# Why do soap bubbles form?

When you open the fosset and water flows into a glass, bubbles form but are shortlived. Now try the same thing but add a drop of soap to your glass before opening the fosset. Now the bubbles formed are larger and stay much longer. Why does soap enhance bubble formation and lifetime? The answer lies within the structure of soap molecules. Macroscopic bubbles emerge from the interaction between water molecules and soap molecules.

# Inside a soap bubble

Each soap molecule contains 2 parts: a hydrophilic (water loving) head and a hydrophobic (water hating) tail. So when soap encounters water, the heads of the molecules bind to water molecules. This causes the soap-water mixture to assemble into bubbles containing trapped water molecules bound by soap molecule heads, and air trapped at the center of the molecule(see below).

Soap is not just useful for making bubble baths. It also removes dirt. When water containing soap encounters hard to remove oils, the water loving head and water hating tail separate oil from water, and oils get trapped at the center of the bubble. My creative students describe the relation between soap, water, and oil as a "love triangle" and "That's all they are going to think about in the shower." This is great, as I want my students thinking about the fascinating science behind routine everyday activities!

# Why are soap bubbles spherical?

Just as soap bubbles emerge from the interaction between soap and water at the molecular scale, bubble shape is also an emergent property. The reason for this emergent property lies in what is called surface tension. Surface tension is simply water (or any other molecule) preferring to stick to each other (cohesion) as opposed to sticking to another surface (adhesion) such as air or glass. Surface tension leads to the generation of shapes which minimize surface area. Turns out a spherical shape minimizes surface area as opposed to any other shape. An example calculation shows that for a droplet of volume 1 cm3, a sphere has lower surface area as opposed to a cylinder of height 1 cm and same volume.

Soap decreases the water-air surface tension by binding to water molecules at the surface, thereby making it possible to have stable longer lasting bubbles. In fact, it's possible to make huge long lasting soap bubbles by adding polymers like guar gum or Polyethylene glycol, which is available in commercial lubricant products like J-lube. A recent scientific study found that these polymers become entangled like a mess of pants after the laundry, and can make huge car-sized long-lasting bubbles!

So how do you make large bubbles? To make the bubble solution, you need to mix 3 liters of warm water, 200 ml of dishwashing liquid dawn (1/3rd of a bottle), and a quarter teaspoon of J-lube. For the large bubble wand, buy a metal hanger and wrap it tightly with cotton twine as in the image below. Make sure to tie the ends together at the end of the wand so that the twine does not unravel. And that's it! Dip your wand in your bubble solution and wave in the air to make large bubbles!

In summary, the science of soap bubbles comes down to how individual water and soap molecules interact. Manipulating these interactions can help make larger, longer lasting bubbles. Now that you know the recipe as well as the science behind making large soap bubbles, its time to have fun! See the video below for a demonstration (and potential lab!)

# Slides

# Lab Activities