Water droplets under freezing conditions do not spontaneously detach from surfaces as they do at room temperature due to stronger droplet-surface interaction and lack of an energy transformation pathway. Since accumulated droplets or ice have to be removed manually or with mechanical equipment, which is costly and inefficient, preventing droplet accretion on surfaces is both scientifically intriguing and practically important. Researchers at The Hong Kong Polytechnic University (PolyU) have invented a ground-breaking self-powered mechanism of freezing droplet ejection that allows droplets to shoot themselves away, paving the way for cost-efficient and promising technological applications.
Published in Nature Chemical Engineering as the cover feature for its last December issue, the research project "Freezing droplet ejection by spring-like elastic pillars" is led 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, and Prof. Haimin YAO, Associate Professor of the PolyU Department of Mechanical Engineering. First co-authors include Postdoctoral Fellow Dr Huanhuan ZHANG, PhD student Mr Wei ZHANG, Research Assistant Professor Dr Yuankai JIN, and PhD student Mr Chenyang WU.
The discovery of the self-ejection phenomenon in freezing droplets was inspired by a fungus capable of shooting its spores away through osmosis-induced volume expansion. Noting that a similar volume expansion occurs when water droplet is freezing, the research team has replicated the self-shooting mechanism found in the fungi and developed a structured elastic surface (SES) with spring-like pillars and wetting contrast that allows for the spontaneous ejection of freezing water droplets.
The SES structure is designed to accelerate the ejection velocity and enlarge the kinetic energy transformation of freezing droplets. When the freezing droplet undergoes volume expansion, it compresses the pillar of SES. The volume expansion work is first converted to and stored as elastic energy in the pillar within tens of seconds, and then to be transformed into the droplet's kinetic energy rapidly within milliseconds. This thousandfold reduction in timescales leads to sufficient kinetic energy to drive freezing droplet ejection.
The simple SES structure, after parameter design, is effective in ejecting freezing droplets without external energy input and even against the forces of wind and gravity. It can be applied to aircraft, wind blades or cable lines to prevent hazards caused by ice accretion. Dr Huanhuan Zhang said, "It is exciting that we, for the first time, introduce a self-powered ice removal concept that will offer a wide range of innovative solutions. We will continuously improve the design of SES, allowing it to be manufactured at various scales and at a low cost to meet societal needs."
Furthermore, the theoretical model developed in the research elucidates the factors determining the successful onset of the freezing droplet ejection phenomenon, with scalable design exhibiting potential practicability in various fields.
Prof. Wang envisions, "This nature-inspired research paved the way for numerous impactful applications. We believe that the freezing droplet ejection, as a prototype, could stimulate the development of self-powered concepts and methods for a wide range of purposes such as de-icing, energy harvesting and soft robotic applications."
Specifically, droplet ejection induced by volume expansion enhances understanding of multi-phase freezing dynamics for anti-icing applications. Prof. Yao remarked, "Our research demonstrates a strategy to efficiently harness and utilise the volume expansion work of freezing droplets to generate ballistic motion. This could subsequently expand the application of energy conversion phenomena, and inspire the development of droplet-based energy generators and soft robotic catapults."
