please explain angle of contact

he contact angle is the angle at which a liguid/vapor interface meets a solid surface. The equilibrium contact angle is specific for any given system and is determined by the interactions across the three interfaces. A single system of drop/medium (say air)/surface has a spectrum of contact angles ranging from the so-called advancing (maximal) contact angle to the receding (minimal) contact angle. The equilibrium contact angle is between those values and can be estimated from them as described below. Most often the concept is illustrated with a small liquid droplet resting on a flat horizontal solid surface. The shape of the droplet is determined by the Young-Laplace equation, with the contact angle playing the role of a boundary condition. Contact angle is measured using a contact angle goniometer. The contact angle is not limited to a liquid/vapour interface; it is equally applicable to the interface of two liquids.

THE ANGLE BETWEEN THE TANGENT PLANES AT THE SOLID SURFACE AND THE LIQUID SURFACE AT THE CONTACT IS CALLED THE CONTACT ANGLE

Case I

(Water and oily surface interface)

Where,

S_la − Interfacial tension of liquid-air interface

S_sa − Interfacial tension of solid-air interface

S_ls − Interfacial tension of liquid-solid interface

• S_la cosθ + S_sl = S_sa

• If S_sl > S_la, then angle of contact is an obtuse angle and the molecules of liquid are attracted strongly to themselves and weakly to those of solids.

• A lot of energy is used to create the liquid-solid interface.

Case II

• If S_sl<S_la, then the angle of contact is an acute angle and the molecules of the liquids are attached strongly to those of the solids.

• Not enough energy is required to create the liquid-solid interface.