Containerless solidification, in which the melt is confined by its own
surface tension, is an important technique by which very pure materia
ls can be produced, The form of the solidified product is sensitive to
conditions at the tri-junction between the solid, the melt and the su
rrounding vapor. An understanding of the dynamics of tri-junctions is
therefore crucial to the modelling and prediction of containerless sol
idification systems. We consider experimentally and analytically the s
imple system of a liquid droplet solidifying on a cold plate. Our expe
rimental results provide a simple test of tri-junction conditions whic
h can be used in theoretical analyses of more complicated systems. A n
ew dynamical boundary condition at the tri-junction is introduced here
and explains the surprising features of solidified water droplets on
a cold surface.