  Freezing of polymer solutions has been extensively investigated from many
aspects, especially the complex pattern formation. The cell/dendrite
micro-structures are believed to be in the type of diffusion-induced M-S
instability. However, the presence of polymer as an impurity in water is
significantly different from that of small ions. The quantitative transient
investigation on directional freezing of polymer solutions remains lack due to
some challenges. For the first time, we observed the planar instability
behaviors during unidirectional freezing of a polymer solution together with a
typical ionic solution with manipulated ice orientation, and their pattern
formation of S/L interface morphology as a function of time in the transient
planar instability process has been revealed and compared to each other. It is
found with real-time observation that the polymer solution exhibits a global
instability mode instead of a local instability mode that is common in ionic
solution during planar instability. W-L model was applied to quantitatively
address the variation of solute recoil of ionic/polymer solution. And it is
found that the W-L model can only reproduce the solute recoil of ionic solution
instead of polymer solution, which indicates the complex physics behind
freezing of a polymer solution. The paper provides a spectacular contrast of
directional freezing process between polymer solution and ionic solution and is
believed to promote relevant investigations in terms of the theoretical
approach to describing the freezing behavior of polymer solution.
