玻璃结构理论GLASS THEORY.docx

玻璃结构理论GLASS THEORY.docx

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玻璃结构理论GLASSTHEORY

TheNatureofGlassRemainsAnythingbutClear（TheNewYorkTimes,2008,Science）

ItiswellknownthatpanesofstainedglassinoldEuropeanchurchesarethickeratthebottombecauseglassisaslow-movingliquidthatflowsdownwardovercenturies.

Wellknown,butwrong.Medievalstainedglassmakersweresimplyunabletomakeperfectlyflatpanes,andthewindowswerejustasunevenlythickwhennew.

Thetalecontainsagrainoftruthaboutglassresemblingaliquid,however.Thearrangementofatomsandmoleculesinglassisindistinguishablefromthatofaliquid.Buthowcanaliquidbeasstrikinglyhardasglass?

“They’rethethickestandgooiestofliquidsandthemostdisorderedandstructurelessofrigidsolids,”saidPeterHarrowell,aprofessorofchemistryattheUniversityofSydneyinAustralia,speakingofglasses,whichcanbeformedfromdifferentrawmaterials.“Theysitrightatthisreallyprofoundsortofpuzzle.”

PhilipW.Anderson,aNobelPrize-winningphysicistatPrinceton,wrotein1995:

“Thedeepestandmostinterestingunsolvedprobleminsolidstatetheoryisprobablythetheoryofthenatureofglassandtheglasstransition.”

Headded,“Thiscouldbethenextbreakthroughinthecomingdecade.”

Thirteenyearslater,scientistsstilldisagree,withsomevehemence,aboutthenatureofglass.

PeterG.Wolynes,aprofessorofchemistryattheUniversityofCalifornia,SanDiego,thinksheessentiallysolvedtheglassproblemtwodecadesagobasedonideasofwhatglasswouldlooklikeifcooledinfinitelyslowly.“Ithinkwehaveaverygoodconstructivetheoryofthatthesedays,”Dr.Wolynessaid.“Manypeopletellmethisisverycontentious.Idisagreeviolentlywiththem.”

Others,likeJuanP.Garrahan,professorofphysicsattheUniversityofNottinghaminEngland,andDavidChandler,professorofchemistryattheUniversityofCalifornia,Berkeley,havetakenadifferentapproachandareascertainthattheyareontherighttrack.

“Itsurprisesmostpeoplethatwestilldon’tunderstandthis,”saidDavidR.Reichman,aprofessorofchemistryatColumbia,whotakesyetanotherapproachtotheglassproblem.“Wedon’tunderstandwhyglassshouldbeasolidandhowitforms.”

Dr.ReichmansaidofDr.Wolynes’stheory,“Ithinkalotoftheelementsinitarecorrect,”buthesaiditwasnotacompletepicture.Theoristsaredrawntotheproblem,Dr.Reichmansaid,“becausewethinkit’snotsolvedyet—exceptforPetermaybe.”

Scientistsareslowlyaccumulatingmoreclues.Afewyearsago,experimentsandcomputersimulationsrevealedsomethingunexpected:

asmoltenglasscools,themoleculesdonotslowdownuniformly.Someareasjamrigidfirstwhileinotherregionsthemoleculescontinuetoskitteraroundinaliquid-likefashion.Morestrangely,thefast-movingregionslooknodifferentfromtheslow-movingones.

Meanwhile,computersimulationshavebecomesophisticatedandlargeenoughtoprovideadditionalinsights,andyetmoretheorieshavebeenprofferedtoexplainglasses.

DavidA.Weitz,aphysicsprofessoratHarvard,joked,“Therearemoretheoriesoftheglasstransitionthantherearetheoristswhoproposethem.”Dr.Weitzperformsexperimentsusingtinyparticlessuspendedinliquidstomimicthebehaviorofglass,andheducksoutofthetheoreticalbattles.“Itjustcangetsocontroversialandsomanyloudarguments,andIdon’twanttogetinvolvedwiththatmyself.”

Forscientists,glassisnotjusttheglassofwindowsandjars,madeofsilica,sodiumcarbonateandcalciumoxide.Rather,aglassisanysolidinwhichthemoleculesarejumbledrandomly.Manyplasticslikepolycarbonateareglasses,asaremanyceramics.

Understandingglasswouldnotjustsolvealongstandingfundamental（andarguablyNobel-worthy）problemandperhapsleadtobetterglasses.Thatknowledgemightbenefitdrugmakers,forinstance.Certaindrugs,iftheycouldbemadeinastableglassstructureinsteadofacrystallineform,woulddissolvemorequickly,allowingthemtobetakenorallyinsteadofbeinginjected.Thetoolsandtechniquesappliedtoglassmightalsoprovideheadwayonotherproblems,inmaterialscience,biologyandotherfields,thatlookatgeneralpropertiesthatariseoutofmanydisorderedinteractions.“Aglassisanexample,probablythesimplestexample,ofthetrulycomplex,”Dr.Harrowell,theUniversityofSydneyprofessor,said.Inliquids,moleculesjigglearoundalongrandom,jumbledpaths.Whencooled,aliquideitherfreezes,aswaterdoesintoice,oritdoesnotfreezeandformsaglassinstead.

Infreezingtoaconventionalsolid,aliquidundergoesaso-calledphasetransition;themoleculeslineupnexttoandontopofoneanotherinasimple,neatcrystalpattern.Whenaliquidsolidifiesintoaglass,thisorganizedstackingisnowheretobefound.Instead,themoleculesjustmoveslowerandslowerandslower,untiltheyareeffectivelynotmovingatall,trappedinastrangestatebetweenliquidandsolid.

