184 votes
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 ...
Floris's user avatar
  • 119k
163 votes

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 ...
Ian_Kinner's user avatar
141 votes
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 ...
John Rennie's user avatar
62 votes
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 ...
Physical Mathematics's user avatar
60 votes

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 $H$ ...
tparker's user avatar
  • 47.2k
38 votes

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 ...
lesnik's user avatar
  • 4,162
31 votes
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 ...
freecharly's user avatar
  • 15.8k
30 votes
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 ...
gandalf61's user avatar
  • 50.5k
29 votes

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 ...
Bob D's user avatar
  • 70.4k
28 votes

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 ...
niels nielsen's user avatar
27 votes
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....
niels nielsen's user avatar
23 votes

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 ...
valerio's user avatar
  • 16.2k
21 votes
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 ...
AlphaLife's user avatar
  • 11.5k
19 votes
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 ...
Thermodynamix's user avatar
17 votes
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. ...
nluigi's user avatar
  • 3,411
17 votes
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 ...
Nat's user avatar
  • 4,600
17 votes

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 ...
U3.1415926's user avatar
16 votes

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 ...
Johannes's user avatar
  • 19k
16 votes

How long would a bubble made in the ISS last?

@CountTo10's answer is great. There are two more factors that will pop the bubble that are worth considering. The first is that the space craft actively circulate air, necessary to prevent pockets of $...
Sean E. Lake's user avatar
  • 22.4k
16 votes

How are curved soap films stable?

Note that in your diagram you take a 2D slice in, let's say, the $xz$-plane where $z$ is the axis of symmetry and $x$ is the other axis you have chosen. This intersects the 2D soap film on a 1D line ...
CR Drost's user avatar
  • 37.6k
16 votes

How does pressure exist inside a drop of water?

The pressure outside a droplet arises from molecules bouncing against the droplet's surface, delivering an inward momentum "kick." Since the droplet doesn't shrink to nothing, there must be ...
Chemomechanics's user avatar
15 votes

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

One of the first places you might encounter tensors, at least one of the more classical applications of them, is in the mathematical theory of rheology. That is an indication that the processes of ...
Li Zhi's user avatar
  • 159
15 votes

How can I make edible gold sink in water without affecting the drinking-water quality?

It's floating because of surface tension, so your options are: reduce the surface tension with an edible surfactant (frequently used surfactants in cooking include: egg yolk, soy lecithin, and mustard)...
Sean E. Lake's user avatar
  • 22.4k
14 votes

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

I think #1 was answered very well by niels nielsen so to answer #2: Is there any plausible reason why a soap with foam can do better cleaning than the same soap without any foam? Time spent rubbing ...
MonkeyZeus's user avatar
14 votes

Are water molecules at the surface closer or farther apart than the molecules inside?

The usual attempt to explain surface tension shows two molecules in a surface and in the bulk, where the surface molecule only experiences attractive forces from one side, while the bulk molecule ...
oliver's user avatar
  • 7,432
14 votes

In physics, when should one use and not use calculus?

You equate the wrong things in the version without $dx$. Namely, you equate the total surface energy of the drop at radius $r$ with the energy to evaporate the entire drop at radius $r$. But what ...
Sebastian Riese's user avatar
13 votes

Could a fish swim out of a sphere of water in a zero-gravity environment?

The experiment has been done. Zero gravity is not required. Fish are observed to jump out of bodies of water on Earth.
John Doty's user avatar
  • 20.2k
12 votes

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

Molecules of soap are composed by one hydrophobic and one hydrophile end. They clean because the hydrophobic end sticks to dirt stuff that is normally greasy, while the hydrophile end allows the ...
Claudio Saspinski's user avatar

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