Scientists turn gold into foam that's nearly as light as air

A piece of the gold foam is light enough to float atop milk froth(Credit: Gustav Nyström and Raffaele Mezzenga/ETH Zurich)

Along with its use in jewellery, gold also has numerous applications in fields such as electronics and scientific research. It's a handy material, but – of course – it's also expensive. That's why researchers at ETH Zurich have developed a new way of making a small amount of gold go a long way. They've created a gold foam that looks much like solid gold, but is actually 98 parts air to two parts solid material. As an added bonus, the aerogel-type foam can also be made in non-gold colors such as dark red.

According to lead scientist Prof. Raffaele Mezzenga, the foam is one one-thousandths the weight of a same-sized piece of a conventional gold alloy. This makes it lighter than water, and almost as light as air. It still looks shiny and metallic, but unlike solid gold, it's malleable enough that it can be shaped by hand.

The solid material within the foam consists of approximately four-fifths actual gold, and about one-fifth milk proteins. Purity-wise, the foam comes out at around 20 carats.

Mezzenga and his team made the foam by first heating the proteins to convert them into tiny fibers, known as amyloid fibrils. When these fibrils were added to a solution of gold salt, they responded by interlacing themselves into a three-dimensional lattice-like structure. As they were in the process of doing so, the gold crystallized into tiny microparticles that became embedded in the fibrils.

The resulting gel-like structure was then dried to create the finished foam, using a labor-intensive process that involved exposing it to carbon dioxide. Air-drying would have been much easier, but could also have damaged the material before it dried.

Additionally, by changing the reaction conditions in the "gold salt" step of the process, the researchers were able to make the gold crystallize into even smaller nanoparticles. Because these had different optical qualities than the larger particles, the completed gold foam took on a dark red color.

Not only could the foam be used in many of the same applications as regular, more costly gold, but it might also be utilized in highly-sensitive pressure sensors. "At normal atmospheric pressure the individual gold particles in the material do not touch, and the gold aerogel does not conduct electricity," says Mezzenga. "But when the pressure is increased, the material gets compressed and the particles begin to touch, making the material conductive."

Source: ETH Zurich

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