Tech gets metal to mimic natural rose petal powers

A new technology can make metallic replicas of soft, natural surfaces such as rose petals, researchers report.

Their metallic surfaces retain properties of the originals, including a rose petal’s sticky, yet water-repelling texture.

The researchers work addresses the question of how we can use natural textures and properties in the engineered world. For instance, could we somehow apply the water-repelling, ultrahydrophobic texture of a lotus plant to an aircraft wing as an anti-icing device?

On the left, a red rose petal has a droplet of water sitting on top of it. On the right, a golden metal petal also has a water droplet on it.
A demonstration shows how a rose petal and a metallic replica of the petal’s surface texture repel water. (Credit: Martin Thuo and his research group)

Previous attempts have involved molding polymers and other soft materials, or etching patterns on hard materials that lacked accuracy and relied on expensive equipment. But what about making inexpensive, molded metallic biostructures?

Pursuing “frugal science/innovation” led the researchers to an answer.

Martin Thuo, an associate professor of materials science and engineering with a courtesy appointment in electrical and computer engineering at Iowa State University and the the corresponding author of a paper on the work in Angewandte Chemie, describes the “frugal science/innovation” approach as “the ability to minimize cost and complexity while providing efficient solutions to better the human conditions.”

For this project, the researchers are taking their previous development of liquid metal particles and using them to make perfectly molded metallic versions of natural surfaces, including a rose petal. They can do it without heat or pressure, and without damaging a petal. They call the technology BIOMAP.

Making metal petals

“This project comes from an observation that nature has a lot of beautiful things it does,” Thuo says.

“The lotus plant, for example, lives in water but doesn’t get wet. We like those structures, but we’ve only been able to mimic them with soft materials, we wanted to use metal.”

Key to the technology are microscale particles of undercooled liquid metal, originally developed for heat-free soldering. The particles are created when tiny droplets of metal (in this case Field’s metal, an alloy of bismuth, indium, and tin), are exposed to oxygen and coated with an oxidation layer, trapping the metal inside in a liquid state, even at room temperature.

The BIOMAP process uses particles of varying sizes, all of them just a few millionths of a meter in diameter. The researchers apply particles to a surface, covering it and form-fitting all the crevices, gaps, and patterns through the autonomous processes of self-filtration, capillary pressure, and evaporation.

A chemical trigger joins and solidifies the particles to each other and not to the surface. That allows researchers to lift off the solid metallic replicas, creating a negative relief of the surface texture. The researchers can create positive reliefs by using the inverse replica to create a mold and then repeating the BIOMAP process.

“You lift it off, it looks exactly the same,” Thuo says, noting the engineers could identify different cultivars or roses through subtle differences in the metallic replicas of their textures.

Hydrophobic materials

Importantly, the replicas kept the physical properties of the surfaces, just like in elastomer-based soft lithography.

“The metal structure maintains those ultrahydrophobic properties—exactly like a lotus plant or a rose petal,” Thuo says. “Put a droplet of water on a metal rose petal, and the droplet sticks, but on a metal lotus leaf it just flows off.”

Those properties could be applied to airplane wings for better de-icing or to improve heat transfer in air conditioning systems, Thuo says.

That’s how a little frugal innovation “can mold the delicate structures of a rose petal into a solid metal structure,” Thuo says.

“This is a method that we hope will lead to new approaches of making metallic surfaces that are hydrophobic based on the structure and not the coatings on the metal.”

Iowa State supported the project with intellectual property royalties Thuo generated.

Source: Iowa State University