Sticky Elastic Spheres: Was Newton Wrong?

Alan Roberts in 1966 was the first person to observe experimentally that elastic spheres made from optically smooth rubber could stick together, but he was not honoured for his unique contribution. The same might be said of Isaac Newton who wrote in his 1704 book Opticks that ‘Two polish’d marbles… by immediate contact stick together’. Newton’s postulate was clearly proved wrong in 1882 by Hertz who produced stress theory and observations to demonstrate there was no adhesion between polished glass or metal spheres. However, Kendall in 1970 produced evidence that iron hydroxide nanoparticles stuck strongly together and suggested that application of the energy criterion could explain this by rejecting the stress criterion of fracture. Ken Johnson had already solved the stress problem of sphere contact 12 years earlier in 1958 but suggested that adhesion was impossible because of the infinite tensile stress at the sphere contact edges. This paper describes how the 1970 collaboration between Johnson, Kendall and Roberts provided the correct solution published in 1971, proving Newton was right 266 years after his adhering spheres prediction.

Graphical Abstract