Polymer-regulated freezing of water droplets revealed by synchrotron X-ray imaging and Raman spectroscopy

During the freezing of polymer-solution droplets, the advancing ice front may act not only as a solidification boundary but also as a polymer-segregation front. We hypothesize that this freeze-induced redistribution converts an initially homogeneous droplet into a spatially heterogeneous frozen matrix with embedded polymer-rich domains. It is these local domains-rather than changes in bulk properties alone-that drive various phenomena observed in polymer-regulated droplet freezing, such as conical-tip suppression.

We freeze sessile aqueous poly(vinyl alcohol) (PVA) droplets on a temperature-controlled substrate, varying the polymer concentration, droplet volume, and molecular weight. Time-resolved synchrotron X-ray radiography tracks the freezing-front propagation, while X-ray computed tomography resolves the frozen internal morphology, and confocal Raman mapping of cryo-sectioned droplets identifies local PVA enrichment.

As the PVA concentration increases, we observe a roughened freezing front, a bumpy frozen surface, reduced bubble entrapment, and a progressively rounded apex across all droplet volumes and molecular weights examined. X-ray tomography reveals interconnected domains of low X-ray transmittance within the frozen droplet interior and near its surface, while Raman spectral mapping independently reveals localized regions with enhanced PVA-associated Raman signal. Our work demonstrates that freeze-induced polymer segregation plays a key role in this interfacial solidification phenomenon, which governs solute redistribution, freezing-front roughening, discrete bubble entrapment, and the overall morphology of the frozen droplets.