Explaining the Mundane V

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.

How does “anti-fog” work?

It’s happened to you before. You are up in space, working on the International Space Station and doing some really awesome science when your helmet visor fogs up. The temperature difference of the glass and your warm, perspiring body temperature causes water vapor in the air to start forming little droplets, a process called condensation. It’s just like on a hot day when a nice ice cold glass of sweet tea begins to sweat; the cold beverage cools the surrounding air and makes the water in the air go from a vapor to a liquid. And you can’t wipe it off from the inside; you’re trapped in your suit.

Okay, so maybe you haven’t had that exact problem, but you might have had some glasses blur your vision a little after you’ve worked up a sweat or had a car windshield get dangerously opaque. Luckily, some smart engineers at NASA were tasked with developing anti-fog technology after an astronaut had an experience similar to the one above.

There are two ways that anti-fog works. First, the concept of “surface tension” should be mentioned. Surface tension is why water, in the absence of the effects of gravity, forms a perfectly spherical drop. Water would rather be next to itself than the surrounding air. So, a water molecule in the center of the drop is pulled on in all directions by other molecules trying to get closer to it. A water molecule on the outside edges is only pulled on from the center and is stretched along the edges just like a balloon. A sphere is the best shape because it minimizes the stretching.

By adding something soapy, the surface tension of water can be decreased. The soapy part doesn’t actually want to be in the liquid and will squeeze between water molecules, disrupting their pull on each other. This would cause the tiny droplets of water to spread out into a really thin coating, just like a soap bubble. That’s one way to get anti-fog; add a soapy material to lower the surface tension of water and spread out the droplets.

Lotus effect - a perfect droplet forms on a hydrophilic leaf (from Wikipedia)

Lotus effect – a perfect droplet forms on a hydrophilic leaf (from Wikipedia)

Another way to get anti-fogging is to make the glass hydrophilic. Hydrophilic comes from hydr-, which means water, and –philos, which means love. Hydrophilic means a surface that loves water. This type of coating can be explained by explaining the opposite: a hydrophobic coating. The lotus flower is self-cleaning; its high water repellence doesn’t let any water adhere to the surface, forming a nearly perfect drop on its leaf. Small dirt particles are picked up by the water droplets as they slide off of the leaf, giving rise to what is called the Lotus effect. A hydrophilic surface would cause just the opposite effect: water molecules love the surface and want to be next to it even more than each other, causing any drop to collapse and spread as a thin, invisible film, perfect for anti-fog coatings.

Does wine have “legs”?

Tears of wine (from Wikipedia)

Tears of wine (from Wikipedia)

The next time you are enjoying a glass of wine (especially one with a high alcohol content) check above the liquid to see if there is a crown of droplets dripping back into the wine. If a quick science experiment doesn’t sound like your ideal way of relaxing with your vintage cabernet, I’ll do the dirty work and explain the effect here. Sometimes called “tears of wine”, “curtains”, or “church windows”, the “legs” that form above the rich, fermented grape juice is due to something called the Marangoni effect.

Remember that whole concept of surface tension that was explained above? Water molecules pull on each other and impart a high surface tension, while soapy molecules have a lower surface tension and can lower the surface tension of water. As it turns out, alcohol is more like soap than water in this aspect; it more often would rather be next to other things than itself.

Wine is just a mixture of water and alcohol. It can climb up the glass wall by capillary action, a gravity-defying phenomenon that is one way paper towels can suck up spills. When it does so, some water evaporates and some alcohol evaporates. If you’ve ever taken a whiff of a bottle of tequila, you probably could guess that the alcohol will evaporate more quickly than the water. Essentially, this creates two different regions on the walls of the glass: one with more alcohol (right next to the top layer of wine) and one with more water (climbing higher and higher by capillary action).

The higher surface tension of the “more water” region lets it pull harder on the water that is in the “more alcohol” region. Eventually, the “more water” region will get too heavy, form drop lets, and flow back down the side of the glass. This gives an effect that looks like tears on the glass walls.

You could actually do your own experiment to see the Marangoni effect in action. Get a shallow dish and fill it with water. Add a little bit of pepper for visual effect. Take a single drop of soap and dab it right in the center of your pepper puddle. The pepper should instantly shoot away from the soap. You are creating a region in the center with low surface tension surrounded by a region of higher surface tension along the edges. The higher surface tension pulls on the fluid in the center in the same way that would cause water droplets to form above a glass of wine.

Water and pepper

Water and pepper

Add a drop of soap

Add a drop of soap

And watch!

And watch!

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