Microtexture topography and freezing characteristics alter freezing outcomes. (a) Schematic of a droplet resting on a superhydrophobic surface (not to scale) during recalescence, introducing β, s, d, h and the contact angle (θ). In the schematic, blue represents supercooled water and the grey area shows the progression of the freezing front. (b), Φ versus s for water droplets in a low-pressure environment. Outcomes are differentiated by colour (red, impalement; blue, expulsion; green, suffusion) for two pillar heights (h = 25 µm and 40 µm). (c), Bar chart of Φ for each s as a function of β (N = 249, n ≥ 19 experiments per data point). Credit: Nature Physics (2023). DOI: https://doi.org/10.1038/s41567-023-01946-3
Supercooled droplets can typically freeze on surfaces in nature, and have broad-scale influence on industries where they can adversely impact technical efficiency and reliability. Superhydrophobic surfaces are therefore a materials engineering solution to rapidly shed water and reduce ice adhesion to form promising candidates that resist icing.
However, the impact of supercooled droplet freezing and their effects on droplet-substrate interactions as well as resultant applications across ice-phobic surfaces remain to be explored in physics and materials engineering.
In a new report in Nature Physics, Henry Lambley and a research team in mechanical and processing engineering at the ETH Zurich, Switzerland, studied frozen supercooled droplets resting on textured surfaces. They induced freezing by evacuating the surrounding atmosphere and determined the surface properties required to promote ice formation.
The team explored these outcomes by balancing anti-wetting surface forces with those triggered by the freezing phenomena to demonstrate textures rationally designed to promote ice expulsion. Additionally, they considered the complementary processes of freezing at atmospheric pressure and sub-zero temperatures to observe bottom-up ice suffusion. In this way, Lambley and colleagues assembled a rational framework to study ice adhesion of supercooled droplets to design and develop efficient ice-repellent surfaces for broad-scale applications in life sciences and processing industries.
Droplet freezing on surfaces occurs quite commonly in nature, effectively impacting the transportation, construction and power generation industries. Existing approaches to generate ice-repellence in the lab and in practice are resource-intensive, relying on chemicals or on high energy consumption. Researchers therefore aim to create ice-phobic surfaces that delay freezing for sustainable industrial applications, while facilitating a low-adhesion surface for ice already in formation. To accomplish this, they explored the physics of droplet freezing; a two-step process of rapid recalescence, i.e., a temporary rise in temperature during the cooling of a substance, followed by slow crystallization.
Rapid expulsion of a supercooled droplet during freezing on a superhydrophobic surface in a low-pressure environment. Credit: Nature Physics (2023). DOI: https://doi.org/10.1038/s41567-023-01946-3
The latent heat released during the process can lead to explosive vaporization and levitation, frost halo formation and cascade freezing phenomena. Additionally, volumetric expansion during crystallization can lead to droplets self-peeling and disintegrating. In this work, the researchers studied freezing behavior of super-cooled droplets on superhydrophobic surfaces across a wide-range of temperatures and pressures. The outcomes provide a blueprint to design robust ice-phobic surfaces across the industries of aviation, civil infrastructure and power transmission.
2023-02-27 07:00:03
Article from phys.org
Recent developments in supercooled droplet experiments have led to the emergence of a revolutionary technology: superhydrophobic ice-repellent surfaces. This cutting-edge technology is set to revolutionize the way we deal with ice formation on surfaces, offering a new way to prevent ice formation and build surfaces that resist icing.
Superhydrophobic surfaces have traditionally been used to prevent water from adhering to surfaces. Due to their low surface tension, such surfaces reduce the binding energy of water droplets and prevent them from forming static droplets that remain on a surface. Superhydrophobic ice-repellent surfaces take this technology a step further by leveraging supercooled droplet experiments. Through these experiments, researchers have identified the conditions in which a superhydrophobic surface will actually repel frozen water droplets.
This technology works by creating an electrostatic field between the surface and the supercooled water droplet. When the temperature is sufficiently below zero degrees Celsius, the droplet forms a vapor layer between itself and the surface. This vapor layer effectively serves as an insulating layer, preventing the formation of ice between the two surfaces. By applying this technology judiciously, scientists can design surfaces which remain ice-free and thus resist icing.
There are a number of potential applications for this technology. It could be used to create surfaces on the exterior of aircrafts which remain ice-free, eliminating the need for anti-icing fluids and drastic temperature changes. It could also be used to create a self-cleaning coating for windows, avoiding the need to scrape them clean of ice or snow.
This technology shows great promise for the future. With further research and development, it could offer innovative solutions to a variety of icing and frosting problems.