Freezing Splats

A droplet hits and then freezes on a cold surface.

When a drop hits a surface colder than its freezing point, there’s a competition between retraction and solidification that determines the final shape of the splat. For many materials, like wax or soldering metals, the contact angle between their liquid and solid phase is zero, so there’s no major shape change once solidification begins. But water — as is so often the case — is an exception.

Water and ice have a non-zero contact angle, which means that retraction can continue even after the drop begins freezing. As a result, the final shape of the splat varies depending on how cold the surface is. For a surface only a little colder than the freezing point, the final splat forms a spherical cap (Image 1). But once the surface is colder, freezing happens before the water can fully retract and the final splat forms a ring (Image 2). (Image and research credit: V. Thiévenaz et al.)

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