Hydrophobic Sand - Understanding Hydrophobicity
Hydro- means water and -phobic means to have a dislike of something, so hydrophobic sand has a strong dislike of water.
There are many hydrophobic materials in nature. One of the most common is the lotus leaf, which has natural tiny microscopic bumps on the surface. Hydrophobic sand works in much the same way.
Sand is made of silica (SiO2) and in Hydrophobic Sand, each grain has tiny siloxane (Si–O–Si) bumps on its surface which repel the water - just like the lotus leaf - so the sand doesn’t get wet.
When you submerge the hydrophobic sand in water it can be pushed together and moulded into structures – this is because it wants to minimise the surface area touching the water so will stick to the sand instead.
Once you remove the sand from the surrounding water, it will be completely dry and flow freely, just like normal dry sand.
Hydrophobic sand was originally created to help clean up oil spills, as the oil would bond with the coated sand grains and then sink to the bottom of the ocean, taking the oil with it. It was found to be too expensive for this use however (and not very good for the fish!).
Siloxane can be damaged by soap, so be careful only to use water with your hydrophobic sand if you want it to stay water repelling for a very long time!
If you would like some Nanogirl hydrophobic sand to experiment with yourself, you can find it here!
A lotus leaf has tiny microscopic bumps on the surface which make it hydrophobic. See how the water forms beads and slides off?