Flatten the water curvature inside a capillary tube

Apparatus and materials

• Capillary tube
• Trough

Technical notes

Use clean apparatus and water. Wash in strong detergent, rinse in clean water, then in distilled water and take care not to put grease from your fingers on it.

The height of the capillary tube should be greater than the height to which water will rise inside it.

Procedure

a Fill the trough with water and place a capillary tube in it. The water will rise inside the tube, to a height 'h' above the water surface in the cup (diagram a).

b Observe the concave curvature of water inside the capillary tube. The curvature forms an angle, say 'θ', on the inside wall of the capillary tube.

c Now, lower the capillary tube such that the height of the capillary tube above the water surface in the beaker is less than 'h' (diagram b).

d Observe the concave curvature starts reducing and the angle at which the water surface meets the inside wall of the capillary tube is greater than 'θ'.

e When the tube is fully lowered to the surface level of water in the cup you will notice that the water surface flattens (diagram c).

Teaching notes

1 Water rises inside the capillary tube due to adhesion between water molecules and the glass walls of the capillary tube. This adhesion, together with surface tension in the water, produces an effect called 'capillarity', with a characteristic concave surface.

The water rises until the weight of the column equals the vertical component of the forces of adhesion.

The weight W of the water column = π r2 h ρ g
where ρ is the liquid density

θ is the angle of contact between liquid and glass
r is the internal radius of the tube
h is the maximum height of the liquid column
g is the gravitational field strength
γ is the surface tension of the liquid

The vertical supporting forces around the circumference of the liquid surface = γ cos θ x 2 π r

so

π r2h g = 2 π r γ cos θ
and h = 2 γ cos θ /r ρ g

The narrower the tube, the higher the water will rise. Nature uses this effect to carry water up from roots to leaves in plants, including trees.
Capillary action

2 When the tube is lowered so that the water surface inside the tube is at any height less than h, θ becomes larger so that the weight of the water column and the forces of adhesion remain balanced.

When the water surface inside and outside the capillary tube are level, the surface is shaped by surface tension alone.