(November 25, 2015) Researchers
at ETH Zurich have created a new type of foam made of real gold. It is the
lightest form ever produced of the precious metal: a thousand times lighter
than its conventional form and yet it is nearly impossible to tell the
difference with the naked eye. There are many possible applications.
A nugget of real 20 carats gold, so light that it does not
sink in a cappuccino, floating instead on the milk foam – what sounds
unbelievable has actually been accomplished by researchers from ETH Zurich.
Scientists led by Raffaele Mezzenga, Professor of Food and Soft Materials, have
produced a new kind of foam out of gold, a three-dimensional mesh of gold that
consists mostly of pores. It is the lightest gold nugget ever created.
"The so-called aerogel is a thousand times lighter than conventional gold
alloys. It is lighter than water and almost as light as air," says
Mezzenga.
The new gold form can hardly be differentiated from
conventional gold with the naked eye – the aerogel even has a metallic shine.
But in contrast to its conventional form, it is soft and malleable by hand. It
consists of 98 parts air and only two parts of solid material. Of this solid
material, more than four-fifths are gold and less than one-fifth is milk
protein fibrils. This corresponds to around 20 carat gold.
Milk protein
filaments and gold salt are the starting materials for the gold foam.
(Ilustration:
Nyström G et al. Advanced Materials 2015)
Drying process a
challenge
The scientists created the porous material by first heating
milk proteins to produce nanometre-fine protein fibres, so-called amyloid
fibrils, which they then placed in a solution of gold salt. The protein fibres
interlaced themselves into a basic structure along which the gold
simultaneously crystallised into small particles. This resulted in a gel-like
gold fibre network.
A foam of
amyloid protein filaments without gold (above),
with gold
microparticles (middle)
and gold
nanoparticles (below).
(Photo: Nyström
G et al. Advanced Materials 2015)
"One of the big challenges was how to dry this fine
network without destroying it," explains Gustav Nyström, postdoc in
Mezzenga's group and first author of the corresponding study in the journal
Advanced Materials. As air drying could damage the fine gold structure, the
scientists opted for a gentle and laborious drying process using carbon
dioxide. They did so in an interdisciplinary effort assisted by researchers in
the group of Marco Mazzotti, Professor of Process Engineering.