1、Published: November 01, 2011r2011 American Chemical Society 15299 dx.doi.org/10.1021/la2039448|Langmuir 2011, 27, 1529915304ARTICLEpubs.acs.org/LangmuirFabrication of Surfaces with Extremely High Contact AngleHysteresis from Polyelectrolyte MultilayerLiming Wang, Jingjing Wei, and Zhaohui Su*State K
2、ey Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, andGraduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. ChinabS Supporting InformationINTRODUCTIONWettability plays a central and fundamental role in numer
3、ouspractical applications, such as cleaning, painting, coating, drying,and adhesion.1,2Contact angle analysis is thus valuable in char-acterizing surfaces both because of its convenience and its highsensitivity to details of interfacial structure at the angstrom scale.For an “ideal” surface that is
4、at, inert, and chemically homoge-neous, an equilibrium liquid contact angle can be uniquely denedby Youngsequation.3The real surfaces are neither perfectly atnor chemically homogeneous, and contact anglesobserved dierfrom the Youngs angle under the eect of free energy barriersintroduced by roughness
5、 and/or chemical heterogeneity. As aresult, observed static contact angles fall between two extremevalues: the advancing contact angle (A) and receding contactangle (R).4Aand Rtogether are characteristic of the surfacechemistry and topography, and the dierence between them isreferred to as contact a
6、ngle hysteresis ( = AC0 R).Contact angle hysteresis plays a decisive role in the motion ofliquid droplets on solid surfaces.5C07The relationship betweencontactanglehysteresisandsurfacehydrophobicitywasreportedbyFurmidge8andrevealstheminimumtiltangle(slide)atwhicha liquid droplet will spontaneously s
7、lide down upon the eect ofits own gravity overcoming the surface tension force holding itonto the surface, as shown by the equationmg sin slide kwlvcos RC0cos Awhere k is a constant, g is the acceleration of the gravity, m and ware the mass and contact diameter of the droplet, and lvis thesurface te
8、nsion of the liquid. The equation suggests that contactanglehysteresisishighlyrelevanttosurfaceadhesionandfriction,and the surface becomes more adhesive to a liquid droplet as thecontact angle hysteresis increases. Usually, a liquid droplet on asurface with low contact angle hysteresis can move easi
9、ly undereven little perturbation, while surfaces with high contact anglehysteresis are very adhesive to liquid droplets. Contact anglehysteresis mainly results from topographic roughness7,9C011andchemical heterogeneity.4,12C014Extrand11,15and McCarthy,16C020in particular, have demonstrated that even
10、ts occurring at thethree-phase contact line during advancing and receding of theliquid droplet are crucial to contact angle hysteresis, such asthe formation of microcapillary bridges during dewetting as thecontact line recedes.10,14C018When a liquid droplet wets a roughhydrophobic surface, one of tw
11、o states of wetting is typicallypresent: the homogeneous wetting (Wenzel) or the compositewetting (Cassie) state. In the Wenzel state, the liquid fullypenetrates into surface asperities, which pins the contact line ofthe liquid droplet and this pinning leads to high contact anglehysteresis as the co
12、ntact line is continuous and stable. Thus, thecontact angle and contact angle hysteresis on a rough hydro-phobic surface increase with surface roughness. On the otherhand,whenthewettingisintheCassiemode,airremainstrappedinthecavitiesoftheroughsurface;theliquiddropletsitspartiallyon air when deposite
13、d on the surface, and the contact angleswould follow the Cassie equation.19In this case, the three-phasecontact line is discontinuous and unstable, which thus causes a lowcontact angle hysteresis, and the contact angle tends to increasewith surface roughness while the hysteresis decreases.7,20C022Re
14、ceived: October 8, 2011Revised: October 31, 2011ABSTRACT: High contact angle hysteresis on polyelectrolyte multilayers (PEMs) ion-pairedwithhydrophobicperuorooctanoateanions is reported.Boththebilayernumberof PEMs and the ionic strength of deposition solutions have signicant inuence oncontactanglehy
15、steresis:higherionicstrengthandgreaterbilayernumbercauseincreasedcontactanglehysteresisvalues.Thehysteresisvaluesof100C176 wereobservedonsmoothPEMsandpinningoftherecedingcontactlineonhydrophilicdefectsis implicatedasthecause of hysteresis. Surface roughness can be used to further tune the contact an
16、glehysteresis on the PEMs. A surface with extremely high contact angle hysteresis of 156C176wasfabricatedwhenaPEMwasdepositedonaroughsubstratecoatedwithsubmicrometerscalesilicaspheres.Itwasdemonstratedthatthisextremelyhighvalueofcontactanglehysteresisresultedfromthepenetrationofwaterintotheroughaspe
17、ritiesonthesubstrate.Thesame substrate hydrophobized by chemical vapor deposition of 1H,1H,2H,2H-peruorooctyltriethoxysilane exhibits high advancingcontact angle and low hysteresis.15300 dx.doi.org/10.1021/la2039448 |Langmuir 2011, 27, 1529915304LangmuirARTICLEFor at but chemically heterogeneous sur
