I've noticed that when I put dish soap, water and a little air into a sealed plastic bag, and then shake the mixture, it forms soap suds and the bag expands significantly in volume. Why does it expand?
After being prompted by the comments, I tried to re-create and measure this phenomenon under more controlled conditions, but the bag does not expand.
The less-controlled conditions, where the bag expanded in the past, were:
- A plastic bag, slightly dirty from crumbs of cooked ground corn, and canola oil (the bag held arepas)
- A little squirt of Ultra Ajax Triple Action Orange dish soap (not automatic dishwasher soap). The ingredients (and purpose) are:
Ammonium C12-15 Pareth Sulfate (Cleaning and Foaming Agent)
Lauramidopropylamine Oxide (Cleaning and Foaming Agent)
SD Alcohol 3-A (Controls Thickness and Clarity)
Sodium Chloride (Controls Thickness)
Poloxamer 124 (Controls Thickness)
Fragrance (Pleasant Scent)
Methylisothiazolinone Benziosothiazolinone (Preservation)
Pentasodium Pentetate (Maintains Product Stability)
Sodium Bisulfite (Maintains Product Stability)
- Chlorinated tap water
- A little air
- Everything at around 30 C (a little warmer than room temperature)
- Seal the bag and shake. Bag volume increased approximately 2x, with no noticeable change in temperature.
I could observe a similar phenomenon in the past by sealing my palm over the open end of a dirty drinking glass containing dish soap water and air, shaking it, and feeling outward pressure on my palm. I don't recall what the glass was dirty from.
Today, under more controlled conditions, I used a clean bag, and a clean drinking glass, and I could not create any expansion. So I think either the "dirt" in the vessel contributed to the phenomenon, or the tap water was more chlorinated on the days where I observed the phenomenon, and that somehow contributed to expansion. In today's experiments, the water doesn't smell chlorinated.
If I can re-create the expansion later, I will note the conditions as well as I can.
At the request of a commenter, "under more controlled conditions" as described above (and notably where the water does not smell chlorinated), I added table salt, and it did not cause expansion. When the local water authority chlorinates the water again, I will retry the experiments.
A similar question has been posted at https://physics.stackexchange.com/questions/617113/why-does-pressure-in-a-thermos-increase-after-shaking-up-hot-water-and-soap