  When a droplet is brought in contact with an undercooled surface, it wets the
substrate and solidifies at the same time. The interplay between the phase
transition effects and the contact-line motion, leading to its arrest, remains
poorly understood. Here we reveal the early solidification patterns and
dynamics of spreading hexadecane droplets. Total internal reflection (TIR)
imaging is employed to temporally and spatially resolve the early
solidification behaviour. With this, we determine the conditions leading to the
contact-line arrest. We quantify the overall nucleation behaviour,
\textit{i.e.} the nucleation rate and the crystal growth speed, and show its
sensitivity to the applied undercooling of the substrate. By combining the
Johnson-Mehl-Avrami-Kolmogorov nucleation theory and scaling relations for the
spreading, we can calculate the temporal evolution of the solid area fraction,
which is in good agreement with our observations. We also show that for strong
enough undercooling it is the rapid growth of the crystals which determines the
eventual arrest of the spreading contact line.