18、faces, high contactangle hysteresis mainly stems from the fact that the receding lineis pinned by high surface energy components, while the advan-cing line is pinned by low surface energy components.13,23Howsparse and dense defects aect hysteresis has been modeled andanalyzed theoretically.24,25Supe
19、rhydrophobic surfaces have attracted tremendous inter-est in the past decade.11,21,26C032Usually, the term superhydro-phobic indicates surfaces that display very high water contactangle,whichoftenexhibitlowcontactanglehysteresisaswellandon which a water droplet can roll o easily, exhibiting antiadhe
20、-sion behavior. However, McCarthy and co-workers also discov-ered a dierent kind of surface that exhibits water contact anglehysteresis as high as 161C176.7Recently, more attention has beenpaidto similarsurfacesthatexhibitbothhigh water contactangleand high adhesion to water droplets.33C039For examp
21、le, inspiredby high adhesive force of geckos feet and rose petals, Jiang andco-workers reported several methods to fabricate dierent kindsof superhydrophobic surfaces with high adhesive force33C035andfurther demonstrated their application in no-loss transfer ofliquid droplets.40Balu et al. reported
22、a sticky superhydrophobicsurfacewith acontact angle hysteresis of 79C176 by coating cellulosepaper with a thin uorocarbon lm.37Sheng et al. demonstratedthat when smearing hydrophobic molecules onto an extendedTeonlm, the surface showed aAof about 140C176 and a contactangle hysteresis greater than 60
23、C176.41Other groups have reportedthat condensation on superhydrophobic surfaces can lead to adramaticincreaseincontactanglehysteresistogreaterthan100C176,which results from severely limiting droplet mobility due topinning of the contact line between surface asperities as thewetting is in Wenzel stat
24、e.5,27,42Despite this progress in fabrica-tion of sticky superhydrophobic surfaces, a systematic study ofthe eects of chemical heterogeneity and topographic roughnesson contact angle hysteresis is still highly desirable.Recently, we demonstrated that the surface of a typicalpolyelectrolyte multilaye
25、r (PEM) can easily be hydrophobizedby ion exchange chemistry;43,44the at surfaces thus obtainedexhibit high contact angle hysteresis due to hydrophilic defectsinherent in the multilayers.23The protocol can be applied toroughsubstrateswithoutsignicantlyalteringsurfacetopology.45In the present study,
26、we systematically investigate the relation-ship between contact angle hysteresis and chemical defects andsurface topographic features for PEMs and compare them withwetting characteristics of surfaces with identical topologies butthat werehydrophobized by achemicalvapordeposition(CVD)method and thus
27、are almost free of hydrophilic defects. Wereport at and rough surfaces with high contact angle hysteresisand show that the presence of surface defects is the predominantfactor leading to high contact angle hysteresis.EXPERIMENTAL SECTIONMaterials. Poly(diallydimethylammonium chloride) (PDDA, Mw=2000
28、00C0350000), poly(sodium 4-styrenesulfonate) (PSS, Mw=70000), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS), per-fluorooctanoic acid (CF3(CF2)6COOH), silicon tetrachloride (SiCl4),and aqueous sodium silicate were all purchased from Sigma-Aldrich.Sodium chloride and sodium hydroxide (99.5+%) were
29、purchased fromSinopharm Chemical Reagent Co., Ltd., and used as received. Sodiumperfluorooctanoate(PFO)(0.10M)waspreparedbyreacting0.010molof the corresponding acid with NaOH in water, and the volume of thesolution was increased to 100.0 mL. An alcohol suspension of silicasubmicrometer spheres with
30、a concentration of about 2.0 wt % contain-ing 90% silica spheres of about 600 nm and 10% silica spheres ofabout 220nmwaskindlyprovided byProf.JunqiSunofJilinUniversity.N-silicon (100) wafers were purchased from Wafer Works Corp.(Shanghai, China). Water (18.2 Mcm) was purified with a MilliporeSimplic
31、ity system and used for all the experiments.SubstrateTreatment.Siliconwaferswerecleanedinahotpiranhasolution (H2SO4/H2O2, 7:3 mixture) at 80 C176C for 30 min, then washedsequentially with copious amounts of acetone, ethanol, and water, anddriedwithaN2flow.Caution:piranhasolutionreactsviolentlywithor
32、ganicmaterials and should be handled with great care.Preparation of the Nanostructured Substrate. A clean siliconwafer was first immersedinto a PDDA aqueoussolution (1.0 mg/mL) for15 min, followed by rinsing with water for 1 min and drying with N2flow,and then the substrate was immersed into an aque
33、ous solution of sodiumsilicate (100 mM, pH 11.5) for 10 min, rinsed with water for 1 min, anddriedwithN2.Thiscyclewasrepeatedtoyielda(PDDA/sodiumsilicate)6film with nanostructure on the Si substrate.46,47Preparation of the Microstructured Substrate. The micro-structuredsubstratewasfabricatedbydeposi
