A water droplet falling onto a cool, solid surface creates a series of distinctive halos around its point of impact.
If a water droplet strikes a surface colder than 0ºC, it can instantly form ice; if the surface is hotter than water’s boiling point, the drop quickly evaporates. To study what happens when the surface is cool, Chun Yang at Nanyang Technological University in Singapore and his colleagues filmed drops of room-temperature water as they struck a surface colder than the dew point — at which condensation occurs — but above water’s freezing point.
The researchers found that vapour evaporating from the droplets condenses on the surrounding surface, helping to form a series of three bands. The edge of the first band marks the extent of the droplet’s expansion on impact. At the instant of impact, water vapour departing the flattened droplet condenses on the surface around it, forming a second band. After the drop contracts and stabilizes, it cools, and vapour wafts outwards, forming a third, outer band.
This behaviour could apply to other processes in which condensation plays a part in heat transfer, the authors write.