The gold bar is as light as air.

Swiss scientists have created gold in the form of a solid foam – the lightest form of precious metals ever created.

The gold nugget is lighter than milk foam. Photo: ETH Zurich

According to an article published in the journal Advanced Materials on November 25th, researchers from ETH Zurich have successfully synthesized a 20-carat gold foam block, so light that it floats on the surface of the milk foam in a cappuccino.

This type of gold foam is thousands of times lighter than regular gold ingots of the same volume, yet it is almost indistinguishable to the naked eye.

"This type of gold foam, also known as aerogel (a gel in a gas-solid state), is thousands of times lighter than regular gold, lighter than water, and as light as air," said research team leader Professor Raffaele Mezzenga on Phys Org.

The gold foam looks no different from a regular gold bar, but it is very soft and easily bent by hand. 98% of the gold foam is air, and only 2% is from a solid mixture. This solid mixture contains only 80% gold, the rest being milk fibers.

Gold foam is synthesized by heating milk to form protein fibers at the nanometer scale. These fibers are then immersed in a gold salt solution. The protein fibers interweave into a network containing crystallized gold particles, resulting in a gel-like network of gold fibers and particles.

"One of the major challenges was how to dry this gold-protein fiber mesh gel without altering its structure," said Dr. Gustav Nyström, a member of the research team.

Because drying can damage the gold foam structure, scientists opted for a more delicate and complex drying process using carbon oxides. This method allows the gold particles to crystallize directly during the production of protein fiber lattice structures, resulting in high uniformity and a shape and color identical to regular metallic gold.

The properties of gold foam can also be altered by influencing the manufacturing process.

"The optical properties of gold foam depend heavily on the size and shape of the gold particles," Nyström said. "Therefore, we can change the color of the gold foam by altering the reaction conditions so that the crystallization process forms gold particles of different sizes. Gold foam can therefore have a deep red color like a rose."

"At normal atmospheric pressure, the gold particles in the material don't touch each other, so the gold foam isn't electrically conductive," Mezzenga said. "But when the pressure is increased, the material compresses and the particles begin to come into contact, making the material electrically conductive."

This new material can be used in many of the current applications of gold. Gold foam has the advantages of being lightweight, small in size, and porous in structure, making it suitable for use in watchmaking, jewelry making, as a catalyst for chemical reactions due to its large surface area, or in light-absorbing or reflective devices. Furthermore, the change in electrical conductivity under pressure is very valuable in the sensing and measurement industries.

According to VnExpress