Future Projects include coloured bubbles

• Liquid Dish-washing Detergent (or other detergent)
• Water
• or Commercial Bubble Solution
• Sodium Hydroxide
• Phenolphthalein
• Thymolphthalein
• Club Soda (optional)

Here’s How:

1. If you are making your own bubble solution, mix the detergent and water.
2. Add the sodium hydroxide and indicator to the bubble solution. You want enough indicator so that the bubbles will be deeply coloured. For each litre of bubble solution (approx 4 cups) this is about 1-1/2 to 2 teaspoons of phenolphthalein (red) or thymolphthalein (blue).
3. Add sodium hydroxide until you get the indicator to change from colourless to coloured (about half a teaspoon should do the trick). A little more sodium hydroxide will result in a bubble that stays coloured longer. If you add too much, the colour of the bubble won’t disappear when exposed to air or rubbed, though you can still react it with club soda.
4. You may find it necessary to dissolve the indicator in a small amount of alcohol before mixing it with the bubble solution. You can use pre-made indicator solution, adding the sodium hydroxide to the indicator rather than diluting with water.
5. You’ve essentially made disappearing ink bubbles. When the bubble lands, you can make the colour vanish by either rubbing the spot (reacting the liquid with air) or by adding a little club soda. Fun!
6. If you have disappearing ink, you could mix it with bubble solution to make disappearing ink bubbles.

Please don’t drink the bubble solution!

 Bubble Science

“Mathematics”

Soap bubbles are physical illustrations of the complex mathematical problem of minimal surface. They will assume the shape of least surface area possible containing a given volume. A true minimal surface is more properly illustrated by a soap film, which has equal pressure on inside as outside, hence is a surface with zero mean curvature. A soap bubble is a closed soap film: due to the difference in outside and inside pressure, it is a surface of constant mean curvature.

While it has been known since 1884 that a spherical soap bubble is the least-area way of enclosing a given volume of air (a theorem of H. A. Schwarz), it was not until 2000 that it was proven that two merged soap bubbles provide the optimum way of enclosing two given volumes of air of different size with the least surface area. This has been dubbed the Double Bubble conjecture.

“Physics”

Merging

When two bubbles merge, they adopt a shape which makes the sum of their surface areas as small as possible, compatible with the volume of air each bubble encloses. If the bubbles are of equal size, their common wall is flat. Else, their common wall bulges into the larger bubble, since the smaller one has a higher internal pressure than the larger one, as predicted by the Young–Laplace equation.

At a point where three or more bubbles meet, they sort themselves out so that only three bubble walls meet along a line. Since the surface tension is the same in each of the three surfaces, the three angles between them must be equal to 120°. This is the most efficient choice, again, which is also the reason why the cells of a beehive have the same 120° angle and form hexagons. Only four bubble walls can meet at a point, with the lines where triplets of bubble walls meet separated by cos⁻¹(−1/3) ≈ 109.47°. All these rules, known as Plateau’s laws, determine how a foam is built from bubbles.

Stability

The longevity of a soap bubble is limited by the rupture of the very thin layer of water which constitutes its surface, namely a micron-thick soap film. It is thus sensitive to :

• Drainage within the soap film: water falls down due to gravity. This can be slowed down by increasing the water viscosity, for instance by adding glycerol. Still, there is an ultimate height limit, which is the capillary length, very high for soap bubbles: around 4 metres. In principle, there is no limit in the length it can reach.
• Evaporation: this can be slowed down by blowing bubbles in a wet atmosphere, or by adding some sugar to the water.
• Dirt and fat: when the bubble touches the ground, a wall, or our skin, it usually ruptures the soap film. This can be prevented by wetting these surfaces with water (preferably containing some soap).