Physics · Mechanical Properties Of Fluids · NEET
Always inside the liquid. Draw the tangent to the liquid surface at the exact point where liquid, solid and air meet. The angle of contact is the angle between this tangent and the solid surface, measured through the liquid. Students often measure it in air and get the wrong shape. For water on glass the tangent leans in, so the inside angle is small (acute). For mercury on glass the tangent leans out, so the inside angle is large (obtuse).
It depends on which force is stronger: adhesion (liquid to solid) or cohesion (liquid to itself). For water on glass, adhesion is stronger, so water climbs the wall, the meniscus curves up (concave) and the angle is acute (about 8 degrees for pure water on clean glass, near 0 for very clean glass). For mercury, cohesion is much stronger, so mercury pulls into a ball, the meniscus curves down (convex) and the angle is obtuse (about 140 degrees).
If the angle is less than 90 degrees, the liquid spreads and wets the solid. If it is more than 90 degrees, the liquid stays as droplets and does not wet the solid. At exactly 90 degrees the surface stays flat. So the single number theta tells you the whole behaviour: water wets glass (acute), mercury does not (obtuse), water beads up on a waxy lotus leaf (obtuse).
Yes, they are two views of the same thing. Acute angle goes with a concave (upward-curving) meniscus and capillary rise. Obtuse angle goes with a convex (downward-curving) meniscus and capillary fall. If theta were exactly 90 degrees the surface would be flat and there would be no rise or fall.
The angle of contact depends on the pair of surfaces, not the liquid alone. It changes with the nature of the solid, the liquid, and any impurity or coating. Adding soap or detergent lowers the angle so water wets and cleans better. Grease, oil or wax on the solid raises the angle so water beads up. Temperature also affects it slightly. It does NOT depend on the amount of liquid or the tube diameter.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the angle inside the liquid between the solid wall and the liquid surface at the point where liquid, solid and air meet. It tells you whether the liquid wets the solid or beads up.
For water on clean glass it is small and acute (about 8 degrees, close to 0 for very clean glass). For mercury on glass it is obtuse (about 135 to 140 degrees).
When the angle of contact is less than 90 degrees. Then adhesion (liquid-solid attraction) is stronger than cohesion (liquid-liquid attraction), the meniscus is concave, and the liquid spreads.
Mercury atoms attract each other much more strongly than they attract glass (cohesion beats adhesion). So mercury pulls into a rounded shape, gives an obtuse angle and a convex meniscus, and does not spread.
Yes. Soaps and detergents lower the angle of contact of water, so water spreads and wets surfaces better. That is why soapy water cleans more effectively.
No. Surface tension is the pull along the liquid surface. Angle of contact is the shape angle where the liquid meets a solid. They are linked (through the balance of three surface tensions) but they are different quantities.
