▁a ▁surface . ▁For ▁example , ▁a ▁drop ▁of ▁liquid ▁will ▁adopt ▁a ▁given ▁contact ▁angle ▁when ▁static , ▁but ▁when ▁the ▁surface ▁is ▁t ilt ed ▁the ▁drop ▁will ▁initially ▁de form ▁so ▁that ▁the ▁contact ▁area ▁between ▁the ▁drop ▁and ▁surface ▁remains ▁constant . ▁The ▁" down hill " ▁side ▁of ▁the ▁drop ▁will ▁adopt ▁a ▁higher ▁contact ▁angle ▁while ▁the ▁" u ph ill " ▁side ▁of ▁the ▁drop ▁will ▁adopt ▁a ▁lower ▁contact ▁angle . ▁As ▁the ▁t ilt ▁angle ▁increases ▁the ▁contact ▁angles ▁will ▁continue ▁to ▁change ▁but ▁the ▁contact ▁area ▁between ▁the ▁drop ▁and ▁surface ▁will ▁remain ▁constant . ▁At ▁a ▁given ▁surface ▁t ilt ▁angle , ▁the ▁adv ancing ▁and ▁re ced ing ▁contact ▁angles ▁will ▁be ▁met ▁and ▁the ▁drop ▁will ▁move ▁on ▁the ▁surface . ▁In ▁practice , ▁the ▁measurement ▁can ▁be ▁influenced ▁by ▁she ar ▁forces ▁and ▁momentum ▁if ▁the ▁t ilt ▁velocity ▁is ▁high . ▁The ▁measurement ▁method ▁can ▁also ▁be ▁challeng ing ▁in ▁practice ▁for ▁systems ▁with ▁high ▁( > 3 0 ▁degrees ) ▁or ▁low ▁( < 1 0 ▁degrees ) ▁contact ▁angle ▁h yst er esis . ▁ ▁Ad van cing ▁and ▁re ced ing ▁contact ▁angle ▁measurements ▁can ▁be ▁carried ▁out ▁by ▁adding ▁and ▁removing ▁liquid ▁from ▁a ▁drop ▁depos ited ▁on ▁a ▁surface . ▁If ▁a ▁sufficiently ▁small ▁volume ▁of ▁liquid ▁is ▁added ▁to ▁a ▁drop , ▁the ▁contact ▁line ▁will ▁still ▁be ▁p inned , ▁and ▁the ▁contact ▁angle ▁will ▁increase . ▁Similarly , ▁if ▁a