High-Resolution 3D reproduction of condensation frost microstructure using UV curable Polymers

• A positive replica method was developed that preserves the shape of frost structures with high fidelity. • Positive replicas overcome limitations of negative replicas by enabling direct observation. • The accuracy and resolution of frost morphology reproduction were experimentally evaluated. • Photocurable resin properties suitable for frost replication were distinguished. Frosting on heat exchanger surfaces causes performance degradation, and quantitative observation of frost microstructures is indispensable for the development of antifrost and defrosting technologies. Among the various approaches, replica methods can reproduce complex frost morphologies at the microscale level, enabling three-dimensional (3D) analysis. However, conventional replica methods preserve the frost structures as negative shapes embedded within the resin. Because such internal cavities are difficult to observe directly, additional measurements using equipment, such as X-ray computed tomography with 3D reconstruction software, are required, and the influence of bubbles inside the resin cannot be fully eliminated. This paper proposes a fabrication process for positive frost replicas using photolithography-based techniques. The positive-type replica has the advantage of preserving the 3D spatial correspondence of frost, while enabling direct observation using scanning electron microscopy (SEM) and confocal microscopy. In this study, frost was laterally grown on AgI stripe patterns, and a low-viscosity ultraviolet-curable resin customized for curing under cooled conditions was employed to fabricate precise and high-strength molds. Positive replicates were transferred from these molds using nanoimprint lithography. The obtained replicas were examined using SEM and confocal microscopy, which successfully reproduced diverse frost morphologies, including leaf-like, wedge-like, zigzag, and hexagonal crystals. The tips of the replicated frost are sharp at a scale of smaller than 2 μm. These results show that positive frost replicas are valuable for direct and artifact-free analyses of frost microstructures and highlight their potential for advancing anti-frost and defrosting research.