My shower curtain is made of some kind of vinyl. It is slick and not sticky. The shower walls are tile. They are slick and not sticky. I shower in clean water. It is not sticky. But if I apply a thin layer of non-sticky water to the non-sticky shower curtain I can stick it to the non-sticky tile like it was glued there. Ok, there’s some kind of adhesion going on here – but how does it work?
Water is, in fact, a fairly weak glue. It is not sticky enough to overcome the force of gravity that would pull a large droplet down the vertical surface of your shower curtain, but sticky nonetheless. When the shower curtain is pressed against the tile, the little spots of water on it are pressed into a thin film, which acts like a weak glue holding the two surfaces together. The film is thin enough that the adhesive force is enough to resist the force of gravity on the water.
The actual mechanism of attraction is called the Van der Waals force.
Water is, in fact, a fairly weak glue. It is not sticky enough to overcome the force of gravity that would pull a large droplet down the vertical surface of your shower curtain, but sticky nonetheless. When the shower curtain is pressed against the tile, the little spots of water on it are pressed into a thin film, which acts like a weak glue holding the two surfaces together. The film is thin enough that the adhesive force is enough to resist the force of gravity on the water.
While water is not sticky enough for you to characterize it as such, the light weight of the shower curtain, the large contact surface area mean, and the shallow hanging angle of the typical curtain mean that “not very sticky” is still enough to hold the curtain against the tile.
The actual mechanism of attraction is called the Van der Waals force. Essentially, a water molecule, while having a net neutral overall electric charge, is not uniformly charged - the hydrogen atoms in each molecule are positively charged and the oxygen atom is negatively charged. This characteristic of molecules is called “polarity”, and many surfaces exhibit it. It is what causes water droplets themselves to stick together, and the cause of water’s weak adhesion to other polar surfaces. If your shower curtain or tile were made of a non-polar material, they would not stick when wet.
This characteristic, incidentally, is why water is a liquid in the first place, and not a gas.
Now, you may ask - if water sticks to my tile because the tile is polar, and water sticks to my curtain because the curtain is polar, why do the curtain and tile not stick naturally? The answer is that the surfaces of the curtain and tile are too rough on a microscopic level for any significant number of molecules on each surface to get close enough for the Van der Waals force to become significant. But water, as a liquid, can flow into all these little nooks and crannies.
Is it Van der Waals? I thought it was more to do with capillary action - the same phenomenon that causes a meniscus to form in a glass of water. But that may be just another way of looking at the same forces, I suppose?
Yes, capillary action occurs because the VDW force is stronger between the water and the container wall than between the water molecules themselves. The water thus creeps up the walls until the VDW attraction balances the force of gravity.
It would not occur if you used a tube with a non-polar inner surface.
Another (though lesser) factor is air pressure. Water prevents air from getting into the center of the wet area, so you aren’t going to have the curtain pulling away from the tile except at the edges. If you wanted to pull up the center first, you’d create a vacuum just like an impromptu suction cup.
I thought it was surface tension.
And this, although it’s obviously not a perfect vacuum, otherwise the curtain would never come off. Another related trick might be when you fill a cup to the brim, cover it with a card and turn it upside down. The card doesn’t fall off.