  An approximate theory is presented describing the propagation of the
ice-water front that develops in droplets of water that are deposited on a
planar surface at a temperature below the melting point of ice. A calculation
based on this theory is compared with our experimental observations of the time
evolution of this front. The results of calculations of this front by Schultz
et al7, obtained by integrating numerically the exact differential equations
for this problem, were published graphically, but only for the time-dependent
velocity of this front. Unfortunately, these theoretical results cannot be
compared directly with our experimental observations. Our experiments were
performed by freezing water droplets directly on a block of dry ice, and in
order to examine the effects of the heat conductivity of a substrate during the
freezing process, such droplets were also deposited on a glass plate and on a
copper plate placed on dry ice. The temperature at the base of these droplets,
and the dependence of the freezing time on their size was also investigated ex
perimentally, and compared with our analytic approximation of the theory. Such
experiment have not been published previously, and reveal that the usual
assumption that the temperature at the base of the droplets is a constant, made
in all previous theoretical papers on this subject, cannot be implemented in
practice.
