Question

What is meant by an angle of contact? State any two characteristics of angle of contact.

Answer 1

The angle between tangent to the liquid surface at the point of contact and solid surface inside the liquid is termed as the angle of contact. The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface.

Complete answer:
A liquid is to be kept in a vessel and thus, liquid is in contact with some solid surface. For a pair of solid and liquid, the angle of contact $\theta$ , is defined as the angle between tangent to the liquid surface drawn at the point of contact and the solid surface inside the liquid.
-If the $\theta < {90^0}$ , the liquid will have a meniscus concave upwards.
-If the $\theta > {90^0}$ , the liquid surface will have a meniscus convex upwards.
-If the $\theta = {90^0}$ , the surface of the liquid at the point of contact is plain.

Characteristics of angle of contact:
-It depends upon the nature of the solid in contact and liquid, and is constant for a given liquid-solid pair, other factors remaining unchanged.
-It is independent of the inclination of the solid to the liquid surface.
-It changes with surface tension. As it changes with surface tension the temperature and the purity of the liquid changes with surface tension. It depends upon the medium above the free surface of the liquid.

PROPERTY	ANGLE OF CONTACT $< {90^0}$	ANGLE OF CONTACT$= {90^0}$	ANGLE OF CONTACT $> {90^0}$
SUBSTANCE	Water and glass	Water and silver	Mercury and glass
ANGLE OF CONTACT	Almost zero.Acute angle	Right angle $= {90^0}$	Obtuse angle $= {135^0}$
MENISCUS SHAPE	Concave	Plane	convex
CAPILLARY ACTION	Liquid rises	No effect	Liquid falls
STICKING TO SOLID	Sticks/wets	Does not wet	Does not wet
RELATION BETWEEN ${F_{a{\text{ }}}}and{\text{ }}{{\text{F}}_c}$	${F_a} < \dfrac{{{F_C}}}{{\sqrt 2 }}$ ${F_a} > {F_c}$	${F_a} = \dfrac{{{F_C}}}{{\sqrt 2 }}$	${F_a} < \dfrac{{{F_C}}}{{\sqrt 2 }}$ ${F_a} < {F_c}$
SHAPE OF LIQUID SURFACE	Almost round	Spreads on the surface	flat

Note: In the static method a drop is deposited on a surface and the contact angle can be measured by looking at the drop through a goniometer (an instrument that measures contact angles). The dynamic method is similar to the static one but the drop of liquid which is deposited on a surface is modified.