Define the angle of contact.

(N/A) The angle of contact is defined as the angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid.
It is measured through the liquid.
For a liquid that wets the solid surface (e.g.,water and glass),the angle of contact is acute $(< 90^{\circ})$.
For a liquid that does not wet the solid surface (e.g.,mercury and glass),the angle of contact is obtuse $(> 90^{\circ})$.