Knudsen force

Hello there! So, have you ever tried to blow up a balloon and noticed that the air is slowly escaping through the tiny opening even though you are not blowing anymore? That's because of something called Knudsen force!

You see, all the air particles inside the balloon are moving around very quickly and bouncing off each other. Some of these particles are hitting the surface of the balloon and bouncing back in, while others are escaping through the small opening. However, the particles that are escaping are doing so because they are moving faster than the ones that are staying inside.

Now, imagine if you were to make the opening even smaller, like the kind of opening you might find in a very small hole or pore. The fast-moving particles that are trying to escape will start bumping into each other more and more as they try to squeeze through. They will eventually slow down and come to a stop, forming a layer of gas close to the surface of the hole. But, because this layer is made up of slower-moving particles, some of the faster-moving particles in the air above it will be able to push against it and move through the opening.

This pressure that the fast-moving particles exert on the slow-moving ones is known as Knudsen force. It's like a little invisible pushing force that helps some particles escape through very small openings, even when they would normally be too slow to do so on their own. This force is very important in many natural and man-made systems, especially when we need to control the flow of gases in very small spaces.