Crystallization fouling – the accumulation of scale forming on surfaces – occurs in equipment like heat exchangers, condensers, boilers and cooling towers, where thousands of gallons of water are flowing. Scale buildup primarily consists of components like calcium carbonate or calcium sulfate.

For thermal power plants, this scaling is an expensive problem. It inhibits heat transfer and flow performance, negatively impacting plant operations. Researchers at ETH Zurich say scale buildup just 1 millimeter thick in heat exchanger pipes can lead to efficiency losses of approximately 1.5%.

But after studying the physics of microfouling adhesion, researchers say a possible solution comes in the form of a hydrogel-based coating with tiny ridges aimed at preventing the adhesion of crystals.

A joint research team from ETH Zurich and the University of California, Berkeley has been studying the effectiveness of such a coating under a European Research Council Grant. The study, led by former ETH Zurich professor Thomas Schutzius (now at UC Berkeley), began in 2019.

Experimenting with water content and microtexture

The research team began examining the interactions among growing crystals, the surrounding water flow and the surface at the microscopic level. They developed and tested several coatings from various soft materials.

“So far, not a lot of people looked into soft coatings,” said Julian Schmid, a PhD Student at ETH Zurich and a study author. “We started to have a look at silicones. But then we thought, let’s try hydrogels.”

Researchers experimented with different coating properties, primarily altering the polymer content. They found the lower the polymer content and the higher the water content, the less well calcium carbonate crystals adhered to the surface.

“We’re talking about lubricating the interface between the crystals and the coating,” said Schmid, in an interview with Power Engineering.

The hydrogel’s surface is also notably covered in tiny ridges. Researchers say they help reduce crystal contact – making the micro foulants easier to remove when water flows over the hydrogel-coated surface.

The team fabricated the microtextured molds using photolithography followed by deep reactive ion etching on a single-side polished silicon wafer.

The hydrogel’s microstructure was inspired by processes taking place in the natural world. For example, shark scales have a ribbed structure to reduce fouling on the shark’s skin.

The team said under shear-driven water flow conditions, up to 98% of the crystals were removed. This was 66% better than when using a rigid, uncoated substrate.

“It was very evident that we had achieved a really good solution,” said Schmid in an interview with Power Engineering.

Schmid stressed that this is only fundamental research he expects others will build upon. He said the next steps would be studying this coating against other types of fouling.

Researchers also plan to test the coating on other substrates, like metal. To this point researchers have only used glass, in order to have a transparent substrate for easier viewing through a microscope.

“I think there there’s more research to do,” said Schmid. “It’s not a final product yet.”

The team’s research was published in the journal Science Advances.

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