The liquid surface is usually curved, when it is in contact with a solid. The particular shape that it takes place depends on the relative strengths of cohesive and adhesive forces. If
(I) Adhesive force > Cohesive force, then liquid wets the solid surface and has concave meniscus.
(II) Adhesive force < Cohesive force, then liquid does not wets the solid surface and has a convex meniscus.
(III) Adhesive force = Cohesive force, then liquid surface is plane.
Adhesive Force
Force of attraction between molecules of the different substances is called a adhesive force.
Cohesive Force
Force of attraction between molecules of the same substances is called Cohesive force.
Angle Of Contact
The angle between tangent to the liquid surface at the point of contact and the solid surface inside the liquid is called as the angle of contact.
The value of angle of contact depends on the following factors.
(I) Nature of the solid and liquid in contact.
(II) Cleanliness of the surface in contact.
(III) Medium above the free surface of the liquid.
(IV) Temperature of the liquid.
For those liquid which wet the wall of the vessel, the angle of contact is acute. For the liquid which do not wet the walls of the vessel the angle of contact is obtuse.
The angle of contact for water and glass is about 8⁰. For Mercury and glass, it is 138⁰ and for pure water and silver, angle of contact is 90⁰.
We consider the three interfacial tensions at all the three interface such as
As the line of contact, the surface forces between the three media must be in equilibrium. Resolving Sla into two rectangular components, we have SlaCosθ along the solid surface and SlaSinθ along the perpendicular to the solid surface.
As the liquid on the surface of solid is at rest, So the molecules of these interfaces are in equilibrium. Thus net force on them is zero.
The following cases arise
(I) If Ssl>Sla, then Cosθ is negative and θ>90. The molecules of a liquid are attracted strongly to themselves and weekly to those of solid. It costs a lot of energy to create a liquid-solid surface. The liquid then does not wet the solid.
Examples: Water-Leaf or Glass-Mercury interface.
(II) Ssl<Sla, Then Cosθ is positive and θ<90. The molecules of the liquid are strongly attracted to those of solid and weekly attracted to them-selves. It costless energy to create a liquid-solid surface and liquid wets the solid.
Examples: When soap or detergent is added to water, the angle of contact becomes small.