When does a liquid 'wet' a solid surface? What is exactly meant when it is said that a liquid wets a solid surface. Has it got only to do with the contact angle ?
 A: The process of wetting of a solid surface is better explained with cohesive and adhesive forces. Wetting of a solid surface by a liquid means the liquid molecules succeeded in maintaining a contact with the surface through the inter molecular attractive forces. You should see that every liquid may not stick on to a given surface and similarly every surface may not get by a given liquid.   
A liquid has it's own inter molecular force of attraction that maintains it's continuity. Such forces existing between same type of molecules is called cohesive force. Now if there is some inter molecular attractive forces between different molecules then it is called adhesive force. When the cohesive force between the water molecules is greater than the adhesive force of water molecules with the solid molecules, then it cannot stick on the surface. You can see a drop of water on such a surface (hydrophobic) forms a round ball and go in crazy directions. So there will be no wetting. Now in some hydrophilic surfaces, the adhesive force between the liquid and solid molecules dominate and the liquid molecules stick on to the surface. This makes the surface wet.  
So the degree of wetting is determined by a force balance between adhesive and cohesive forces.  
 
The contact angle which you mentioned and as shown in the figure depends on the balance between the adhesive and cohesive forces. So one could say that the degree of wetting depends on the angle of contact.If the adhesive forces dominate, then the drop spreads out on the surface, thereby decreasing the angle of contact. In such case, the wetting will be higher. If the cohesive forces dominate, the angle increases as the drop forms a bubble or round up into a spherical body which means the degree of wetting will be low. Thus the contact angle provides an inverse measure of wettability.  
Hence if the contact angle between the liquid surface interface is zero, i.e., $\theta=0^0$, then the degree of wetting is maximum and is said to be perfect wetting. In such case the solid-liquid interactions are much stronger than the liquid-liquid interactions. If $0^0<\theta<90^0$, then the degree of wettingis high. If $90^0<\theta<180^0$, then the degree of wetting is low. If $|theta=180^)$, then the degree of wetting becomes minimum and the surface is said to be perfectly non-wetting. In such a case, the liquid-liquid interactions will be much higher than solid-liquid interactions.
