Fluid Surface energy and surface tension
The potential energy per unit area of the surface film is called surface energy.
Consider a metal frame ABCD in which AB is movable. The frame is dipped in a soap solution. A film is formed which pulls AB inwards due to surface tension. If T is the surface tension of the film and l is the length of the wire AB, this inward force is given by 2 × T l . The number 2 indicates the two free surfaces of the film.
If AB is moved through a small distance x as shown in Fig. to the position A′ ′ B , then work done is
W = 2Tlx
Work down per unit area = T
Surface energy is numerically equal to surface tension.
Angle of contact
When the free surface of a liquid comes in contact with a solid, it becomes curved at the point of contact. The angle between the tangent to the liquid surface at the point of contact of the liquid with the solid and the solid surface inside the liquid is called angle of contact.
In Fig. , QR is the tangent drawn at the point of contact Q. The angle PQR is called the angle of contact. When a liquid has concave meniscus, the angle of contact is acute. When it has a convex meniscus, the angle of contact is obtuse.
The angle of contact depends on the nature of liquid and solid in contact. For water and glass, θ lies between 8o and 18o. For pure water and clean glass, it is very small and hence it is taken as zero. The angle of contact of mercury with glass is 138o.
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