  Thermal effect is essential to regulate the interfacial instabilities for
diverse technology applications. Here we report the fingering instability at
the propagation front for a spreading liquid film subjected to the supercooling
at the vertical direction. We find the onset timescale of hydrodynamic
instability is strongly correlated with that of the vertical solidification
process. This correlation is further validated in a non-uniform geometry,
demonstrating the capability of controlling fingering instability by structure
design. We attribute the identified interfacial instability to a pronounced
thermo-viscous effect, since the rapidly increased viscosity of propagation
front undergoing solidification can significantly enhance the mobility contrast
locally in the vicinity of the spreading front, consequently producing the
instability analogous to viscous fingering. This work offers another valuable
dimension by gating the vertical temperature to exploit the interfacial
stabilities and steer liquid flow, consequently shedding light on the
microfluidic cooling for electronics, and the advanced functional fibers and
fabrics.
