I noticed sometime ago that bubbles formed in puddles/streets whenever it rained. I am wondering if someone could tell me what could cause such bubbles. I examined the bubbles again today more closely and they didn't look "soapy". I know that is not a really valid answer to rule out that leftover detergent/soap could be the cause of such bubble formation. However, the bubbles are short-lived. Furthermore, there aren't "frothy". In other words, there are no clumps of small bubbles.

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    $\begingroup$ So by "streets" I'm picturing asphalt. Is that right? The bubble are short-lived; how big do they get? $\endgroup$
    – Ryan
    Dec 31 '13 at 5:45
  • $\begingroup$ Yes. They get sort of big. Usually they are roughly between 2 to 4 centimeters in diameter. $\endgroup$ Dec 31 '13 at 5:58
  • $\begingroup$ I see this too sometimes. I second the idea that there's probably no soap involved, although I always thought that there's probably something else in there. $\endgroup$
    – chipbuster
    Jan 2 '14 at 3:03
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    $\begingroup$ When you turn on the faucet into a bucket of water, bubbles appear also. It's because the rush of water brings air down and cohesive tension creates a bubble for the air. $\endgroup$
    – jeremy
    Jan 2 '14 at 7:57
  • $\begingroup$ I see. But I am not quite convinced that raindrops would be large enough to trap enough air to form a bubble of 2 to 4 centimeters. Maybe the small bubbles coalesce into a large one, but I don't think I've seen such thing happen. $\endgroup$ Jan 3 '14 at 8:20

Bubbles consist of a gas trapped by a liquid, where the liquid has a surface tension high enough to encapsulate the gas. This gives bubbles three ingredients: the liquid, the gas, and surface tension.

The first ingredient, the liquid, is obviously the rainwater.

The second ingredient, the gas, is (also obviously) air. But what isn't obvious is how the gas becomes entrained in the liquid. Asphalt is porous, meaning the microstructure of asphalt consists of lots and lots of tiny random tubes through the solid material. Thanks to the capillary effect, water is sucked into those tiny tubes, and gas is forced out. This gas being forced out of the porous asphalt material and through the liquid on the surface creates bubbles.

Surface tension is the third ingredient, and controls the bubbles' shape, size, and strength. Surface tension can be enhanced by surfactants (big molecules which, when added to a liquid, agglomerate and increase the surface tension). The surface of a road accumulates lots of surfactants - mostly, oil. Oil consists of many long-chain hydrocarbons, which are perfect surfactants (most detergents are actually long-chain hydrocarbons that are polarized at one end). Surfactants tend to agglomerate (that's why, if you pour a little oil on top of water, it will tend to clump together as it spreads out over the surface of the water). Having oil act as a surfactant increases the surface tension of the rain water on the surface of the road, and makes any bubbles formed in the water stronger. That explains the large bubbles you saw.

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    $\begingroup$ The high-level concept here may be correct. Unfortunately, there are at least three factual problems: (1) surfactants decrease surface tension; related, (2) surface tension must be low in order for bubbles to form; and (3) oils themselves are not surfactants; only the fatty acids derived from them are. There may still be surfactant residues on roadways, but they are definitely not oils. $\endgroup$
    – hBy2Py
    Jun 16 '15 at 3:27

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