Discussion of "measuring and Understanding Contact Area at the Nanoscale: A Review" (Jacobs, T. D. B., and Ashlie Martini, A., 2017, ASME Appl. Mech. Rev., 69(6), p. 061101)

Jacobs and Martini (JM) give a nice review of direct measurement methods (in situ electron microscopy), as well as indirect methods (which are based on contact resistance, contact stiffness, lateral forces, and topography) for measurement of the contact area, mostly at nanoscale. They also discuss simulation techniques and theories from single-contact continuum mechanics, to multicontact continuum mechanics and atomistic accounting. As they recognize, even at very small scales, "multiple-contacts" case occurs, and a returning problem is that the "real contact area" is often an ill-defined, "magnification" dependent quantity. The problem remains to introduce a truncation to the fractal roughness process, what was called in the 1970s "functional filtering." The truncation can be "atomic roughness" or can be due to adhesion, or could be the resolution of the measuring instrument. Obviously, this also means that the strength (hardness) at the nanoscale is ill-defined. Of course, it is perfectly reasonable to fix the magnification and observe the dependence of contact area, and strength, on any other variable (speed, temperature, time, etc.).