# Question #6a4f5

Feb 7, 2017

First of all let us define two types of molecular forces.

1. Adhesive force: It is the attractive force acting between molecules of different substances.
2. Cohesive force: attractive force acting between molecules of same substance.

Both these forces are van der Waals' forces and are inversely proportional to seventh power of the distance between the molecules.

The angle of contact is the angle $\theta$ as shown in the figure below

It is the angle between the tangents (1) drawn at the liquid surface at the point of contact and (2) at the solid surface; measured inside the liquid.
It depends on the nature of liquid-solid contact and medium above the free surface of liquid.

If the liquid molecules exhibit strong adhesive force with the solid molecules then the liquid drop will completely spread out on the solid surface. This corresponds to a ${0}^{\circ}$ contact angle.

Contact angles are highly sensitive to contamination. Measured values are reproducible to an accuracy of a few degrees only under laboratory conditions with samples of purified liquids and very clean solid surfaces.

Therefore, for pure water and perfectly clean glass (no contamination), the angle of contact is ${0}^{\circ}$.
Whereas for ordinary water and glass, it lies between ${8}^{\circ} \mathmr{and} {18}^{\circ}$.