MadSci Network: Chemistry

Re: Would bubbles ever flow downwards?

Area: Chemistry
Posted By: Philip Plait, Astronomer/Programmer
Date: Mon Aug 11 15:19:57 1997
Area of science: Chemistry
ID: 869565841.Ch

Bubbles can indeed flow downwards, but you have to trick them!

Bubbles flow upwards because they are less dense than the liquid they are in. They "know" which way down is because that is the way gravity pulls them, and they tend to flow away from that direction.

Gravity does more than just pull on you, it accelerates you. In other words, if the ground were to suddenly drop away, you wouldn't simply fall at one speed, you would fall faster and faster as time went on. It is the acceleration of gravity that the bubbles feel, and they flow in the other direction.

So you could make bubbles flow down if you could trick them into thinking gravity was pulling them up. They would respond by flowing down, the opposite direction. But how can you do that? Well, have you ever noticed that when you sit in a car that is accelerating forward, you feel a backwards push into your seat? And when it slows down, you feel a push forwards. The direction you feel the push in is opposite the direction of the acceleration. So if you could accelerate the bottle with the soda downwards, the bubbles would feel a push upwards, and try to move against it. They would flow down! The problem with this is the bottle would hit the ground pretty quickly and you'd have a messy experiment.

I noticed something like this just the other day. I bought a helium ballon for my daughter, and was driving home in my car. I had to stop quickly, and while I was pushed forward, the balloon moved to the back of the car! Like the bubbles in a soda bottle, the helium in the balloon is lighter than air and wants to move opposite the direction of gravity. The sudden stopping of my car tricked the balloon for a second, making it feel an acceleration forwards. So it went in the opposite direction. Actually, it was weird to see it move in the opposite direction than everything else in the car. You can do this yourself. Ask your folks to drive you around with a helium balloon in the car, and see how it behaves. You'll be surprised!

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