Discussion
Which of the following are directly related to surface tension phenomena ?
(I) Spherical shapes of droplets in the absence of external forces
(II) A sail boat in water
(III) A needle floating in water
(IV) Water rising in the xylem of plants
Answer: C
The phenomena directly related to surface tension are (I), (III) and (IV).
- (I) A free droplet takes a spherical shape because surface tension minimizes its surface area, and a sphere has the least area for a given volume.
- (III) A steel needle laid gently on water floats, supported by the surface film, although steel is far denser than water.
- (IV) The rise of water in the narrow xylem vessels of plants is treated as capillary action, which depends on surface tension together with the adhesion of water to the vessel walls.
(II) A sailing boat floats by buoyancy (Archimedes' principle) and moves by wind on its sails, neither of which involves surface tension. Every option containing (II) is therefore wrong.
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