Physics · Mechanical Properties Of Fluids · NEET
Cohesion is the attractive force between molecules of the SAME substance, for example water molecule to water molecule. It is what holds a drop together and causes surface tension. Adhesion is the attractive force between molecules of DIFFERENT substances, for example water molecules and the glass wall. The meniscus shape is simply a contest between these two forces at the edge where the liquid meets the solid.
For water in glass, adhesion (water to glass) is stronger than cohesion (water to water). So the edge molecules are pulled up along the glass wall more than they are pulled inward. The liquid climbs the wall, making the surface dip in the middle and rise at the sides. This upward-curving cup shape is called a concave meniscus, and water is said to wet the glass.
For mercury in glass, cohesion (mercury to mercury) is stronger than adhesion (mercury to glass). The mercury molecules pull each other inward more than the glass pulls them outward, so mercury pulls away from the wall. The middle bulges up like a hill and the edges drop. This is a convex meniscus, and mercury does not wet glass.
The angle of contact (theta) is the angle inside the liquid between the tube wall and the liquid surface at the point of contact. Concave meniscus (water): theta is acute, less than 90 degrees. Flat surface: theta equals 90 degrees. Convex meniscus (mercury): theta is obtuse, greater than 90 degrees. So the same cohesion vs adhesion rule that shapes the meniscus also fixes the angle of contact.
Yes. In capillary rise h = 2T cos(theta) / (rho g r). If the meniscus is concave (theta acute), cos(theta) is positive and the liquid RISES, like water. If the meniscus is convex (theta obtuse), cos(theta) is negative and the liquid FALLS below the outside level, like mercury. So a curve-up meniscus means climb up, a curve-down meniscus means drop down.
Three liquids of densities rho1, rho2 and rho3 (with rho1 > rho2 > rho3), having the same value of surface tension T, rise to the same height in three identical capillaries. The angles of contact theta1, theta2 and theta3 obey:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In water touching glass, adhesion (water to glass) is stronger than cohesion (water to water). That is why water wets glass, climbs the wall, and forms a concave meniscus with an acute angle of contact.
Mercury has very strong cohesion (mercury to mercury) and weak adhesion to glass. The mercury molecules cling to each other and pull away from the wall, so mercury does not spread on glass. This gives a convex meniscus and an obtuse angle of contact of about 140 degrees.
For water in a glass tube (concave meniscus), read the level at the LOWEST point of the curve. For mercury (convex meniscus), read the level at the HIGHEST point of the curve. Always keep the eye level with that point to avoid parallax error.
Yes. The meniscus depends on both the liquid and the solid. Water is concave in clean glass but can behave differently on a waxy or oily surface where adhesion to the solid is weak, so it may bead up instead of wetting.
A perfectly flat liquid surface at the wall means cohesion and adhesion effects balance, giving an angle of contact of exactly 90 degrees. Here cos(theta) is zero, so there is no capillary rise or fall.
