perfect sphere, which is non-wetting. 45° is a partial wetting angle. Each of these is incorrect because they don't represent complete wetting.
The clinical pearl here is remembering that contact angle is a key factor in determining wettability. Zero degrees means complete wetting, which is important in applications like coatings or drug delivery systems where the liquid needs to spread evenly.
Putting it all together, the correct answer is 0°, and the explanation should highlight the relationship between contact angle and wettability.
**Core Concept**
The contact angle measures the wettability of a solid surface by a liquid. A contact angle of **0°** indicates complete wetting, where the liquid spreads uniformly over the solid surface. This occurs when the liquid has a high affinity for the solid, overcoming surface tension forces.
**Why the Correct Answer is Right**
When a solid is **completely wet**, the liquid spreads out into a flat film with **no distinct boundary** between the liquid and solid. This corresponds to a contact angle of **0°**, as the cosine of the contact angle (cos θ) equals 1 (Young's equation: γ_s = γ_sl + γ_l cos θ). Zero degrees signifies maximal adhesion between the liquid and solid phases.
**Why Each Wrong Option is Incorrect**
**Option A:** A contact angle >90° indicates poor wettability (e.g., water on wax).
**Option B:** 90° is the threshold angle for partial vs. complete wetting.
**Option C:** 180° represents perfect non-wetting (e.g., water on a hydrophobic surface).
**Clinical Pearl / High-Yield Fact**
Remember: **"Zero angle, full spread"**—complete wetting (0°) is critical in applications like drug absorption through mucosal surfaces or ensuring proper adhesion of coatings.
**Correct Answer: A. 0°**