The photos above show how a drop of water freezes on a cold surface:
by forming a sharp point at the top:
Researchers at the University of Twente, in the Netherlands, placed
water droplets on a plate chilled to -20 degrees Celsius and captured
images as a freezing front traveled up the droplet.
The photos are published in the American Institute of Physics' (AIP)
journal Physics of Fluids. The approximately 4-millimeter diameter droplets
took about 20 seconds to freeze. During the final stage of freezing,
the ice drop developed a pointy tip, as can be seen in Figure 1d. The
effect, which is not observed for most other liquids, arises because
water expands as it freezes. The vertical expansion of the ice, in combination
with the confining effect of surface tension on the spherical cap of
remaining liquid, leads to the point formation.
But that's not all! After the water has frozen, the sharp tip of the
ice attracts water vapors in the air and grows a "tree" of ice
crystals!
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