What it shows
Water climbs up a narrow glass tube and goes higher in a narrower one. The height is inversely proportional to the radius, and the measured heights give the surface tension of water.
The science
Water wets clean glass, so its surface curves upward at the wall and surface tension pulls it up the tube. The upward pull 2πr T cos θ balances the weight of the lifted column π r² h ρ g, so h = 2 T cos θ / (ρ g r), where T is the surface tension, θ the contact angle (almost 0 for clean glass), ρ the density and r the radius. Hence h × r is a constant, and the surface tension is T = h r ρ g / 2. For water at 25 degrees C, T is about 0.072 N/m.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Glass capillary tubes or pen refill tubes of three or four different bores (from a lab shop or snapped from fine glass droppers)
- Water coloured with a drop of ink or food colour
- A shallow dish
- A ruler or a graph-paper strip fixed on a card as a scale
- A magnifier to read the bore
- Clay to hold the tubes upright
- A thermometer
Safety
For this project: An adult must cut or snap any glass and smooth the ends; do not touch broken edges.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wash the tubes with soap, rinse them well and dry them so that the water wets the glass evenly.
Measure the inner diameter of each tube by looking at its end through a magnifier against a mm scale, and find its radius r.
Stand the tubes upright in the dish of coloured water with clay, each with its lower end well below the surface.
Wait a minute for the water to settle and measure the height h of the water inside above the surface of the water in the dish.
Record h for each tube and note the water’s temperature.
Plot h against 1 / r and check that the graph is a straight line through the origin.
Calculate T = h r ρ g / 2 for each tube and average the values.
What you should see
The narrowest tube lifts the water highest, and the graph of h against 1 / r is a straight line through the origin. The surface tension comes out near 0.06 to 0.075 N/m.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is capillarity?
The rise or fall of a liquid in a narrow tube caused by surface tension.
2Why does water rise but mercury is pushed down?
Water wets glass and has a small contact angle, while mercury does not wet glass and has a large contact angle.
3How is h related to the radius?
h is inversely proportional to the radius, from Jurin’s law.
4Where does capillary action help in nature?
It helps water rise in the narrow vessels of plants and in soil.
Take it further
Add a drop of detergent to the water and show that the capillary rise falls because the surface tension has dropped.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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