 Real surfaces are not atomically smooth or chemically homogeneous so a drop will assume contact angle hysteresis.The equilibrium contact angle (formula_18) can be calculated from formula_19 and formula_20 as was shown theoretically by Tadmor  and confirmed experimentally by Chibowski  as,

where

On a surface that is rough or contaminated, there will also be contact angle hysteresis, but now the local equilibrium contact angle (the Young equation is now only locally valid) may vary from place to place on the surface.According to the Young–Dupré equation, this means that the adhesion energy varies locally – thus, the liquid has to overcome local energy barriers in order to wet the surface.