Capillary Action

A glass tube with a very fine uniform bore is called a capillary tube. When a capillary tube is dipped vertically into a liquid contained in beaker, the liquid immediately rises or falls in the tube. The rise or fall of a liquid in a very narrow capillary tube is given by


Where:

T is the surface tension of the given liquid.

  • r is the radius of the capillary tube
  • is the density of the liquid
  • g is acceleration due to gravity
  • is the angle of contact for the given pair of solid and liquid

The angle of contact is defined as the angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid. The angle can be acute or obtuse. If the angle of contact is acute, the level of liquid inside the capillary tube is higher than that in the beaker. This capillary rise is observed in the case of water. If the angle of contact is obtuse, the level of liquid inside the tube is lower than that in the beaker.

This capillary fall is observed in mercury (θ 140°).

For water in silver tube, θ =90° and h = 0. The level of liquid remains the same.

For pure water and clear glass θ = 0°

Applications of Capillary Action in daily life

  • The rise of sap in trees and plants: The Xylem or Bark has such structure that the water rises to reach from roots to leaves via capillary action, although some other theories also persist to explain this. When the bark of a tree (Xylem) is removed in a circular fashion all around near its base, it gradually dries up and dies because water from soil cannot rise to aerial parts.
  • The rise of kerosene or oil in the wick of an oil lamp or stove.
  • The absorption of ink in a blotting paper.
  • Sandy soil is gets drier earlier than clay: The interspaces between the particles of the clay form finer capillaries and water rises to the surface quickly.
  • The purpose of applying soap to clothes is to spread it over large area. When soap is dissolved in water the surface tension of water is lowered. Surface tension always opposes the spreading of a liquid. By reducing surface tension we facilitate the liquid to spread over larger surfaces. This is why soap is used for washing.
  • For the same reason the paste spreads more freely in the mouth and facilitates cleaning of the mouth.
  • When we pour oil on the surface of water it lowers the surface tension of water. Hence the mosquito breed sinks down and perishes.
  • In voyage at the high seas, when there are violent waves the sailors pour tins of oil around their boats or ships. Due to oil the surface tension of sea water is reduced thereby the height of water waves is also reduced.
  • A pen nib is split at the tip to provide the narrow capillary and the ink is drawn upto the tip continuously.
  • When molten lead is allowed to fall through the end of a narrow tube, lead drops assume spherical shape due to surface tension. In factories lead shots are manufactured in this way.
  • Rain drops assume spherical shape due to surface tension of water.
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