static and dry friction due to multiscale surface roughness

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1、StaticandDryFrictionduetoMultiscaleSurfaceRoughnessJ.B.SokoloffDepartmentofPhysicsandCenterforInterdisciplinaryResearchonComplexSystemsNortheasternUniversity,Boston,Massachusetts02115,U.S.A.,E-mailaddress:j.sokoloff@neu.eduMay13,2008AbstractItisshownonthebasis

2、ofscalingargumentsthatadisorderedinterfacebetweentwoelasticsolidswillquitegenerallyexhibitstaticand”dryfriction”(i.e.,kineticfrictionwhichdoesnotvanishastheslidingvelocityapproacheszero),becauseofTomlinsonmodelinsta-bilitiesthatoccurforsmalllengthscaleasperi

3、ties.Thisprovidesapossibleexplanationforwhystaticand”dry”frictionarevirtuallyalwaysobserved,andsuperlubricityalmostneveroccurs.1IntroductionAfewyearsago,MuserandRobbins[1]proposedthatstaticfrictionbe-tweencleancrystallinesolidsurfaces,whosecrystalaxeswererot

4、atedbyanarXiv:0805.1915v1[cond-mat.mtrl-sci]13May2008arbitraryanglewithrespecttoeachothersothatthesurfacesareincom-mensurate,wouldbezero.Thisiscommonlyknownas”superlubricity”[2].Inordertoexplainthefactthatsuperlubricityisrarelyobserved,theythenproposedthatth

5、elikelysourceofstaticfrictionforsuchsurfacesistheexis-tenceofmobiledirtmolecules,alwayspresentatanyinterface.Themobilemoleculesseekoutlocalenergyminimaattheinterface,whichresultsinthesurfacesbeingpinnedwithrespecttoeachother,i.e.,thereisstaticfriction.1Actua

6、lly,therecanalsobesuperlubricityforsmoothdisorderedsurfaces.InRefs.[3,4]itwasarguedthataflatdisorderedinterfacebetweentwomacroscopicsizesurfaceswhichdonotinteractchemicallywillexhibiteffec-tivelynostaticfrictionforinterfaceinteractionperunitinterfaceareasmallc

7、omparedtotheshearelasticconstantandhighfrictionforinterfaceinterac-tionabovethisvalue.Thisfrictionlessregimeisknownastheweakpinningregimeandthelargefrictionregimeisknownasthestrongpinningregime.InRef.[3],Iarguedthattheexistenceofmicronlengthscaleasperitiesat

8、aninterfacecanresultinbothslowspeedkinetic(oftenreferredtoas”dryfriction”)andstaticfriction,resultingfromthefactthattheseasperitiescanoccurinmultistableequilibriumconfigurations,whichresultin”dry