▁small ▁ ▁amount ▁of ▁liquid ▁is ▁removed ▁from ▁a ▁drop , ▁the ▁contact ▁angle ▁will ▁decrease . ▁ ▁The ▁Young ' s ▁equation ▁assumes ▁a ▁hom ogeneous ▁surface ▁and ▁does ▁not ▁account ▁for ▁surface ▁texture ▁or ▁outside ▁forces ▁such ▁as ▁gravity . ▁Real ▁surfaces ▁are ▁not ▁atom ically ▁smooth ▁or ▁chem ically ▁hom ogeneous ▁so ▁a ▁drop ▁will ▁assume ▁contact ▁angle ▁h yst er esis . ▁The ▁equilibrium ▁contact ▁angle ▁() ▁can ▁be ▁calculated ▁from ▁ ▁and ▁ ▁as ▁was ▁shown ▁theoret ically ▁by ▁T ad mor ▁ ▁and ▁confirmed ▁experiment ally ▁by ▁Ch ib owski ▁ ▁as , ▁ ▁where ▁ ▁On ▁a ▁surface ▁that ▁is ▁rough ▁or ▁cont amin ated , ▁there ▁will ▁also ▁be ▁contact ▁angle ▁h yst er esis , ▁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 – D u pr é ▁equation , ▁this ▁means ▁that ▁the ▁ad hes ion ▁energy ▁var ies ▁locally ▁– ▁thus , ▁the ▁liquid ▁has ▁to ▁overcome ▁local ▁energy ▁bar riers ▁in ▁order ▁to ▁wet ▁the ▁surface . ▁One ▁consequence ▁of ▁these ▁bar riers ▁is ▁contact ▁angle ▁h yst er esis : ▁the ▁extent ▁of ▁wet ting , ▁and ▁therefore ▁the ▁observed ▁contact ▁angle ▁( aver aged ▁along ▁the ▁contact ▁line ), ▁depends ▁on ▁whether ▁the ▁liquid ▁is ▁adv ancing ▁or ▁re ced ing ▁on ▁the ▁surface . ▁ ▁Because ▁liquid ▁adv ances ▁over ▁previously ▁dry ▁surface ▁but ▁re