Every household item that comes into touch with (hot) water, including kettles, washing machines, and hot water tanks, develops limescale. This is especially true in regions where the water is hard, or high in calcium.
Test set-up with which the researchers tested how lime crystals adhere to different surfaces. Image Credit: Julian Schmid/ETH Zurich
Often, the only thing that works is to dissolve the extremely hard deposits and get the appliance working again with vinegar or a specialized descaler.
This is an annoyance in homes, and it may be costly in thermal power plants such as those that produce electricity, where the buildup of limescale is referred to as fouling. The formation of limescale in heat exchangers is a common occurrence that significantly lowers system efficiency.
A 1 mm thick layer of limescale in the heat exchanger's pipework can lower power generation efficiency by almost 1.5%. It would be necessary to burn an extra 8.7 million tons of hard coal to make up for these losses, which is costly for the companies that produce the electricity and detrimental to the climate and carbon impact.
Innovative Limescale-Repellent Surface
Researchers from the University of California, Berkeley, and ETH Zurich have recently discovered a potential remedy for this issue: a unique covering that repels limescale that includes minuscule ridges that stop limescale crystals from adhering. Science Advances has published the team's work.
There has not been much basic study done on creating surfaces that repel limescale. Thus, the scientists, under the direction of former ETH Professor Thomas Schutzius, examined in great detail the microscopic interactions that occur between individual growing limescale crystals, the surrounding water flow, and the surface.
Julian Schmid, a doctorate student of Schutzius, and his colleagues used this information to create several coatings using different soft materials, which they then tested in an ETH Zurich lab.
Hydrogel with Microstructure is the Most Effective
The most successful coating proved to be a polymer hydrogel, whose surface has microscopic ridges on it because the researchers used photolithography to create microtextured molds.
The ribbed shape of shark scales, which also feature a hydrogel microstructure to prevent fouling on their skin, is evocative of natural models.
Water running over the hydrogel and through the ribbed structure transports the limescale crystals away. In kettles or boilers, the riblets ensure that the crystals have less contact with the surface, meaning they cannot adhere and are, therefore, easier to remove. The coating cannot completely stop limescale crystals from forming, but it can inhibit them from growing together to form a tenacious layer by continuously removing the tiny crystals passively.
The researchers mainly changed the polymer composition to create the various coatings. The calcium carbonate crystals' ability to stick to the surface decreases with increasing water content and decreasing polymer content.
Polystyrene model particles used in tests demonstrate that the surface structures of the coating need to be smaller than the particles that are deposited on it. As a result, there is less contact surface and less adhesive force.
We varied the material’s surface structure to achieve the greatest efficiency, then carried out the crystal experiments with this optimum structure size.
Julian Schmid, Assistant Professor, Department of Mechanical Engineering, University of California
The hydrogel coating is particularly successful, as demonstrated by the team's research, which removed up to 98 % of the previously developed limescale crystals with a size of around 10 µm when water ran across the hydrogel-coated surface.
Eco-Friendly Solution
The researchers stress that their technique of descaling is more environmentally friendly and effective than current methods, some of which use harsh and hazardous chemicals. The hydrogel, on the other hand, is ecologically benign and biocompatible. This solution's technology ought to be scalable as well because the coating can be used in a variety of methods that are now in use in industry.
The researchers have consciously chosen to publish their development in a scientific journal rather than seeking a patent. This implies that the novel coating can be developed and used by any interested parties without restriction.
Thomas Schutzius received the grant in 2019 from an ERC Starting. He is an Assistant Professor of Mechanical Engineering at UC Berkeley currently and is no longer working at ETH Zurich.
Journal Reference
Schmid, J., et.al., (2023). Imparting scalephobicity with rational micro-texturing of soft materials. Science Advances. doi.org/10.1126/sciadv.adj0324