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WHEN does a raindrop splash? Surprisingly, the process begins before it makes contact.

A liquid drop hitting a surface often flattens into a thin sheet that then bounces to form a crown shape. Previous experiments have suggested the surrounding air plays a role: a droplet doesn’t splash so easily when the air pressure is low, instead spreading out into a thin pancake.

Shreyas Mandre of Harvard University and colleagues ran computer simulations of liquid drops hitting a solid surface, accounting for factors such as air pressure and the drop’s surface tension. Their results showed that a typical raindrop – roughly 2 millimetres wide and travelling at a few metres per second – compresses air in front of it a few microseconds before hitting a solid surface. This creates an air cushion that causes the raindrop to flatten and spread out, which the team believe would be less likely if the drop was striking a higher friction surface, and may help the splash evolve into a crown ().

Splashing is still poorly understood, says Mandre, so describing such processes could help create splash-suppressing materials for use in kitchens, for example.