When a solid is dipped inside a liquid, the liquid surface becomes curved near the contact. For some solid–liquid interfaces, the liquid is pulled up along the solid, while for others, the liquid surface is convex or depressed near the solid surface. This phenomenon can be explained using the concept of cohesive and adhesive forces.
The adhesive force is the molecular force between molecules of different materials, that is, between the molecules of the solid and the liquid. The cohesive force is the molecular force between molecules of the same material—in this context, between the liquid molecules of the same species.
The contact angle is the angle between the tangent planes at the solid and the liquid surface. It is drawn such that the tangent to the solid is toward the liquid, and the tangent to the liquid is away from the solid. The contact angle is acute if the liquid rises along the solid's surface and obtuse if the liquid is depressed.
The contact angle depends on the net force on the small part of the liquid near the contact. The forces on it are the adhesive force of the solid, which is usually attractive, the cohesive force from the molecules near the surface, and its weight. The liquid rests in a way that it is perpendicular to the resultant force. Due to the material dependence of the adhesive and cohesive forces, the contact angle depends on the pair of surfaces considered. This phenomenon is similar to friction.
The contact angle measurement provides a practical way to evaluate the interfacial strengths and interactions between a solid and liquid, and it has numerous applications in various fields, such as surface science, material science, and engineering.
