Explaining the Mundane IV

I started my undergraduate career set on getting a business degree from a local college and working as a branch manager somewhere. Then, I changed paths and decided I wanted to enter medical school and become a doctor. Like most 18-year-olds, I had no idea what I really wanted. As I started taking more science courses, I soon realized I had discovered my calling.

One appealing aspect of science and engineering was how seemingly mundane occurrences could be explained by scientific concepts that apply to a wide range of phenomena. I could wonder about the tiniest details of everyday life and try to use my knowledge of physics to explain how stuff works. I learned a lot this way and enjoyed testing my physical intuition. I’ve gathered a few of my favorite mundane events that are explained by seemingly-esoteric (but ultimately intuitive) scientific concepts.

Why are root beer floats so foamy?

Mmm…let’s first all take a moment to think back to warm summer nights when our parents would come home from work with a single brown paper bag. Inside the bag was sweet, caramel-colored root beer and delicious golden vanilla ice cream. You grab a frozen mug, scoop one, two, three giant spoonfuls of ice cream, and pour cold soda carefully along the side of the glass. The result: a creamy, foamy concoction most people call a root beer float. Most likely, a lot of us had similar memories (maybe with a slight variation, like the purple cow, which has grape soda instead of root beer), and we all might have stopped to think where all that foam comes from.

Soda and other fizzy drinks have dissolved carbon dioxide in them. To get the typically gaseous compound into sodas, a large amount of pressure is required. A high pressure is needed to get carbon dioxide in, and when the pressure is removed (like when a can of soda is opened), gas bubbles start escaping. Typically, this happens slowly, but if you shake your can of soda, gas bubbles bump into the walls of the can. The walls act as “nucleation sites”, which is just a place for something (in this case, carbon dioxide bubbles) to start to form. When you open the can, the bubbles attached to the wall also act as nucleation sites, wanting to expand, set off a chain reaction, and cause a messy explosion.

When ice cream and soda are mixed, the ice cream 1) provides an additional nucleation site for carbon dioxide bubbles, 2) melts due to the soda, which uncovers microscopic air pockets that further enhance nucleation, and 3) contains fatty compounds and proteins that coat the bubbles and trap them at the top when they float. So, that top foamy layer is a fatty, bubbly mixture that forms due to nucleation of carbon dioxide on the ice cream.

The fat and proteins encapsulate the carbon dioxide in a similar way as a soap bubble forms. A soap bubble has two layers of thin layers of soap molecules sandwiching a layer of water. The water-loving functional groups in the soap will point toward the water layer while the water-adverse groups will point away from the water. Oleic acid, for example, is common in cream (used to make ice cream) and has a carboxylic acid end group that loves water and a carbon chain that will avoid water. A bubble acts like an elastic sheet with a surface tension that resists external forces. Surfactants like soap actually reduce the surface tension of water though by increasing the space between water molecules. However, a soap bubble (and by analog, a root beer float bubble) lasts longer in relation to a water bubble (or just a normal root beer bubble) because soap stabilizes the bubbles through the Marangoni effect.

External factors like gravity act upon the bubble as soon as it forms. This means that the top part of the bubble gets thinner as the liquids flow down. If a bubble gets too thin, it will pop! The Marangoni effect explains that since water has a higher surface tension than soap, a bubble has a surface tension gradient (similar to a temperature gradient, like when you touch a hot stove and the heat transfers to your colder hand). The higher surface tension water pulls on the lower surface tension soap, and the soap flows toward the water – towards the thin top of the bubble. The soap boosts the overall integrity of the bubble by strengthening where the bubble is the weakest.

It’s hard to say exactly what compound in ice cream (although it’s likely contained in the milk fat) is contributing to the bubbles’ coating, but a good comparison can be made between the ice cream foam and sea foam, which is due to protein and fat from aquatic microorganisms. Now, it’s time to go get some root beer and ice cream.

How does ironing clothes smooth out the wrinkles?

Clothes are made out of polymers. Cotton, polyester, wool, silk, nylon: these are all polymers (some natural, some synthetic). Polymers differ from other materials in their chemical structure and the resultant properties. Instead of single atoms sharing electrons to make a structured material (like diamond, which is just carbon bonded to other carbon in a neat array), polymers can consist of many different atoms that form a long, repeating chain. If take a single paper clip, and then loop another paper clip to the end (and another and another), you’ll eventually have a perfect analogy to a polymer.

Since polymer chains are so long, there are many different shapes the individual chain can take. If you take your paper clip chain, you could extend it so that it stretches straight. You could also fold it in half. Or crumple it up. With a polymer, the individual units can dictate the shape it will take. Are the individual paper clips big or small? Are they made of magnetic material so they attract or repel one another? Polymers often orient randomly and can be weakly bonded to other nearby chains, like loosely clumped paper clip chains, but other environmental factors (e.g., temperature) can also impact the structure.

Ironing clothes increases the temperature. Temperature can be simply thought of as a measurement of how much molecules are moving. When temperatures a really high, molecules are zipping around really fast. When temperatures are low, molecular motion can be nearly frozen; at absolute zero, a single atom would be completely stationary. When the iron is hot enough, the temperature causes the polymeric fibers to move around and break the weak bonds they have with their neighbors. As the temperature cools again, the polymers form new bonds.

This is also why clothes straight from the dryer become wrinkled after they sit in a pile. For natural fibers like cotton (and other cellulose-based fabrics), water can also play a role in wrinkling. Cellulose contains oxygen, which likes to make strong bonds with hydrogen. Hydrogen in relatively tiny water molecules can slip between polymer chains and attach itself to the oxygen. When the water squeezes in, the polymer chains change their shape. Eventually, the water evaporates, but the fabric stays wrinkled. That’s why most modern irons have a steaming function.

Another interesting aspect of the science behind ironing and wrinkling is permanent press, which is a modern method of increasing the wrinkle resistance of fabric polymers. To do this, another small molecule, which often contains hydrogen and oxygen (just like water) but also likes to keep a stiff shape, is added while manufacturing clothing. This small molecule forms “crosslinks”, which attach one long polymer chain to another long polymer chain . An example is given in the image below (with the polymer represented as “fiber” on the image). You can see that the small molecule looks almost like a person with long arms grabbing two giant polymer chains and holding them tightly.  This helps the clothing resist wrinkling due to water. The permanent press cycle on a dryer lets the dryer cool down first since hot clothes that sit in a pile can still wrinkle.

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