Angle of Contact and Adhesion vs Cohesion

The angle of contact is an important parameter in understanding the behavior of liquid droplets on a solid surface. It is defined as the angle formed between the liquid-vapor interface and the solid-liquid interface at the three-phase contact line.

Definition of Angle of Contact

The angle of contact depends on the relative strengths of adhesive forces between the liquid and solid (adhesion) and cohesive forces within the liquid (cohesion). Adhesion refers to the attractive forces between unlike molecules, while cohesion refers to the attractive forces between like molecules.

Explanation of Option A: Adhesion is more than cohesion

If the angle of contact of a drop of liquid is acute, it means that the liquid wets the solid surface well, spreading out and forming a thin film. In this case, the adhesive forces between the liquid and the solid are stronger than the cohesive forces within the liquid. Therefore, option A, which states that adhesion is more than cohesion, is the correct answer.

Reasoning behind Option A

When the angle of contact is acute, the liquid molecules have a strong affinity for the solid surface. This is typically observed with liquids that have high surface tension and low contact angle. The adhesive forces between the liquid and the solid overcome the cohesive forces within the liquid, causing the liquid to spread out and wet the surface.

Example

For example, consider a drop of water on a clean glass surface. Water has a high surface tension and low contact angle on glass. As a result, the water wets the glass surface, forming a thin film. The adhesive forces between the water and the glass are strong, allowing the water to adhere to the surface. The cohesive forces within the water are weaker compared to the adhesive forces, leading to a more spread-out shape of the droplet.

Conclusion

In conclusion, when the angle of contact of a liquid droplet is acute, it indicates that the adhesive forces between the liquid and the solid are stronger than the cohesive forces within the liquid. This leads to the liquid wetting the solid surface and spreading out, forming a thin film. Therefore, in this scenario, adhesion is more prominent than cohesion.