Explaining The Mould Of Microscale Molecules

When water freezes it is well known for being one of the few liquids that will expand.

Hexagonal ice unit cell
Thank you to the London South Bank University for this image

This is due to the ability for hydrogen bonds to form when the water is cooled which force the water molecules to arrange in a certain pattern described as being hexagonally symmetric. Although it is difficult to see the origin of that name looking at the diagram here, the hexagonal structure is clear when looking at a large relatively pure crystal of ice, such as a snow flake. Although these snow crystals clearly show symmetry and ordered layout, clouds, mists and liquid water really have no order at all with all the particles randomly orientated and moving independently.

However it has been found that when water is heated at its surface, tiny droplets, less than a hundred of a millimetre, have been observed to form clusters that match the hexagonal pattern both on the surface and also levitating a small distance above the thin film of water. This is a very important discovery as microdroplets of water are what make up clouds and the water cycle as well as the mechanics of microfluidics. Yet we still are not sure what determines the size and regularity of the droplets that form these clusters or what mediates the distance in between them. These hexagonal droplet formations have also been observed in liquids such as Glycerol and Benzyl alcohol with the clusters levitating over the hottest part of the surface in each case.

This recent experiment involved taking a layer of water below a millimetre thick and heating it to between 50°C and 70°C by laser irradiation or filament heating. A single layer of condensed droplets was found above the water level supported by the vapour pressure from the heat flux at the liquids surface. When the temperature was kept constant the radius of the droplets were seen to gradually increase over time. They all remained as spheres and filled out the already observed honeycomb shape containing hundreds of the microscale molecules. The reason for this was concluded to be one of energy. The densest packing configuration while maintaining a set distance due to repulsion is that of the hexagonal lattice and densest packing configuration also minimises the predicted potential energy of the molecules making it the most natural formation to take in this situation.

Paper links: Self-assembled levitating clusters of water droplets: pattern-formation and stability

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