WhetherdropletstransitfromtheCassietotheWenzelstate(C–W)onatexturedsurfaceisthetouchstonethatthesuperhydrophobicityofthesurfaceisstillmaintained.However,theC–Wtransitionmechanism,especiallythespontaneoustransitionofsmalldroplets,isstillnotverycleartodate.TheinterfacefreeenergygradientofasmalldropletisfirstlyproposedandderivedasthedrivingforceforitsC–Wevolutioninthisstudybasedontheenergyandgradientanalysis.ThenthephysicalandmathematicalmodeloftheC–WtransitionisfoundaftertheC–Wdrivingforceortransitionpressure,theresistance,andtheparametersofthemeniscusbeneaththedropletareformulated.Theresultsshowthatthemicro/nanostructuralparameterssignificantlyaffecttheC–Wdrivingforceandresistance.Thesmallerthepillardiameterandpitch,theminortheC–Wtransitionpressure,andthelargertheresistance.Consequently,theC–Wtransitionisdifficulttobecompletedforthedropletsonnano-texturedsurfaces.Meanwhileifthepostsaretooshort,thefrontofthecurvedliquid–airinterfacebelowthedropletwilltouchthestructuralsubstrateeasilyeventhoughthethreephasecontactline(TPCL)hasnotdepinned.Whenthepostsarehighenough,theTPCLbeneaththedropmustmovefirstlybeforethemeniscuscanreachthesubstrate.Asaresult,thedropletonatexturedsurfacewithshortpillarsiseasytocompleteitsC–Wevolution.Ontheotherhand,thesmallerthedroplet,theeasiertheC–Wshift,sincethetransitionpressurebecomeslarger,whichwellexplainswhyanevaporatingdropwillcollapsespontaneouslyfromcompositetoWenzelstate.Besides,bothintrinsicandadvancingcontactanglesaffecttheC–Wtransitionaswell.Thegreaterthetwoangles,thehardertheC–Wtransition.Intheend,theC–Wtransitionparametersandthecriticalconditionsmeasuredinliteraturesarecalculatedandcompared,andthecalculationsaccordwellwiththeexperimentalresults.
