Capillarity

When a tube of small bore is inserted into a container of liquid, the level will either rise or fall within the tube, as shown below, depending upon the angle of contact between the liquid surfaces.

For liquids, such as water, which wet the tube, the conditions are as shown in (a) and result in a capillary elevation, while for liquids which do not wet the tube, such as mercury, a capillary depression results as shown in (b).

The gravitational force on the column of liquid elevated must be supported by the surface tension σ, acting around the periphery of the tube.

Resolving vertically,

Thus:                                                               Eqn 1

When the liquid wets the wall of the tube, θ is zero.

Then Eqn 1 becomes:                                            Eqn 2

This capillary action can cause serious errors when measuring pressures in terms of a head/column of liquid such as in a piezometer tube of manometer, if the bore of the tube is too small.

Observation of effect of capillarity

The object of this exercise is to observe the effect of the size of the gap between two flat plates on capillary elevation.

To achieve this, you must use a Parallel Plate Capillary Apparatus as shown below.

The two glass plates are thoroughly cleaned and a piece of fine wire is wrapped around one of the plates, near to one end.

The two glass plates are brought together and placed between the supporting clips.  Slide the two plates down to the bottom of the trough containing water.

Observe and sketch the pattern of the capillary rise as indicated between the plates.

Note that where the gap between the plates is at its smallest, the rise is greatest and conversely where the gap is widest, the capillary rise is at its smallest.

This experiment can be refined by using Capillary Tube Apparatus as shown below.

With this piece of apparatus, you can actually measure the height h in each of the tubes.

The internal diameter of each tube is different.

From Eqn 2, h can be calculated.

First you must make sure that each of the capillary tubes are thoroughly clean.  This may have to be done by placing them in alcohol.

Fill the water trough to the level of the bottom support plate and insert the capillary tubes.

Place a piece of white card behind the tubes and mark the card with the height of the capillary elevation in each tube.

With a pair of dividers, measure the capillary rise 'h' for each tube.

Given that the surface tension of water σ = 0.074 N/m

Calculate what the rise should have been.

 ID of Tube  (mm) Measured Capillary Rise, h  (mm) Calculated Capillary Rise, h (mm) 0.5 0.8 1.1 1.7 2.0 2.2

You should comment on the difference between measured and calculated rises.