Editing Soap bubble (section)

== Physics ==
{{broader|Bubble (physics)}}
=== Merging ===
[[File:Ggb in soap bubble 1.jpg|thumb|Soap bubbles can easily merge]]
[[File:Soap bubbles being formed by a bubble wand - slow motion - 2022 July 28.webm|thumb|Slow motion video of soap bubbles being formed by a bubble wand]]
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. If they are not the same size, 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°.  Only four bubble walls can meet at a point, with the lines where triplets of bubble walls meet separated by cos<sup>−1</sup>(−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 ease of rupture of the very thin layer of water which constitutes its surface, namely a [[micrometre|micrometer]]-thick [[soap film]].
It is thus sensitive to :
* Drainage within the soap film: water falls down due to gravity. This can be slowed 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 13 feet (4 meters). In principle, there is no limit in the length it can reach.
* [[Evaporation]]: This can be slowed by blowing bubbles in a wet atmosphere, or by adding some sugar to the water.
* Dirt and fat:  When the bubble touches an object, it usually ruptures the soap film. This can be prevented by wetting these surfaces with water (preferably containing some soap).

After experiments, researchers found that a solution containing:

* 85.9 % water
* 10 % [[glycerol]]
* 4 % [[dishwashing liquid]]
* 0.1 % [[guar gum]]

gave the longest lasting results as it minimised the [[Marangoni Effect]].<ref>[https://www.newscientist.com/article/2338803-whats-the-best-recipe-for-bubble-mixture-scientists-have-the-answer/ New Scientist: What’s the best recipe for bubble mixture? Scientists have the answer 22 September 2022 By Chris Simms]</ref>

=== Wetting ===

When a soap bubble is in contact with a solid or a liquid surface [[wetting]] is observed. On a solid surface, the [[contact angle]] of the bubble depends on the [[surface energy]] of the solid.<ref>{{Cite journal | doi=10.1016/j.jcis.2009.05.062| pmid=19541324| title=Contact angle of a hemispherical bubble: An analytical approach| journal=Journal of Colloid and Interface Science| volume=338| issue=1| pages=193–200| year=2009| last1=Teixeira| first1=M.A.C.| last2=Teixeira| first2=P.I.C.| bibcode=2009JCIS..338..193T| s2cid=205804300| url=http://centaur.reading.ac.uk/29242/1/contact_rev3.pdf}}</ref><ref>{{Cite journal |doi = 10.1063/1.4812710|title = Wetting of soap bubbles on hydrophilic, hydrophobic, and superhydrophobic surfaces|journal = Applied Physics Letters|volume = 102|issue = 25|pages = 254103|year = 2013|last1 = Arscott|first1 = Steve|bibcode = 2013ApPhL.102y4103A|arxiv = 1303.6414|s2cid = 118645574}}</ref> A soap bubble has a larger contact angle on a solid surface displaying [[ultrahydrophobicity]] than on a hydrophilic surface – see [[Wetting]]. On a liquid surface, the contact angle of the soap bubble depends on its size - smaller bubbles have lower contact angles.<ref>M.A.C. Teixeira, S. Arscott, S.J. Cox and P.I.C. Teixeira, Langmuir 31, 13708 (2015).[http://pubs.acs.org/doi/abs/10.1021/acs.langmuir.5b03970]</ref><ref>{{cite web |url=https://ciencias.ulisboa.pt/pt/noticia/08-02-2016/o-despertar-da-bolha |title=O despertar da bolha |access-date=2016-02-09 |url-status=live |archive-url=https://web.archive.org/web/20160212130252/http://ciencias.ulisboa.pt/pt/noticia/08-02-2016/o-despertar-da-bolha |archive-date=2016-02-12 }}</ref>

<gallery mode=packed heights=180>
File:Bubble on an ultrahydrophobic surface.jpg|A soap bubble wetting an ultrahydrophobic surface
File:Bubble on a liquid surface.jpg|A soap bubble wetting a liquid surface
</gallery>

=== Floatation ===
The gas inside a bubble is less dense than air because it is mostly water vapor. Water vapor is a gas that is formed when water molecules evaporate. When water molecules evaporate, they escape from the liquid state and enter the gas state. In the gas state, water molecules are further apart than they are in the liquid state. This is because water molecules are attracted to each other. When they evaporate, they break away from these attractions and move further apart.

The further apart water molecules are, the less dense they are. This is why water vapor is less dense than air. The gas inside a bubble is mostly water vapor, so it is also less dense than air.

The density of a gas can also be affected by its temperature. As the temperature of a gas increases, the molecules of the gas move faster. This causes them to spread out and become less dense. The opposite is also true. As the temperature of a gas decreases, the molecules of the gas move slower. This causes them to bunch together and become more dense.

The temperature of the gas inside a bubble is affected by the temperature of the water around it. The warmer the water, the warmer the gas inside the bubble. This means that the gas inside a bubble will be less dense if the water is warm than if the water is cold.