Stage-wise freezing of droplets on cold superhydrophobic microstructured surfaces

Abstract

The freezing of droplets on cold solid surfaces poses severe challenges to transportation, energy, and infrastructure systems. While superhydrophobic microstructures are widely considered effective passive anti-freezing strategies, their behavior under extreme low-temperature impact remains insufficiently understood. Here, through combining high-speed experiments with lattice Boltzmann simulations, we successfully revealed the complete freezing pathway of droplets impacting cold superhydrophobic microstructures. We show that impact inertia drives droplet penetration into microcavities and triggers rapid freezing of the trapped liquid within hundreds of milliseconds. Meanwhile, the spreading area remains in the Cassie–Baxter state, where limited contact at the microstructure tops induces localized pinning, and leads to the formation and subsequent rupture of elongated liquid bridges during retraction. We further identify a stage-wise freezing sequence: inertia-dominated impact, microstructure-dominated heat transfer, filling transition stage, and cold-air-dominated freezing. This evolution explains how microstructures simultaneously accelerate internal icing and allow external air cooling to govern final shell formation. The findings provide new mechanistic insight into enhanced adhesion and freezing on cold superhydrophobic microstructured surfaces, offering novel guidelines for anti-icing design in extreme environments.