  Contact angle hysteresis and generation of dynamics angle are two fundamental
phenomena about the contact angle deviation from the equilibrium state.
Roughness on the solid surface, disjoining pressure in the thin film, and
liquid-solid adhesion have all been considered as the origins of the phenomena.
This work for the first time made a reduction to absurdity by employing
absolutely smooth solid surfaces in ultra-large-scale molecular dynamic
simulation. The results showed that the equilibrium angles were well
established on the absolutely smooth surface just as regular, while the
hysteresis and the dynamic angle vanished. The critical structure of the convex
nanobending for advancing contact lines vanished as well. In contrast, the
solids that made of atoms, even at the minimum roughness, would bring
significant angle deviation and convex nanobending. A 3D observation was
further made using state-of-the-art helium ion microscopy for the first time
revealing the ubiquitous nanoscopic distortion along the contact line on
atomically smooth surfaces. The results answer the question of the origin of
the angle deviation, that the hysteresis and dynamic angle can be unified to
originate from the friction, either static or dynamic; and at the same time
resolve the puzzle of the hysteresis existence on reported smooth surfaces,
showing that even the minimum i.e. atomic roughness is great enough to be
effective on the contact line. The results are against the disjoining pressure
theory which ignored the roughness and predicted the occurrence of the
hysteresis on absolutely smooth solids.
