Which of the following phenomena does not involve surface tension ?

Options

A. Mercury drops acquire spherical shape .
B. Liquids tend to rise in the capillary .
C. A liquids flows over a fixed surface .
D. Moist soil grains are pulled together .

Explanation

To determine which of the following phenomena does not involve surface tension, we will analyze each option based on the definition and properties of surface tension.

### Step-by-Step Solution:

1. **Understand Surface Tension**:
- Surface tension is a property of liquids that allows them to resist external forces due to cohesive forces between liquid molecules. It causes the liquid surface to minimize its area, forming shapes like droplets.

2. **Analyze the Options**:
- **Option A**: Mercury drops acquire a spherical shape.
- **Explanation**: This phenomenon involves surface tension because the cohesive forces in mercury cause its surface to minimize, resulting in a spherical shape.

- **Option B**: Liquids tend to rise in a capillary tube.
- **Explanation**: This phenomenon also involves surface tension. The cohesive forces between the liquid molecules and adhesive forces between the liquid and the tube walls lead to the rise of the liquid in the tube.

- **Option C**: A liquid flows over a fixed surface.
- **Explanation**: This does not involve surface tension. The flow of liquid over a surface is primarily due to viscosity, which is the resistance of the liquid to flow. There are no cohesive forces acting to minimize the surface area in this case.

- **Option D**: Moist soil grains are pulled together.
- **Explanation**: This phenomenon involves surface tension as the moisture creates a cohesive force between the soil grains, pulling them together.

3. **Conclusion**:
- The only option that does not involve surface tension is **Option C**: A liquid flows over a fixed surface.

### Final Answer:
**Option C: A liquid flows over a fixed surface does not involve surface tension.**