# Tag Info

Accepted

### When separating an Oreo cookie, why does the cream stick to just one side only?

The "stuff" sticks to itself better than it sticks to the cookie. Now if you pull the cookies apart, you create a region of local stress, and one of the two interfaces will begin to unstick. At that ...
• 119k

### Can ants walk on liquid mercury?

Needing any excuse to break away from the work I was doing, I immediately assumed the task of answering this question. Yes, ants can walk on mercury with no trouble at all. I bet it was even kind of ...
• 471
Accepted

### Why doesn't water actually perfectly wet glass?

In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract ...
• 354k
Accepted

### Can ants walk on liquid mercury?

Whether or not a small animal/insect can walk on a liquid is determined much more by surface tension than by density. To see why this is consider a dense liquid without any surface tension. You would ...
• 1,036

### If the pressure inside and outside a balloon balance, then why does air leave when it pops?

For an inflated and tied balloon, the inner and outer pressures aren't equal. The inner pressure is higher by an amount $2 \gamma |H|$, where $\gamma$ is the inflated balloon's surface tension and $H$ ...
• 47.2k

### Why do water drops form spheres in space?

No, it's not because of gravity. You need to take quite a lot of water in order for gravitational effects to become significant. It's because of surface tension. Sphere is a shape which minimizes the ...
• 4,162
Accepted

### What causes the water in this fountain to reverse direction?

This is probably caused by a ring-shaped whirl of air (similar to a "smoke ring") under the water sheet which is driven by the speed of the water. This whirl flow produces a lower air pressure at the ...
• 15.8k
Accepted

### How does an air bubble have 2 surfaces and a liquid drop has 1 surface?

Think about what a microscopic observer would experience as they moved from somewhere outside the bubble to its centre. With an air bubble they go from air (outside) to water (the "shell" of ...
• 50.5k

### If the pressure inside and outside a balloon balance, then why does air leave when it pops?

But after that equality is obtained why does the air goes out when we penetrate the balloon? If there is pressure equality what causes the air flow? You need to take into account that the elastic ...
• 70.4k

### Why do water particles appear to be exiting a stream of water from a faucet, instead of the stream being held together by surface tension?

Did your faucet nozzle contain a bubbler (which mixes air into the water and produces a white, frothy stream down to the bottom of the sink)? Most kitchen faucets do, and when the bubbles closest to ...
• 92.1k
Accepted

### How does rubbing soap on wet skin produce foam, and does it really enhance cleaning?

The soap bubbles are a side-effect of the cleaning process. It is the mixing of air with the soapy water, and the film stability of the resulting bubble walls, that generates and maintains the bubbles....
• 92.1k

### Can a droplet of water bounce back when it hits a water surface?

Yes, as you can see in this video. As you can see, the droplet will hit the surface, partially coalesce (merge) with the bulk, re-emerge as a smaller droplet, bounce 1-3 times, partially coalesce ...
• 16.2k
Accepted

### Why does water fall sort of helically from a cup?

You have made a very interesting observation. In short, this is due to the shape oscillations of the flowing stream of water. I did the experiment and observed the pattern too. But I'd say that the ...
• 11.5k
Accepted

### Why do bubbles initially stick to the side or the bottom of a glass while boiling?

The bubbles are already on the surface, they are just too small to see with the naked eye. Wetting a surface, even at room temperature, results in tiny gas/vapor bubbles at defect sites due to ...
• 1,742
Accepted

### Does surface tension play a role in planet's shape?

Let's throw some numbers at this. The Eotvos (or Bond) number is a dimensionless ratio of the body forces to surface tension forces often used in the sciences to characterize certain flows regimes. ...
• 3,411
Accepted

### Does soap repel water?

tl;dr- When the soap touched the water, part of it dissolved, reducing the local surface tension. Since the higher surface tension further away from the soap pulled more strongly than the weaker ...
• 4,600

### How does an air bubble have 2 surfaces and a liquid drop has 1 surface?

I think there is a little confusion here between a bubble and a membrane. What is referred here as an air bubble, is actually a membrane of liquid form enclosing a volume of gas, and floating inside ...
• 311

### Does surface tension play a role in planet's shape?

The gravitational binding energy for a spherical object of mass $M$ and radius $R$ is given by: $$E_{grav}=\frac35 \frac{GM^2}{R}$$ The interfacial energy for a spherical droplet is simply ...
• 19k

• 11.5k