The Physics and Chemistry of Bubbles

Topics: Surface tension, Soap, Water Pages: 10 (2593 words) Published: March 18, 2011

By Olivia Kenyon

Table of contents

Table of contents2
Table of figures3
Background knowledge4
The film4
The soap or detergent4
The shape5
The colours7
Different fabrics as wands9
My investigation10
Part 110
Part 212
Other sources13

Table of figures

Figure 1. Soap molecules surrounding a layer of water making the film of a bubble4 Figure 2. The general structure of a soap molecule5
Figure 3. A table explaining that the sphere has the smallest surface area to volume ratio.5 Figure 4. A flat interface between two bubble films6
Figure 5. A cubic bubble film4, and a diagram of a wand that could form it26 Figure 6. Another cubic bubble film6
Figure 7. A helical bubble film4, and a diagram of a wand that could form it26 Figure 8. Tetrahedral bubble films4, and a diagram of a wand that could form it27 Figure 9. A tubular bubble7
Figure 10. Constructive interference of light7
Figure 11. Destructive interference of light8
Figure 12. Constructive and destructive interference8
Figure 13. The patterns created in a bubble film with the constructive and destructive interference of white light8 Figure 14. The thin film at the top of the bubble appears black9 Figure 15. High ‘wettability’9

Figure 16. Low 'wettability'9
Figure 17. David Stein's Bubble Thing10

Background knowledge

As a small child, I was always fascinated by bubbles, and they seem to make everyone happy. The way they float so beautifully; the way they are so colourful, yet come from a colourless solution; and the way they miraculously morph themselves into the most incredible and ever-changing shapes. And then suddenly they burst and the magic disappears. You can never get the same bubble twice.

The film

The film of the bubble is basically made of three layers, the middle layer is water molecules, and the inner and outer layers are the soap film (see figure 1). The cohesion between the water molecules is caused by the hydrogen bonds that from between the ?+ charge on the hydrogen atom and the ?- on the oxygen atom, due to the electrons in the electron cloud being pulled more by the oxygen atom than the hydrogen atoms. This happens because of the large difference in electronegativity between the oxygen and the hydrogen atoms. This cohesion means that there is surface tension, which causes the film to act like an elastic sheet.[1] In the simplified diagram below (based on an image from “Chemistry in the toy store”[2]), the inner layer of water realistically would contain other ingredients in the bubble recipe, such as glycerine and sugar. [pic]

Figure 1. Soap molecules surrounding a layer of water making the film of a bubble

The soap or detergent

The soap or detergent molecules have a non-polar hydrocarbon tail and a polar head, for example a carboxylate end as in soap molecules, which can dissolve in water (see figure 2).

Figure 2. The general structure of a soap molecule

The fatty non polar end can dissolve non-polar substances such as oils and grease, and the polar end is soluble in water, making it perfect for washing dishes. This is also what makes the film of water of the bubble stable. It lowers the surface tension of the film to approximately one third of the surface tension of a water film. As the soap film stretches, the concentration of soap molecules decreases, so the surface tension increases, therefore the soap effectively strengthens the weakest parts of the film. Another benefit of soap in the bubble solution is that is reduces evaporation of the water molecules from the water film.1 As well as reducing the surface tension of the bubble, detergent molecules also add stabilizing elastic properties to the liquid surface and a greater total surface area (as a result of the non-polar ends of the soap molecules sticking out of the surface). This causes an increase in the surface energy (or tension) of the solution, but it is still less than...
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