Everyone knows that water likes to stick to water - we see it all the time in the form of surface tension and in the spherical shape of water droplets. Now, a rather surprising result from Greg Kimmel and colleagues at Pacific Northwest National Lab shows that sometimes water can be hydrophobic.

The team studied what happens when water molecules are added to create a monolayer on a platinum substrate at temperatures in the range 20 to 155 K. At temperatures above about 60 K small islands of two-dimensional ice form, with successive water molecules falling off these islands and sticking to the platinum until a solid monolayer is formed. The researchers found that at temperatures less than 120 K additional monolayers of water will stick as expected to this initial layer of crystalline ice films.

However, above 135 K the system ceases to behave in this way, and becomes more like a waxy leaf or polished table top, where water forms large drops rather than wetting the surface. It seems that of the four bonds that water could form with other water molecules, one is used up to stick to the platinum and the other three are used to stick to each other. The net result is a hydrophobic layer of ice. Only when up to 50 additional monolayers have been added are all the non-wetting portions of the first entirely covered.

Whether this sort of phenomenon ever happens in nature is not yet clear. However, it could be relevant in understanding how ice crystals form in clouds after seeding.

Further reading

Greg Kimmel et al. 2005 Phys. Rev. Lett. 95 166102.