Jo blocks are an amazing object for the study of friction and adhesion

Johansson gauge blocks (“Jo blocks”) are made of steel and used for precision length measurement. Their surface is very smooth, and two blocks can adhere to each other, however, the strong adhesion occurs only after the sliding (wringing). Various hypotheses explaining wringing and adhesion mechanisms in the blocks have been suggested in the literature, including the role of intermolecular forces, oil surface tension, and air pressure. We study the frictional sliding of two Jo blocks against each other to obtain insights into the mechanisms of wringing. The results show an increase in the friction force with the sliding distance, which is consistent with the removal of the oxide film from the steel surface by wringing. This is likely the dominant mechanism of Jo block adhesion.

The unusual ability of Johansson blocks to stick together has amazed physicists and engineers. Richard Feynman mentioned them in The Feynman Lectures on Physics and also in his now-famous 1959 essay “There is plenty of room at the bottom,” in which he predicted the advent of nanotechnology. This is what Feynman said in the lecture on molecular forces: “These molecular forces can be demonstrated in a fairly direct way: one of these is the friction experiment with a sliding glass tumbler; another is to take two very carefully ground and lapped surfaces which are very accurately flat so that the surfaces can be brought very close together. An example of such surfaces is the Johansson blocks that are used in machine shops as standards for making accurate length measurements. If one such block is slid over another very carefully and the upper one is lifted, the other one will adhere and also be lifted by the molecular forces, exemplifying the direct attraction between the atoms on one block for the atoms on the other block.”

The length standard is by itself a remarkable object in the history of sciecne. Although today the frequency standard is used (the meter is defined as 1/299792458 of the distance travelled by light in vacuum in one second, while second is defined in terms of the caesium frequency), the standard platinum bar metre (Mètre) was introduced after the the French Revolution. The nature of the meter bar was investigated by Einstein when he developed the Relativity Theory. Philosopher Ludwig Wittgenstein who studied self-referential paradoxes, wrote: “There is one thing of which one can say neither that it is one metre long, nor that it is not one metre long, and that is the standard metre in Paris.” Jo blocks originate from the concept of a rigid bar length standard.

Please see our new paper where we suggest a new explanation of the misterious wringing mechanism
A. Breki and M. Nosonovsky 2022 “Friction and adhesion of Johansson gauge blocks” Surface Innovations (in press)