Sand & Water

[ll_billym]

I would like an explanation of the following please:

Why does sand stick together when it is mixed with water?

Sand is just gound down shells and rocks, larger pieces of shells and rocks do not stick together when they are mixed with water.

Is there a particular size of particles where they stop behaving like sand and stop sticking together?

[newtron]

This has to do with the adhesion forces between the water and the sand grains, which are related to the surface tension between these two phases.  These forces can be demonstrated with capillary tubes. when the end of a small capillary tube is submersed in water, the rise of water within the tube indicates that there is an upward force actiing on the water that is equal to the weight of the water column.  The distance that water moves up the tube increases as the radius of the tube decreases.  The curviture of the meniscus within the capillary tube implies that a pressure loss exists across it.  In fact,  the fluid beneath the meniscus is at a pressure less than atmospheric and is thus under tension.  With sand, you can consider the pores between grains as many small capillary tubes.  If this tension is greater than the gravitational force on the individual grains, then the grains will stick together.  Obviously, as the grains and pores get larger, at some point the force of gravity on the grains will be greater than  the surface tension between the water and the grains and they will not stick together.  I'm sure you can calcuate the particale size where they stop sticking toghether, but I don't have time right now.  I'm sure you can find more detailed information very easily by using goolge.