Archimedes’ principle does not work in the nanoworld. So say Roberto Piazza, from Milan Polytechnic in Italy, and his colleagues. The principle, a law of physics established 23 centuries ago, states that a body immersed in a fluid is buoyed up by a force equal to the weight of the fluid it displaces, but, as Piazza has found, this does not hold for objects a millionth of a millimetre in size.

‘It works for footballs, but not in the microscopic world,’ Piazza said in a recent interview. ‘What we have done is just a generalisation; had Archimedes had such small particles at his disposal, he could have done the same,’ he added.

Piazza’s team noticed that when they added gold nanoparticles (20 times more dense than water) to an aqueous suspension of plastic particles just slightly denser than water and six times larger than the nanoparticles, the gold nanoparticles floated to the top, forming a thin layer on the surface after a few days. ‘What happens is that not only is the liquid displaced, but the submerged object gets an additional upward push owing to the perturbation induced by the distribution of the other particles,’ explained Piazza. The larger particles slowly push the nanoparticles towards the surface.

The finding may have implications for biology and geology research, Piazza said. These include techniques to separate biological fluids from nanomaterials and getting a better idea of how sedimentary rocks form.

Elinor Hughes

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