There are a lot of misconceptions related to hydrostatic pressure. One of them is that if you took a jug like the one below (or a U-tube, as in my post on letter tubes and misconceptions around hydrostatic pressure) the water level would have to be higher in the narrow snout of the jug than in the main body. So when I saw a cheap-ish fat separator jug recently, I had to get it “for my blog” (ok, because I wanted to play with it) to show that water, indeed, seeks its level.
But it turns out it is really difficult to take pictures of the water level! My first attempt (above) was with dyed water because I thought that might make it easier to see what is going on. Turns out that the adhesion of water makes it really difficult to observe the water level: The water is pulled up along the walls of the jug, leading to these weird changes in color.
In the picture below, taken from slightly above water level, you can see the curvature of the water surface both in the main body of the jug and in the spout:
Using clear water turns out to be the best way to photograph this phenomenon (below).
Who has an understanding of the effect of washing-up liquid on surface tension on a molecular level? Please help!
I’ve recently shown a lot of experiments on the topic of surface tension. And while it is a helpful analogy to think of a thin membrane on top of the water that lets water striders or paper clips sit on the water and not sink, and that rips open when washing-up liquid is introduced – this is really not satisfactory to me. But I am having a hard time understanding surface tension on a molecular level.
So let’s go back to the basics. Water molecules have a polar structure that allows each water molecule to form up to four hydrogen bonds to neighboring water molecules. A water molecule in midst of other water molecules will hence experience strong cohesive forces in all directions, which vanish in sum.
A water molecule at the very water surface will only experience strong cohesive forces from water molecules underneath it or from water molecules sitting at the surface right next to it, since there are no (or hardly any) water molecules above it, and adhesion with air molecules is much less strong. A water molecule at the surface will hence not easily leave the surface, and the surface itself will try to minimize its area, since that’s the best configuration energetically. If small weights are put on the water surface, the water surface will be deformed slightly, but not break, and this behavior will indeed look similar to a membrane spread over the water surface.
So far, so good. But why does this “membrane” rip when we put washing-up liquid on it?
That is because the molecules in dish soap attach to the uppermost water molecules with their hydrophil end, hence the resulting force on the uppermost layer of water molecules isn’t downward any more. The hydrophob ends of the soap molecules make sure the soap stays at the surface and prohibit formation of a new “membrane”.
Do you hate these graphics, too? My excuse: no computer allowed on this vacation and the ipad doesn’t have a better app. Do you have suggestions for ipad graphics apps that can deal with typed text well? I’m all ears!
Occasionally one notices water levels in straws that are slightly above the water levels in the glass. And of course – even though we always talk about water seeking its level and hydrostatics and stuff – we know that that’s how it should be because of the capillary effects. And then we probably all did that experiment in school where we had a very thin glass tube and the water rose really really high. But have you ever wondered how heights between straws with different diameters would differ? (Really? Only me?)
Anyway, here is how:
I do realize that the diameter of “typical” straws differs from country to country, but these are the Norwegian – and German – typical straws, so I herewith define this as universally typical. Anyway, from left to right: 8mm, 4mm and 3mm diameter on the outside. Unfortunately I don’t have the tools to measure the inner diameter. Plus I really need to get clear thin straws! Sorry the water level is so hard to see in the yellow straw – I even dyed the water for you!
But even with the imperfect materials I have – isn’t this quite an impressive result?
Btw, this is what it looked like when I did the experiment in my kitchen.
Remember how in this post my parents sent me a picture of the experiment that I didn’t get to work out?
Later the same day they sent me the movie below, demonstrating first how to put stuff on the surface without it sinking, and then how to destroy the surface tension using a tooth pick that is dipped in washing-up liquid.
Isn’t it curious how sometimes the surface tension breaks down right away and sometimes it doesn’t? I need might have to try this for myself again. Like right now. It’s bugging me so much that it didn’t work the first time round!
Lots of stuff an be made to float on water just because of surface tension.
This morning, I was taking pictures of heaps of waters on coins. I was planning to follow up on that post with pictures of a dome of water on a full mug. So far so good.
Then, I was planning on putting paper clips on top to show how surface tension would keep them afloat.
Except it DID NOT WORK. Maybe there was dish soap residue in the glass? Maybe I was too clumsy? I have no idea what was wrong. Anyway, I was on the phone with my mom later that day, and within half an hour I had the picture below in my inbox.
I guess you can make almost anything float on the surface if you put your mind to it… ;-)
More pictures from the same spot at the banks of the Pinnau.
Looking more closely, you can see the water strider:
And now a real close-up from the pond in my parents’ garden (because those pesky little bugs are too fast to take pictures off when you are ashore and they are on the water, and the water is wider than a meter in each direction).
See how you can see the impression its feet make on the water surface?