eli5: why does the second shampoo make more foam ?
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Because there’s less oil/dirt in your hair.
You can try an experiment to show how it works: Take a pan and put some oil in it, then wash it with dish soap. There won’t be as much foam, but it breaks down the oil. Once the oil and soap are rinsed off, add dish soap again and it will foam a lot more.
Shampoo works by binding to certain molecules like magnets. If you have oil in your hair, it will bind to those and create tiny, microscopic oil-in-shampoo bubbles, which you can wash away with water. If you don’t have oil in your hair, the shampoo will find something else to bind to; in the absence of anything else, it will bind to air molecules (oxygen and/or nitrogen gas). These will make larger, more visible air bubbles the same way soap does, so they’re more noticeable and foamier.
Shampoo+water is thicker than shampoo+water+oil. Exactly why is…complicated. After the first wash there’s much less oil and dirt in your hair so you’re dealing with a mix more like the stuff they use for bubble toys.
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