The freezing of water droplets on cold surfaces is a significant problem across numerous engineering applications, from aircraft icing to the efficient operation of power lines and other outdoor infrastructure. Current active de-icing methods, such as heating or mechanical scraping, are energy-intensive, expensive, and often impractical. Passive methods utilizing superhydrophobic coatings have shown promise, enabling droplets to rebound before freezing. However, these methods fail on sufficiently cold surfaces where the droplets freeze despite reduced contact time. This research explores an alternative passive de-icing method focusing on exploiting the inherent thermal-mechanical stresses that develop within a freezing water droplet. Previous studies have shown that such stresses can lead to the self-peeling or cracking of solidified materials such as liquid metals and alkane droplets upon impacting a cold surface. This study investigates whether these principles can be applied to the efficient removal of ice from cold surfaces. The overarching goal is to provide experimental evidence and a theoretical framework for a new passive anti-icing strategy based on the self-peeling of ice.

Existing literature extensively documents the challenges of ice accretion on various surfaces and the limitations of active de-icing techniques. Superhydrophobic coatings have emerged as a promising passive approach, leveraging water repellency to minimize contact time and reduce ice formation. Studies demonstrate the effectiveness of superhydrophobic surfaces in preventing ice adhesion under certain conditions, primarily by promoting droplet rebound. However, the limitations of this method on very cold surfaces are also well established. Research on the self-peeling of other materials, like liquid metals and alkanes, when impacting cold substrates highlights the potential of harnessing thermal-mechanical stresses for material removal. These studies indicate that the internal stresses generated during rapid solidification can overcome adhesive forces, leading to spontaneous detachment. This paper builds upon this existing body of research by focusing specifically on the self-peeling of ice and developing a comprehensive theoretical model to understand the process.

Experiments were conducted within a controlled nitrogen-filled chamber. Deionized water droplets of various sizes were released from controlled heights onto copper substrates with varied surface wettability (contact angles ranging from 22° to 119°). The substrate temperature was precisely controlled between -10°C and -47°C. High-speed imaging captured the droplet impact, freezing, and subsequent peeling or cracking behavior. The impact parameters (height, droplet size) and surface wettability were systematically varied to investigate their influence on the freezing behavior. A theoretical model was developed to complement the experimental observations. This model incorporated thermodynamics and elasticity theory to describe the self-peeling process. The model considers the freezing droplet as a two-layer cylinder (water film over ice), analyzing the temperature distribution and the resulting thermal stresses within the ice. Kirchhoff-Love plate theory was applied to model the elastic deformation of the ice disk, calculating the thermal elastic moment density. The model predicts the peeling time and the bending behavior of the ice sheet as a function of material properties, temperature difference, and impact parameters. The model's predictions were compared to the experimental observations to validate its accuracy and to extract key parameters characterizing ice-substrate adhesion.

The experiments revealed a stark contrast in the freezing behavior of water droplets on hydrophobic versus hydrophilic surfaces. On hydrophobic surfaces (contact angle ≥ 104°), the frozen droplets exhibited complete self-peeling, easily removable by a gentle air flow. This self-peeling behavior was dependent on several factors. Complete peeling transitioned to partial peeling, then no peeling or cracking as the temperature difference (ΔT) between the freezing point and substrate temperature decreased. Hydrophilic surfaces (contact angle < 104°), in contrast, showed ice cracking but no self-peeling, with the ice remaining firmly adhered to the substrate. The critical temperature difference required for cracking was independent of the contact angle. The theoretical model successfully captured the key features of the self-peeling process. The model predicted the peeling time (τp) which scaled inversely with (ΔT)², aligning well with experimental data. The model also accurately predicted the early-time evolution of ice deflection (δ), demonstrating the importance of both thermal stresses and ice-substrate adhesion. A general variable, the ice disk's bending curvature (κD), was identified as a useful predictor of complete versus partial peeling. Complete peeling was observed when κD exceeded a threshold value, which slightly increased with increasing contact angle, suggesting that more hydrophobic surfaces promote complete peeling. The model also showed that increasing the temperature difference (ΔT) and droplet impact height (*H*) promoted complete peeling.

The findings demonstrate a novel passive de-icing method based on harnessing the thermal-mechanical stresses within freezing droplets. The strong dependence of the freezing behavior on surface wettability highlights the importance of surface engineering for effective ice removal. The success of the self-peeling mechanism on hydrophobic surfaces stems from the significantly lower ice-substrate adhesion compared to the ice cohesion. The theoretical model provides a valuable tool for designing surfaces with enhanced anti-icing properties. By controlling surface wettability and substrate temperature, the conditions for complete self-peeling can be optimized. The model can help predict the effectiveness of different surface treatments and guide the selection of materials with appropriate mechanical and thermal properties for anti-icing applications.

This research provides compelling experimental evidence and a theoretical framework for a passive ice removal method based on the self-peeling of frozen water droplets on hydrophobic surfaces. The study highlights the importance of surface wettability and temperature difference in determining the freezing behavior. The developed thermal-mechanical model accurately predicts the peeling and bending behaviors of ice, providing a valuable tool for the design of effective anti-icing surfaces. Future research could explore the application of this method to various practical scenarios and investigate the long-term durability and performance of such surfaces under realistic environmental conditions.

The current study focused on idealized experimental conditions using deionized water and smooth copper substrates. The influence of impurities in water, surface roughness, and complex geometries on the self-peeling behavior requires further investigation. The model's accuracy might be affected by the assumptions made about temperature distribution and ice behavior during the rapid freezing process. Further refinements to the model, incorporating more complex material behavior and interfacial phenomena, are warranted for improved prediction accuracy.