Monday, January 26, 2009

Why do bubbles form in water

How and why do bubbles form in water?

Son and dad played with some water the other day. First, we blew into a glass of tap water with a straw. A few bubbles formed on the surface and quickly popped.

Why?

Surface tension.

Surface tension arises from the fact that a liquid is held together by cohesion. The molecules that make up the liquid are attracted to all adjacent molecules. Well, at the surface of a liquid, there are fewer adjacent molecules. This means the cohesive forces between the remaining adjacent molecules are enhanced. This is surface tension.

When you blow air through the water, the air displaces the water and we see buoyancy in effect. We also see surface tension take action (yes, even at the bottom of the glass of water). This is because there is a surface at the air-liquid interface, and the water molecules on that surface have fewer adjacent molecules; the forces are enhanced. The tension on the bubble would like to be minimized and the best way to do that is to maximize the surface area over which the force is distributed, giving you a roughly spherical shape to the air bubble.

So, due to surface tension a roughly spherical bubble forms. Due to buoyancy, that bubble moves upward through the water.

Why does a bubble form at the surface? Surface tension again. When the air moves through the top "layer" of water, the water tries to stick together and a bubble forms.

It pops so quickly because the surface tension of water is so great that the water is pulled back to itself quickly. Here's a poor-quality, high-speed-camera video of a soap bubble popping. If you squint and watch closely, you'll see that the bubble does not just collapse. It pulls back upon itself, like a retracting dome. This is due to the surface tension keeping the water together.


video

So, after that, we put a little dish soap into the water and again blew air through. This caused a lot more bubbles to form on the surface, and most of those bubbles lasted a long time.

Why?

Surface tension.

Dish soap is a detergent, which decreases surface tension. Because the surface tension is decreased, the bubbles on the surface of the water are able to stay together longer; the cohesive forces are not as strong now that the detergent has been mixed in and the water+soap doesn't pull itself back together as quickly. If the surface tension was 0, the liquid would be a gas...

Detergents are used specifically because they lower the surface tension of water, which stops its beading behavior and allows it to soak into clothing or better dissolve junk on dirty dishes. Also, higher temperatures decrease water's surface tension, allowing better cleaning behavior.

4 comments:

Jennifer said...

Do you just know all this stuff, or do you have to do research for these posts? Either way, your brain amazes me. Also, I love the astronomy pic of the day!

I am Moses. said...

Well, the nuclear stuff I had to do a lot of reading to make sure I got (most of) the details right. I'm sure I missed a lot and I know I overly simplified a lot. Also, I'm still missing a nuclear post.

Almost anything that includes biology is just me faking it after reading but barely understanding.

The other stuff I generally just need a little reminder of the equations or concepts or whatever. That's why biology is so difficult; there are no equations... ;) There are obviously times when I get things wrong or simplify so much that it's incorrect, but I think for the most part these posts are correct and possibly even illuminating.

The hard part is making it at all understandable to people who don't do this every day. Obviously the early posts full of equations were learning experiences... :)

I like to search for interesting links to include, and I'll often (re-)discover something cool while doing so.

So... Yes?

deborah said...
This comment has been removed by the author.
deborah said...

I had a spelling error and posted before it was corrected. So what I want to know is if my granson understood all of this? And my question is ...oh wise one =)..How come soap bubbles seem to float if you blow on them and keep their shape in the air for a length of time? Does that also have to do with surface tension?