It seems as if my experimental period is still not over now.
- Is water always wet?
- Does fabric always gets wet when you dip it into a fluid?
- Does rain always make leaves wet?
- What do detergents do?
- Does a sponge soak up every kind of fluid?
- Why does chalk stick on a a chalkboard? Why not when the chalkboard is greasy?
All these questions have in common, that there are several kinds of material that come into close contact.
There are adhesive forces that are responsible for different things sticking together. Between all materials there are such forces, but they are stronger or less strong depending on what materials get in contact.
There are also forces between the molecules of the same kind, kohesive forces. They are less or more strong depending on which material it is. They are responsible for the fact that fluids form drops, when they are poured on the ground.
Lets make an example!
Put a drop of water on a desk or on glass. It looks like this. Well, It's Cola I poured on my desk.
If you put some mercury on a desk or on a plate of glass it looks like this:
Can you spot the difference?
It's the angle between the drop and the material the drop is in contact with:
If it is bigger than 90° the kohesive forces are stronger than the adhesive. The fluid does not get much in contact with the material, because the stronger the adhesive forces the more it gets the shape of an orb and an orb touches the base in only one small point. If the angle is smaller than 90° the cohesive forces between the fluid and the base is stronger and therefore the fluid covers the base. Such fluids are called wetting fluids.
The smaller the angle the wetter the fluid makes the base, because it covers a big area. The bigger the angle is the dryer stays the base, because the fluid covers only a small area.
And this is already the answer: Only wetting fluids can make something wet! If you would try to wash your shirt in mercury (who would do so!?), it wouldn't even get wet.The mercury would simply drip of. A sponge of course would never soak up a non wetting fluid. Chalk sticks on a chalkboard, because the adhesive forces are stronger than the koherent forces, but it's different when the base changes. A greasy chalboard makes the adhesive forces much smaller. So the chalk wouldn't write on such a chalkboard. By the way, a prominent joke of pupils, cleaning the chalkboard with some kind of detergent. The chalk won't stick any more on this chalkboard.
This leads us to the question, what detergents do. Mixed with water they diminish the kohesive forces, so that the water gets more wetting and can therefore resolve more dirt than before. But detergents on a chalkboard make it sort of greasy and therefore they diminish the adhesive forces.
Because it depends on both, the kind of fluid and the base, it is possible that leaves can't get wet although water is poured over them:
They stay dry! The leaves do this by a very special structure of their surface. This effect is well known as the lotus effect, but not only lotus plants do this, also this lupine.
So water is not always wet.
Because water always contains resolved minerals and dirt, surfaces that can't be wetted by water stay clean much longer. Ceramic in bathrooms and glass is sometimes finished with nano particles which cause this lotus effect. You need not clean them as often as normal ceramic or glass.
The wettest fluid I know is fluid helium. If it is extremely cold, shortly above absolute zero (0 K or - 273°C or -459° F) it nearly has no kohesive forces any more. And because there are only adhesive forces between the fluid helium and the vessel it is in, it starts to creep up the vessel and gets out of it this way. It's sort of superfluid at that temperature.
Thank you for hearing my short lecture in phyics. ;o)
I need a coffee now and you too, I guess! I'm glad that it's not wetting enough to creep out of my cup!























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