Why do wet things stick together?

[Kenan]

I know pretty well what is surface tension,adhesion and cohesion.

I do know that at the very top of the water molecules of water are not balanced by other pulling forces so they create a straight line but this line bends into a circle because of another layer of water molecules below them,that's why soap bubbles have such round form and that's why water in the space is round,this line also allows slight deformations so other bodies can be put on it like insects who just stand on water or the needle on the water example.

But I cannot understand why does wet sand stick to other wet sand,why do wet hair stick together,why does wet glass stick to another wet glass.I don't understand what does wet sand stickiness have to do with the surface tension,it just does not really make any sense to me.

I know that it depends,like glass won't get wet if cohesion is stronger than adhesion and vice versa - glass will get wet cause adhesion is stronger than cohesion but these examples are for 2 bodies - water and some other material but in sticky examples we have 3 bodies - water,glass number 1,glass number 2 - that's why I'm so puzzled right now.

 

[Dr. Courtney]

Water is a polar molecule. The positive parts of one water molecule want to stick to the negative parts of other water molecules.

 

[Andy Resnick]

<snip>But I cannot understand why does wet sand stick to other wet sand,why do wet hair stick together,why does wet glass stick to another wet glass.I don't understand what does wet sand stickiness have to do with the surface tension,it just does not really make any sense to me.

The concept is called 'disjoining pressure' and/or 'capillary pressure', and it can be quite large. Disjoining pressure typically applies to the molecular scale, while capillary pressure can be used once you can treat the fluid as a continuum.

https://en.wikipedia.org/wiki/Disjoining_pressure
https://en.wikipedia.org/wiki/Capillary_pressure

For capillary pressure, the origin of the high pressure is the pressure jump that occurs across the edges of the film, where the fluid curvature is very high- it is very difficult to displace the (say) water with (say) air- or displace oil with water, in the case of oil recovery from porous rock.