The planar instability during unidirectional freezing of a macromolecular polymer solution: Diffusion-controlled or not?

Freezing of polymer solutions has been extensively investigated from many aspects, especially the complex pattern formation. The cellular/dendritic microstructures occurred in freezing of polymer solutions are usually believed to belong to the type of diffusion-induced Mullins–Sekerka (M − S) instability. However, the presence of macromolecule as an impurity in water is significantly different from that of small ions. The quantitative investigation on transient process of directional freezing of polymer solutions remains lack due to some challenges. For the first time, we in-situ observed the planar instability behaviors during unidirectional freezing of a polymer solution together with a typical ionic solution with manipulated ice orientation. It is found that unlike the ionic solution exhibiting typical diffusion-controlled planar instability, the solute recoil of a polymer solution deviated severely from the predictions of Warren-Langer (W-L) model. Meanwhile, the real-time observation shows that the polymer solution exhibits a global instability mode instead of a local instability mode. These results reveal the complex physics behind freezing of a polymer solution, and are believed to promote relevant investigations in terms of the theoretical approach to describing the freezing behavior of a polymer solution.