Water droplets in freezing temperatures do not easily come off surfaces as they do at warmer temperatures. This is because the interaction between the droplets and surfaces is stronger, and there isn’t a way for energy transformation. Removing accumulated droplets or ice can be costly and inefficient since it often requires manual labor or mechanical tools. Therefore, finding ways to stop droplets from building up is both a fascinating scientific challenge and an important practical need. Researchers have now developed a revolutionary self-powered mechanism that enables freezing droplets to eject themselves, opening up possibilities for cost-effective technological applications.
Water droplets in freezing conditions do not detach from surfaces spontaneously as they do at higher temperatures due to increased interaction with the surface and the absence of energy transformation pathways. Because accumulated droplets or ice require manual or mechanical removal—which is both expensive and ineffective—preventing droplet buildup on surfaces presents significant scientific interest and practical significance. Researchers from The Hong Kong Polytechnic University (PolyU) have created a groundbreaking self-powered mechanism for ejecting freezing droplets, allowing them to shoot off surfaces and leading to promising and cost-efficient technological applications.
Featured as the cover story in the last December issue of Nature Chemical Engineering, the research titled “Freezing droplet ejection by spring-like elastic pillars” is spearheaded by Prof. Zuankai Wang, Associate Vice President (Research and Innovation), Kuok Group Professor in Nature-Inspired Engineering, and Chair Professor of the PolyU Department of Mechanical Engineering, along with Prof. Haimin Yao, Associate Professor in the same department. The team includes first co-authors Postdoctoral Fellow Dr. Huanhuan Zhang, PhD student Mr. Wei Zhang, Research Assistant Professor Dr. Yuankai Jin, and PhD student Mr. Chenyang Wu.
The team drew inspiration from a fungus that propels its spores away through osmosis-induced volume expansion, recognizing that a similar expansion occurs when water droplets freeze. They replicated this self-ejection mechanism and engineered a structured elastic surface (SES) featuring spring-like pillars and a wetting contrast to facilitate the spontaneous ejection of freezing droplets.
The SES structure is crafted to enhance the speed of droplet ejection and maximize the conversion of stored elastic energy into kinetic energy during freezing. When a droplet freezes and expands in volume, it compresses the SES pillars. This expansion generates elastic energy in the pillars, which is stored quickly within seconds, then converted to kinetic energy in a matter of milliseconds. This drastic reduction in time significantly provides the kinetic energy necessary for the ejection of freezing droplets.
The straightforward SES design, after careful parameter adjustments, effectively ejects freezing droplets without requiring any external energy, even in the presence of wind and gravity. This mechanism has potential applications in areas such as aircraft, wind turbine blades, and cable lines to mitigate risks associated with ice buildup. Dr. Huanhuan Zhang expressed enthusiasm about introducing the self-powered ice removal concept, which could lead to many innovative solutions. “We aim to further enhance SES design to allow for various production scales and low-cost manufacturing to address societal needs,” she said.
Additionally, the research established a theoretical model that clarifies what factors enable the successful ejection of freezing droplets, suggesting that this scalable design may have practical applications across multiple fields.
Prof. Wang believes that this research, inspired by nature, could pave the way for significant applications. “We foresee that the freezing droplet ejection mechanism could spark the development of self-powered technologies for diverse uses such as de-icing, energy harvesting, and soft robotics,” he remarked.
Particularly, the ejection of droplets through volume expansion enhances the understanding of multi-phase freezing processes, which can be valuable for anti-icing technologies. Prof. Yao commented, “Our findings offer a method to effectively harness and utilize the energy from the volume expansion of freezing droplets to create movement. This could broaden energy conversion applications and lead to developments in droplet-based energy generators and soft robotic launching systems.”
