Why does damp sand stick together?

[neonsignal]

Why does damp sand stick together?

[DrP]

I'd say that it is probably the surface tension of the water.

[neonsignal]

I'd say that it is probably the surface tension of the water.

 

Presumably you are talking about some sort of adhesion between the water and the sand, perhaps dispersive adhesion?

Edited December 4, 2009 by neonsignal

[DrP]

Just surface tension really. Here is wiki's entry for surface tension: http://en.wikipedia.org/wiki/Surface_tension

 

QUOTE: "Surface tension is a property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to another surface........"

[swansont]

Surface tension is a force of cohesion, which is between like molecules. The force between unlike molecules is adhesion, as neonsignal has suggested.

 

http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html#c4

[D H]

Surface tension and capillary action. Have you driven a vehicle on a beach? Drive too far from the water and you risk getting stuck in the loose, dry sand. Drive too *close* to the water and you risk getting stuck in the loose, saturated sand. There is an optimal distance from the water where the sand has the greatest cohesion.

 

Dry sand is only sand+air, and without the water there is nothing to bind the grains of sand together. Saturated sand is only sand+water, and without the air the water exerts little cohesive force to bind the grains together. To have wet, sticky sand all three phases need to be present: solid sand + liquid water + gaseous air.

 

Imagine two grains of sand in close proximity being wetted. The water will move toward the closest points of contact due to capillary action. If there's enough water in the mix the water will bridge the gap between the grains. It is the shape of that bridge that makes the sand sticky.

 

Suspend a droplet of water in the air and it forms a nice sphere. The surface has positive curvature. That bridge between the grains of sand has negative curvature. Instead of being fat in the middle (a sphere), the bridge between grains of sand is skinny in the middle. It forms a hyperboloid. The wasp-waisted shape that results from surface tension in turn results in the water being at a slightly reduced pressure compared to the air, and that in turn results in the grains being drawn to one another. Repeat that many, many times over throughout the volume of slightly wet sand and you have a substance with significantly greater cohesion than dry sand.

 

What about saturated sand? Start with dry sand and gradually add more and more water. That first bit of water spreads a bit to thin to accomplish much. Add a bit more and those bridges start forming. These are ultra-wasp-waisted bridges, so they have a *lot* of cohesion. Add a bit more water and more bridges form. The sand becomes even more cohesive. Add even more water and those bridges become a bit less wasp-waisted, so a bit less cohesion per bridge. However, more bridges still form, and this more or less counteracts the fact that each bridge is a bit weaker. The cohesion will remain more ore less constant as you keep adding water -- up to a point. Eventually, those bridges become tubes, then those tubes join. The cohesion goes down. By the time the sand is saturated there is no longer much holding it together.