Frozen water droplets take on a whole new shape when they freeze: Instead of staying round, they form a pointy tip, and eventually sprout a tiny forest of ice crystals on their surface. In order to observe these effects, researchers dripped tiny beads of water on a plate kept at a chilly -20°C. In the 18 seconds that it took the 4-millimeter-diameter droplets (top row) to solidify, researchers snapped photos of the water freezing from the bottom up. During the final stage of freezing, the ice drops developed a pointy tip (middle row), which continued to grow and eventually formed delicate ice crystals on the surface, the team reported last month in Physics of Fluids. Researchers believe the unusual pointy tip is caused by the vertical expansion of the ice combined with the surface tension on remaining liquid. Once frozen, the sharp tip of the drop attracts water vapor from the air, and produces treelike ice crystals (bottom row).
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