ANS:D - complete wetting

Complete wetting refers to a situation where a liquid completely covers the surface of a solid material, forming a uniform and continuous film. In this state, the liquid spreads evenly across the surface, maximizing contact between the liquid and the solid. Here's a more detailed explanation of complete wetting:

1. Surface Energy: Every material has a characteristic surface energy, which determines how readily it interacts with other substances. When a solid material is in contact with a liquid, the balance of forces between the solid-solid, liquid-liquid, and solid-liquid interfaces influences whether wetting occurs.
2. Contact Angle: The degree to which a liquid spreads on a solid surface is quantified by the contact angle. If the contact angle is less than 90 degrees, the liquid wets the surface, spreading out to form a thin film. If the contact angle is greater than 90 degrees, the liquid forms droplets on the surface, indicating partial wetting.
3. Complete Wetting: In the case of complete wetting, the contact angle is zero degrees. This means that the liquid spreads out completely on the solid surface, forming a continuous film. In other words, the entire surface of the solid is covered by the liquid without any gaps or droplets.
4. Thermodynamic Equilibrium: Complete wetting occurs when the adhesive forces between the liquid and the solid are strong enough to overcome the cohesive forces within the liquid. Once complete wetting is achieved, further addition of the liquid does not produce any additional thermal effect, as the system has reached thermodynamic equilibrium.
5. Applications: Complete wetting is desirable in various applications, such as in painting, coating, and lubrication processes, where uniform coverage of the solid surface by the liquid is necessary for effective performance. It also plays a crucial role in phenomena such as capillary action and surface tension.