Coupled density–volume and solid–liquid interaction effects in the freezing dynamics of water droplets

When a water droplet freezes, its transformation is far more complex than a simple volumetric expansion. Rather than expanding smoothly, the coupled dynamics of phase change and interfacial stress can produce unexpected morphologies—ranging from subtle contraction in hydrophilic droplets to pronounced tip sharpening in hydrophobic ones. In this Letter, we combine droplet freezing experiments with a lattice Boltzmann model that couples density–volume variation (DVV) and solid–liquid interaction distinction (SLID) mechanisms to uncover the underlying physics. The joint experimental–numerical analysis demonstrates that the DVV and SLID framework accurately reproduces the full freezing process and resolves long-standing discrepancies between theory and observation. Results reveal that contraction in hydrophilic and intermediate droplets originates from interfacial stress redistribution driven by liquid–solid coupling, while in hydrophobic droplets, although contraction is absent, the same interaction enhances ice-tip elevation and sharpening. Together, these findings establish that liquid–solid interfacial interaction is a curvature-dependent governing factor in droplet freezing. The synergy between experiment and simulation provides a unified physical explanation for the anomalous morphologies of freezing droplets and offers guidance for the design of anti-icing and de-icing surfaces and controlled solidification in multiphase systems.