The surface of liquid near the plane of contact with another medium is curved.
Angle of contact : The angle between tangent to the surface at the point of contact and solid surface inside the liquid is know as angle of contact .it is denoted by `theta`.
It is different in interfaces of differernt pairs of liquids and solids.
In figure (a) and (b) water forms and droplets on lotous leaf is shown in (a) which spreads over a clean plastic plate a shown in (b)
In figures the surface tension of liquid air intefaces `=S_(la)`
The surface tension of solid air interfaces `=S_(sa)`
The surface tension of solid -liquid interfaces `-S_(sl)`
At the line of contact , the surface forces between the three media must be in equilibrium.
In figure (a) `S_(sa)+S_(la)cos(180^(@)-theta)=S_(sl)`
`thereforeS_(sa)-S_(la)costheta=S_(sl)`
`thereforeS_(sl)ltS_(sa)`,
means `theta` obtained negative
In this situation angle of contact `theta` is an obtuse, because cohesive forces between the molecules of liquid , while weak adhesive force between molecules of liquid and solid.
If angle of contact is obtuse liquid does not wet the surface of solid body and the liquid will be depressed in a capillary of small bar and meniscus becomes convex.
Illustration :
(1) Mercury and any solid surface .
(2) Water and wing of duck.
(3) Lotus leaf and water.
Glass and mercury.
(5) Solid surface with wax and oil and water.
(6) Raincoat and water.
In figure (b) `S_(sl)+S_(la)costheta=S_(sa)`
`thereforecostheta=(S_(sa)-S_(sl))/(S_(la))`
`thereforeS_(sl)ltS_(sa)` means `theta` is positive.
In this situation angle of contact become acute because adhesive force is more predominant than cohesive force is more predominant than cohesive force.
If angle of contact is acute ,then liquid wets the solid surface and the liquid rise in vertical capillary and the meniscus is concave.
Illustrations :
(1) Water is spread with glass , plastic or other body .
(2) Kerosene is spread over any surface.
the angle of contact between the silver capillary and water is `90^(@)` Hence in this capillary is placed vertical in water , water neither rise nor depressed.
