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• Droplet freezing, solidification, Laser Induced Fluorescence. When a droplet impinges onto a subcooled surface, it starts freezing very rapidly if the surface is already covered with frost. In this study, the freezing process is investigated for water droplet and two subcooled substrates having different thermal properties: duralumin (high heat conductivity) and N-BK7 glass (low heat conductivity). Experiments performed on single droplets and modelling are carried out together. A new experimental technique based on laser-induced fluorescence is developed to reconstruct the evolution of the ice front within the freezing droplets with and without taking into account the volume expansion during the freezing process. Comparisons between the two substrates reveals how the freezing dynamics can have a significant effect on the shape of the freezing front. Early in the freezing process, the ice front displays a spherical shape for both substrates. However, the curvature of the ice front and the angle with the free liquid surface are dependent on the freezing rate and the substrate material. The formation of a pointy tip is observed only for the N-BK7 substrate, as the solidification takes a much longer time in this case. The contact angle at the solid/liquid/air tri-junction point allows explaining the formation of this pointy tip. A simplified 2D axisymmetric model, accounting for the enthalpy released at the phase change and the thermal diffusion in both the solid substrate and the freezing medium is compared to the experimental results. Although it is in general not able to mimic the exact shape of the ice front, it allows predicting the overall freezing rate with a good accuracy. [ABSTRACT FROM AUTHOR]