Adhesion and Cohesion
Have you ever wondered why you cannot build a sand castle with dry sand?
Why does water allow you to build a sand castle?
Why does too much water cause a sand castle to collapse?
Updated: 26 March 2025
The explanation will be easier starting with bbs and ball bearings, which are large enough to be seen easily.
BBs are spherical and have little surface area, thus they have little friction. Gravity pulls them into a single layer.
Water adds two more forces to the mix (other than gravity and friction): adhesion and cohesion. Adhesion is the attraction of unlike materials, and is the force behind adhesives such as tape and glue. It is the force of attraction that causes water to stick to ball bearings, sand grains, and soil particles. Cohesion is the attraction of water to itself.
The combined forces of adhesion and cohesion create a capillary effect, so that water is held in small pores, but not in large pores. The capillary effect is the reason water will move up into a straw or curve up slightly where it touches the sides of a glass. There is a pull, a negative pressure (potential energy) that causes the water in the bridges to be concave on both sides (a negative curvature).
The water “bridge” where the bearings touch is the result of capillarity, the film of water around the bearings due to adhesion of water to the ball bearings and the cohesion of water to itself. Cohesion is the reason the smaller pores are filled, but cohesion is not strong enough to pull water across the biggest pores, so they are empty.
Adhesion and cohesion also allows building this small pyramid.
Sand particles are small (0.05 to 2.00 mm in effective diameter) and have rough edges and irregular shapes. Very fine sands still feel a little gritty to the touch, but the dry silt and clay feel like fine powder. Your eyes have to be really good to see silt particles without a magnifying glass or microscope, and clay particles are less than 0.002 mm (2 microns), smaller than bacteria and invisible to the human eye without a very strong microscope.
Gravity and friction are the only two forces at work in dry sand. Gravity tries to pull the sand into a flat layer, but the friction of the rough edges causes the sand to form a mound. The shape of the mound, angle of repose, is related to particle size and shape – more about that below.
Just like with the bbs, water is attracted to the sand particles due to adhesion and cohesion of water pulls the particles together, like the water bridges between the ball bearings. So, if the sand is not too dry – or too wet – you can build a sand castle.
But when there is too much water, the conditions change. Adhesion still holds water to soil particles, but as the pores fill with water, the pressure changes from negative to positive, the water pushes the particles apart, and the sand castle slumps (begins to collapse). Look carefully and contrast the sand castle and the slumping one: No water is visible in the standing one, but slumping one glistens on the surface because the pores are full of water, and there is water in the dish.
This is one reason not to build a house on the sand.
Angle of Repose
Notice all these volcanoes look a bit like an inverted cone. The angle of repose is the equilibrium state of a mound of particles, determined by their size and shape as they resist the pull of gravity. The next examples use dry bbs, sand and silica flour.
The round bbs have no friction and cannot resist the pull of gravity, so they make a flat layer.
This is a mixture of very fine to coarse sands (0.05 to 1.00 mm). These particles have irregular shapes, so there is some friction to keep them from spreading out. They form a low mound with shallow angles.
Silica flour is essentially ground sand. The particles are very fine and feel and act a bit like powdered sugar. Because the particles have been ground, they have irregular shapes, so they have a lot of friction. Because they are so small, they have a lot of surface area and some cohesion. This causes the silica flour to form a taller mound with very steep sides.
Updated: 26 March 2025