Figure 4.

Schematic illustration of the surface force balance (SFB) setup, where normal and shear forces between two surfaces are directly measured via the bending of two orthogonal springs. White light undergoes multiple beam interference in passing through the half-silvered mica sheets, resulting in Fringes of Equal Chromatic Order (FECO, top right inset). These yield absolute separation between the surfaces to within ±0.2 nm. A sectored piezoelectric tube can move the surfaces normally and laterally relative to each other. The bending of normal and shear springs in response to applied motion, monitored via the change in an air gap capacitor or through changes in the interface fringes, directly reveals the shear and normal forces, respectively.