Why is the head of a pint of beer white not amber?
I discussed how water entraps air bubbles in the post when is water white.
Disorganized and different-sized air bubbles reflect white light back to us.
Pockets of air are not only encompassed with water. Other natural materials hold air bubbles hostage. For example, molecules such as protein, fats, and soaps are really good at this. Even glass (feature image) can trap air bubbles. The images above show:
- Egg white proteins forming whipped meringue.
- Fermentation proteins of beer forming the “head” of a pint of beer. The gas that produces the "head" in a pint of beer is carbon dioxide, though nitrogen is sometimes added to produce a longer lasting, less acidic taste.
- Fat globules of whipped cream: one needs at least 30% fat in cream to create enough air bubbles that are held in place for a whipped texture.
- Detergents/soap foams: A white foam develops when we use soap to lather our hands. Contrast the picture of the white foam with its multitude of bubbles with the see-through single soap bubble shown here.
Polar bear skin is black not white
What are other ways we can create white color in our natural world?
There are two well-know examples: Polar bears and the Mynah birds with their ultra-white feathers. What these two creatures have in common is that their protective covering encases air pockets in translucent materials.
- Polar bear skin is not white. The hairs that make up the bear's fur are transparent and hollow; this fur covers a black skin! However, when sunlight hits a polar bear's fur, the light scatters from the air mass within the straw-like strands of hair. The result is a white coat.
- Mynah bird feathers are note white: the feathers of the Bali Mynah bird tend to be hollow entrapping pockets of air.
The common thread through those elements is that their molecules form a casing that entraps air. In both the fur and feather examples, the casing is made of protein molecules that are form of keratin. This is the same protein that makes up our hair and nails.
What is the biggest soap bubble one can make?
On another note, one can make single large bubbles that maintain their shape for a longer time. One way of doing this is through adding corn syrup to the soapy mixture.
Keep in mind that the netting that holds air bubbles in place - in soap, cream, or egg whites - contains a layer of water. When one incorporates a sugary molecular substance into that water layer, everything becomes many times more cohesive.
The increased stickiness of the water layer allows the soap bubble to expand to considerable dimensions without popping.
Moreover, since the glucose molecules attract the water molecules, the soap bubble takes longer to dry out. It lasts longer without popping.
- Whipped cream that lasts for days
Both techniques will result in whipped cream that has stiffer peaks that keep their shape for longer than 1 day.
- Sea foam and a beer head are white for the same reasons
The same concept - adding cohesive molecules to foamy liquids makes the foaming bubbles last longer - is the reason sea foam gets created.
Proteins from broken down biodegradable material, like algae, mixes with the water. Thin layers of protein and water hold innumerable bubbles in place.
The result is a long lasting foam.
Conclusion and related articles
In effect, gas molecules (like nitrogen N2 and Oxygen O2) reflect light.
Keep in mind that air - like most gasses - is colorless. It doesn't absorb any wavelengths of light. Without a visible container encompassing gas molecules, the gas would be transparent.
However, an interface develops when bubbles of gas are entrapped within another material. Even if the substance holding the bubbles in place is transparent (like water), light can be bounced at the level of the interface.
Some of the light echoes back to our eyes. This creates the whiteness, as if the bubbles were painted.
Thus, to have whiteness due to reflection off gas molecules, three things must take place:
- Gas bubbles are held captive in the netting of a material (water, protein, fat, or soap)
- Light photons hit the gas molecules and bounce around haphazardly
- Light is echoed without absorption; It comes back as white light
"Whiteness" is produced depending on the level of chaos around bubbles. The more refraction (bouncing around) of light, the whiter the bubbles will appear.
- How about white butterfly wings?
Keep in mind, that not all white things in nature are due to reflection off a bunch of gas bubbles. The microscopic structure and arrangement of the fibers can reflect light back to us, creating a structural white color.
Examples include wings of butterflies or bright-white beetles or the white bark of birch trees. In some of those cases, the actual material is colorless and transparent. However, because it scatters white light, it will appear white.
Finally, some flowers create a pigment that echoes all light; , creating white flowers.
- By LVV. Transparent glass ball with air bubbles inside on a blue background. Shutterstock. ID: 45809566
- Scott Mindeaux. whipped_egg_whites. Flickr photo-sharing. Taken July 6, 2005.
- Martin Garrido. Beer. Flickr - photosharing. Taken on Dec 27, 2011.
- Brian Teutsch. Whipped cream. Flickr - photo sharing, taken on Nov 22, 2007.
- By jocic. Soapy foam on man's hand. Shutterstock. ID: 160713749.
- kismihok. Bubble: Giant soap bubble and the sky before storm. Flickr photo-sharing. Taken July 17, 2012.
- Jessica Merz. Polar bear. Flickr photo-sharing. Taken on Feb 20, 2006.
- Charles Barilleaux Bali Mynah. Flickr photosharing..March 31, 2014
- elPadawan. Giant Soap Bubble: Giant soap bubble outside the New Yorker shop, Můstek subway station, Prague. Flickr photo-sharing. July 12, 2012.
- James St. John. Lake foam along the shoreline of Storr's Lake (San Salvador Island, Bahamas) . Flickr photo-sharing; taken on March 24, 2007. ".... The dissolved organic matter (from biodegraded Algae) increases the surface tension of the water, so wind-induced wave agitation creates more trapped bubbles in the water, resulting in a relatively persistent foam."
- Jevgenijs Slihto. Birch. Flickr - photosharing. Taken on March 7, 2015