Theglasstransitiondiffersfromausualphasetransitioninseveralotherkeyways.Energy,whatiscalledlatentheat,isreleasedwhenwatermoleculeslineupintoice.Thereisnolatentheatintheformationofglass.

Theglasstransitiondoesnotoccuratasingle,well-definedtemperature;theslowerthecooling,thelowerthetransitiontemperature.Eventhedefinitionofglassisarbitrary—basicallyarateofflowsoslowthatitistooboringandtime-consumingtowatch.Thefinalstructureoftheglassalsodependsonhowslowlyithasbeencooled.

Bycontrast,water,cooledquicklyorcooledslowly,consistentlycrystallizestothesameicestructureat32degreesFahrenheit.

Todevelophistheory,Dr.Wolyneszeroedinonanobservationmadedecadesago,thattheviscosityofaglasswasrelatedtotheamountofentropy,ameasureofdisorder,intheglass.Further,ifaglasscouldbeformedbycoolingataninfinitelyslowrate,theentropywouldvanishatatemperaturewellaboveabsolutezero,violatingthethirdlawofthermodynamics,whichstatesthatentropyvanishesatabsolutezero.

Dr.Wolynesandhiscollaboratorscameupwithamathematicalmodeltodescribethishypothetical,impossibleglass,callingitan“idealglass.”Basedonthisidealglass,theysaidthepropertiesofrealglassescouldbededuced,althoughexactcalculationsweretoohardtoperform.Thatwasinthe1980s.“Ithoughtin1990theproblemwassolved,”Dr.Wolynessaid,andhemovedontootherwork.

Noteveryonefoundthetheorysatisfying.Dr.Wolynesandhiscollaboratorssoinsistedtheywererightthat“youhadtheimpressiontheyweretryingtosellyouanoldcar,”saidJean-PhilippeBouchaudoftheAtomicEnergyCommissioninFrance.“IthinkPeterisnotthebestadvocateofhisownideas.Hetendstooversellhisowntheory.”

Aroundthattime,thefirsthintsofthedichotomyoffast-movingandslow-movingregionsinasolidifyingglasswereseeninexperiments,andcomputersimulationspredictedthatthispattern,calleddynamicalheterogeneity,shouldexist.

Dr.WeitzofHarvardhadbeenworkingforacoupleofdecadeswithcolloids,orsuspensionsofplasticspheresinliquids,andhethoughthecouldusethemtostudytheglasstransition.Astheliquidissqueezedout,thecolloidparticlesundergothesamechangeasacoolingglass.Withthecolloids,Dr.Weitzcouldphotographthemovementsofeachparticleinacolloidalglassandshowthatsomechunksofparticlesmovedquicklywhilemosthardlymoved.

“Youcanseethem,”Dr.Weitzsaid.“Youcanseethemsoclearly.”

ThenewfindingsdidnotfazeDr.Wolynes.Around2000,hereturnedtotheglassproblem,convincedthatwithtechniqueshehadusedinsolvingproteinfoldingproblems,hecouldfillinsomeofthecomputationalgapsinhisglasstheory.Amongthecalculations,hefoundthatdynamicalheterogeneitywasanaturalconsequenceofthetheory.

Dr.Bouchaudandacolleague,GiulioBiroli,revisitedDr.Wolynes’stheory,translatingitintotermstheycouldmoreeasilyunderstandandcomingupwithpredictionsthatcouldbecomparedwithexperiments.“Foralongtime,Ididn’treallybelieveinthewholestory,butwithtimeIbecamemoreandmoreconvincedthereissomethingverydeepinthetheory,”Dr.Bouchaudsaid.“Ithinkthesepeoplehadfantasticintuitionabouthowthewholeproblemshouldbeattacked.”

ForDr.Garrahan,theUniversityofNottinghamscientist,andDr.Chandler,theBerkeleyscientist,thecontrastbetweenfast-andslow-movingregionswassostrikingcomparedwiththeotherchangesnearthetransition,theyfocusedonthesedynamics.Theysaidthatthefundamentalprocessintheglasstransitionwasaphasetransitioninthetrajectories,fromflowingtojammed,ratherthanachangeinstructureseeninmostphasetransitions.“Youdon’tseeanythinginterestinginthestructureoftheseglassformers,unlessyoulookatspaceandtime,”Dr.Garrahansaid.

Theyignorethemoresubtleeffectsrelatedtotheimpossible-to-reachidealglassstate.“IfIcannevergetthere,thesearemetaphysicaltemperatures,”Dr.Chandlersaid.

Dr.ChandlerandDr.Garrahanhavedevisedandsolvedmathematicalmodels,but,likeDr.Wolynes,theyhavenotyetconvincedeveryoneofhowthemodelisrelatedtorealglasses.Thetheorydoesnottrytoexplainthepresumedconnectionbetweenentropyandviscosity,andsomescientistssaidtheyfoundithardtobelievethattheconnectionwasjustcoincidenceandunrelatedtotheglasstransition.

Dr.Harrowellsaidthatintheproposedtheoriessofar,thetheoristshavehadtoguessaboutelementaryatomicpropertiesofglassnotyetobserved,andhewonderedwhetheronetheorycouldcoverallglasses,sinceglassesaredefinednotbyacommon