  Freezing or solidification of impacting droplets is omnipresent in nature and
technology, be it a rain droplet falling on a supercooled surface, be it in
inkjet printing where often molten wax is used, be it in added manufacturing or
in metal production processes or in extreme ultraviolet lithography (EUV) for
the chip production where molten tin is used to generate the EUV radiation. For
many of these industrial applications, a detailed understanding of the
solidification process is essential. Here, by adopting a totally new optical
technique in the context of freezing, namely TIR (Total-Internal-Reflection),
we elucidate the freezing kinetics during the solidification of a droplet while
it impacts on an undercooled surface. We show for the first time that at
sufficiently high undercooling a peculiar freezing morphology exists that
involves sequential advection of frozen fronts from the centre of the droplet
to its boundaries. This phenomenon is examined by combining elements of
classical nucleation theory to the large scale hydrodynamics on the droplet
scale, bringing together two subfields which traditionally have been quite
separated. Furthermore, we report a peculiar self-peeling phenomenon of a
frozen splat that is driven by the existence of a transient crystalline state
during solidification.