34、tionofsubmicrometerscalesilica spheres onto a silicon wafer according to a previous report.46Analcoholic silica suspension was first sonicated for 10 min to uniformlydisperse the silica spheres. A clean silicon wafer was immersed into thesuspension for 10 s at room temperature and then withdrawn fro
35、m thesuspensionatarateof1.5mm/s.Afterthealcoholicsolventsufficientlyvolatilized in a few seconds, silica spheres were successfully depositedontothesubstratesurface.Thedepositionprocesswasrepeatedforthreetimes to produce a rough surface. In order to make the surface structuremorerobust,across-linking
36、reactionwascarriedouttostabilizethesilicaspheres. Specifically, a silica-sphere-coated substrate was dipped into atoluene solution of SiCl4(1 wt %) and triethylamine (0.6 wt %) for30minandthenwashedwithtolueneseveraltimes,hydrolyzedinwater,and dried with a flow of nitrogen.PEM Fabrication and Counte
37、rion Exchange. PEMs wereassembled at room temperature by alternate dipping of a substrate inPDDA(1.0mg/mL)andPSS(1.0mg/mL)aqueoussolutionsfor15mineachwithwaterrinsingandN2dryinginbetweenuntiladesirednumberoflayers was obtained. NaCl of various concentrations was maintainedin thepolyelectrolyte solut
38、ions. All PEMs were capped with a PDDA outermostlayer, and the ClC0counterion in the PEMs was exchanged by immersingthe PEMs in an aqueous PFO solution (0.10 M) for 1 min, followed byrinsing with water and drying with N2. In this work, the contact angles andcontactanglehysteresiswerealwaysmeasuredaf
39、terthePEMsurfaceswerehydrophobized by counterion exchange with PFO anions.Chemical Modification of the Substrate. POTS was used tomodify substrate surfaces by the chemical vapor deposition (CVD)method. A sealed vessel containing the substrate and several drops ofPOTS was heated in an oven at about 1
40、20 C176C for 3 h to enable thereaction between the OH groups on the substrate surfaces and thePOTS and then maintained at about 150 C176C for 1.5 h to remove theunreacted POTS molecules.Characterization. Microstructures of the nanoscale asperities andthe microstructured surface coated by submicrosca
41、le silica spheres wereobserved on a field emission scanning electron microscope (FESEM,Micro FEI Philips XL-30-ESEM-FEG) operating at 20 kV. Topographyand roughness of the PEMs were assessed with a tapping mode atomicforce microscope (AFM, SPA-300HV, with a SPI3800N Probe Station,Seiko Instruments I
42、nc.). Probes with a resonant frequency of 60C0150 kHzand a spring constant of 3 N/m were used. Root-mean-square (rms)roughness was calculated as follows:rms 1NNi1ziC0zav2s15301 dx.doi.org/10.1021/la2039448 |Langmuir 2011, 27, 1529915304LangmuirARTICLEwhere ziis the z value of a specific pixel, zavis
43、 the average value of thez values in the scan area, and N is the number of pixels in the same area.Water contact angles were measured using a Kruss DSA10-MK2 dropshape analyzer at room temperature using water as the probe fluid(4 L). Each contact angle value reported was an average of at least fivei
44、ndependent measurements.RESULTS AND DISCUSSIONBetween the two factors which primarily contribute to surfacecontact angle hysteresis, roughness is believed to have a greaterinuence on apparent contact angles and thus contact angle hyster-esis compared to chemical defects (heterogeneities).10According
45、ly,the eect of roughness or topographic features on contact angleshasattractedpersistentattention,9,10,20,26C029,34C037,48whereasthestudies of chemical defects on surface wettability are rare.4,12C014Surfaces of PEMs assembled by the layer-by-layer (LbL) tech-nique are known to be heterogeneous due
46、to interpenetration ofconstituent layers and desorption of assembled molecules.Recently, in a preliminary study we reported a method to revealand quantify surface heterogeneities (surface defects) in PEMsanddemonstratedthatcontactanglehysteresismaybecorrelatedto area fraction of the defects.23Thus,
47、PEMs are suitable targetsfor studying the eect of defects on wettability. All PEMs used inthis work were assembled from PSS and PDDA with a PDDA-capped layer and then hydrophobized with PFO anion viacounterionexchange.43C045ThesurfacesofthesePEMsaremostlyhydrophobic PFO units, with smallfractions of
48、 areas where PFOunits are absent, hydrophilic defects.23Because ionic strengthin polyelectrolyte solutions49,50and number of bilayers depo-sited50C052are crucialto the PEM buildup and properties,werstprobed the eects of these two factors on the contact anglehysteresis. Figure 1 displays contact angl
49、es and hysteresis for thePEMs deposited on at substrates from solutions containing0.1 M NaClwithnumber ofbilayers n ranging from 0to8.Whilethe receding contact angle remained low at about 20C176, both theadvancing contact angle and the hysteresis rose with n rapidlyand reached plateau values of 120C176 and 100C176, respectively.The surfaces of these PEMs were at and featureless with rmsroughness values of 1 nm (Supporting Information). There-fore,theincreasesofbothadvancingcontactangleandhyst
