This post is based on a demonstration I gave in the Skepchick Space Lab at CONvergence 2014 on the science of bubbles. If you saw the demo and came to check out the blog, welcome! (My post about the molecular gastronomy Sandbox will be up next week!) Below is a video of the demo, filmed by our wonderful volunteer Chris Pederson. A written version of the presentation is below the fold, followed by a recipe for the cocktail.
Water molecules consist of two hydrogen atoms and one oxygen atom—H2O. The molecule looks like sort of like a Mickey Mouse head, with the oxygen atom making up the face and the hydrogen atoms serving as the ears. It turns out that the single electron in each of those hydrogen atoms tends to hang out at the bottom of the ear, near the oxygen atom. This means that the “top” of the molecule is positively charged. Meanwhile, the eight electrons in the oxygen atom tend to hang out toward the bottom of the face, away from the hydrogen atoms. This means the “bottom” of the molecule is negatively charged. Consequently, in a volume of water you’ve got molecules with one positively charged end and one negatively charged end floating around together. This results in a big tug of war, with attraction taking place and molecules pulling on one another from all directions.
Except, that is, for at the surface of the volume of water, because there are no molecules above those at the surface to pull up on them, so they are only pulled down. This causes surface tension. You’ve seen surface tension in action if you’ve ever poured water into a glass all the way up to the brim and seen how, instead of sitting flat or spilling over immediately, the water forms a curved surface called a meniscus. This happens because the molecules below those at the surface are pulling down on the ones above them, holding them in place.
Surface tension is what allows bubbles to form and gives them their spherical shape. There’s a specific reason you’ve never seen bubbles shaped like cubes, pyramids, or any other shape besides a sphere. To understand why, let’s compare a bubble to a balloon. Each consists of a thin skin that surrounds a volume of air. When a rubber balloon deflates, the skin goes slack. However, thanks to surface tension, the skin of a bubble stays stretchy no matter the volume of air it contains, which means that it will always shrink down to the smallest surface area possible that will contain that volume of air. And it turns out that the geometric shapes that allows it to minimize its surface area is a sphere.
When two bubbles meet, they merge to share the wall between them, because it allows them to further minimize that surface area. Bubbles always merge at 120° angles. This means that when you have a cluster of bubbles, they will form hexagons, with one bubble in the middle and six around. The same principal explains why honeycomb is made up of hexagonal cells: hexagons allow bees to be efficient and minimize the number of walls they need to build and therefore the amount of wax they need to use.
If you were to blow into a straw in a glass of water, bubbles would form but immediately pop—the surface tension is too high for them to remain stable. However, there are ways to stabilize bubbles, by adding substances that lower that surface tension. One stable bubble everyone should be familiar with is a soap bubble. Soap molecules are made up of chains of hydrogren and carbon atoms. One end of this chain is hydrophilic, which means it loves being in water. The other end is hydrophobic, meaning that it shuns water. It’s attracted to grease instead, which I what allows to soap to work to clean things in the first place. The hydrophobic end of the soap molecule wants to escape the water, so it pushes its way to the surface, pushing past the water molecules, separating them and relieving some of that tensions caused by the attraction between them.
There are substances besides soap that lower surface tension as well. In the case of the bubbles I served in the Skepchick Space Lab at CONvergence, I added two ingredients to help stabilize the bubbles. The first is Versawhip, a modified soy protein that acts to lower surface tension The second is Xanthan gum, a thickener that serves to make the skin of the bubbles sturdier and less prone to popping.
Bubble Universe Cocktail
Cranberry bubbles like the ones I served in the Skepchick Space Lab are a perfect garnish for a class Cosmopolitan cocktails—which is even thematically appropriate if we shorten the name to “Cosmos!” Below is a recipe for a tasty Cosmos and directions for the beautiful foam that turns these cosmos into a Bubble Universe.
2 oz Vodka
1 oz Cranberry juice
1 oz Cointreau or other orange liqueur
1 oz Rose’s Lime
Combine all ingredients in a cocktail shaker filled with ice. Shake to chill, then strain into a cocktail glass.
Cranberry bubbles (recipe from MolecularRecipes.com):
380 g cranberry juice
1 g Versawhip
1 g Xanthan gum
An aquarium air pump and a length of clean PVC tubing
- Blend the cranberry juice and Versawhip using an immersion blender.
- Add the Xanthan gum and blend again until fully dissolved.
- Connect the tubing to the air pump and insert the other end in the cranberry mixture.
- Turn the pump on and let bubbles collect. Allow bubbles to sit for at least thirty seconds to stabilize (you can let the pump keep running during this time).
- Scoop the bubbles onto your cocktail.
Bubble science source:
Featured photo by Shahveer Press.