At what condition the liquid spread over the solid surface and drop will not be formed.
Answers
Explanation:
A liquid surface in contact with a solid boundary, e.g. with the vertical wall of the container, will in general not meet it at right angles. The angle α between the bounding plane and the tangent to the liquid surface at the point of contact is called the angle of contact between the particular liquid and solid. It is measured, as indicated in Fig. 2.5, between the solid wall below the liquid surface and the tangent to the liquid surface by passing from one to the other through the liquid. For pure water and clean glass α = 0. For mercury and glass α > 90° as shown. When α > 90° the liquid does not spread over the solid and will form drops as does mercury on glass, the surface of the drops curving under before making contact with the glass. When α = 0 there is an infinite capacity for spreading. The upward curvature of the water at a vertical glass boundary can be interpreted as an attempt on the part of the liquid to spread over the glass. It is prevented from doing so by the weight of the small quantity of water which is thus raised above the general level. When a liquid tends to spread in this way it is because the forces between the liquid molecules among themselves are weaker than the forces between the liquid molecules and those of the solid, i.e. the forces of cohesion between the liquid molecules are weaker than the forces of adhesion between the liquid and solid molecules. The reverse is true for mercury and glass.