Laser textured surface gradients

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warwick.ac.uk/lib-publications

Original citation:

Ta, Van Duong, Dunn, Andrew, Wasley, Thomas J., Li, Ji, Kay, Robert W., Stringer, Jonathan,

Smith, Patrick J., Esenturk, Emre, Connaughton, Colm and Shephard, Jonathan D.. (2016)

Laser textured surface gradients. Applied Surface Science, 371 . pp. 583-589.

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Laser

textured

surface

gradients

Van

Duong

Ta

a,∗

,

Andrew

Dunn

a

_,

_Thomas

_J.

_Wasley

b

_,

_Ji

_Li

b

_,

_Robert

_W.

_Kay

b

_,

Jonathan

Stringer

c

_,

_Patrick

_J.

_Smith

c

_,

_Emre

_Esenturk

d

_,

_Colm

_Connaughton

d,e

_,

Jonathan

D. Shephard

a

a_Institute_of_Photonics_and_Quantum_Sciences,_Heriot-Watt_University,_Edinburgh_EH14_4AS,_UK b_Additive_{Manufacturing}_Research_Group,_Loughborough_University,_{Leicestershire}_LE11_3TU,_UK

c_Laboratory_of_Applied_Inkjet_Printing,_Department_of_Mechanical_Engineering,_University_of_Shefﬁeld,_Shefﬁeld_S1_4BJ,_UK d_Warwick_Mathematics_Institute,_Zeeman_Building,_University_of_Warwick,_Coventry_CV4_7AL,_UK

e_Centre_for_Complexity_Science,_Zeeman_Building,_University_of_Warwick,_Coventry_CV4_7AL,_UK

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received11January2016

Receivedinrevisedform3March2016 Accepted5March2016

Availableonline9March2016

Keywords: Roughnessgradients Wettabilitygradients Lasersurfacetexturing Nanosecondlaser Chemicalsensors

a

b

s

t

r

a

c

t

Thisworkdemonstratesanoveltechniqueforfabricatingsurfaceswithroughnessandwettability gradi-entsandtheirsubsequentapplicationsforchemicalsensors.Surfaceroughnessgradientsonbrasssheets areobtaineddirectlybynanosecondlasertexturing.Whenthesestructuredsurfacesareexposedtoair, theirwettabilitydecreaseswithtime(upto20days)achievingbothspatialandtemporalwettability gradients.Thesurfacesareresponsivetoorganicsolvents.Contactanglesofaseriesofdiluteisopropanol solutionsdecayexponentiallywithconcentration.Inparticular,afallof132◦_in_contact_angle_is_observed

onasurfacegradient,oneorderofmagnitudehigherthanthe14◦_observed_for_the_unprocessed_surface,

whentheisopropanolconcentrationincreasedfrom0to15.6wt%.Asthewettabilitychanges gradu-allyoverthesurface,contactanglealsochangescorrespondingly.Thiseffectoffersmulti-sensitivityat differentzonesonthesurfaceandisusefulforaccuratemeasurementofchemicalconcentration.

1. Introduction

Surfaceswithspecialpropertiesincluding

superhydrophobic-ity,superhydrophilicityandgradientareimportantfornumerous

applicationsinbiomedical,microﬂuidics,sensorsandsuppression

of the coffee-stain effect [1–6]. While

super-hydrophobic/-hydrophilicsurfacesexhibiteitherincompleteorcompletewetting,

surfaceswithwettabilitygradientsaremoreinterestingbecause

theirwettabilitychangesgraduallyovertheirlengthinspaceand

may even develop in time [5]. Surface gradients are common

innaturewhichdemonstrateuniqueabilitiessuchasdirectional

watercollection(fromhumidairorfog)[7,8].Artiﬁcialgradients

havealsobeenfabricatedwithpotentialincontrollingliquid

move-ment, solvingheat transfer problemsand, pH sensitivedevices

[9–12].

Gradientscanbesimplyclassiﬁedintotwo categorieswhere

surfacespossesseitheragradualvariationofchemicalorphysical

properties[5].Physicalgradientshavegradualpatternsor

rough-nessprocessedonthesurface[13–17].Chemicalgradientsaremore

∗Correspondingauthor.

E-mailaddress:[email protected](V.D.Ta).

common,whichareprimarilyformedbycoatingordepositingthin

chemicallayersonoriginalmaterial[9–12,18–24].

Recently,lasertexturinghasbeendemonstratedasanexcellent

toolformodifyingsurfaceroughnessonnearlyalltypesofmaterials

[25–29].Comparedwithchemicalmethods,directlasertexturing

isalowwaste,single-stepprocedurewithpotentiallyhigh

process-ingrateandimportantly,theabilitytocontrolsurfaceroughness

orwettabilitydirectlyontheoriginalmaterialswithoutcoating

[30–32].However,directlylasertexturingthesesurfacegradients

havebeenrarelystudiedwithafewreportsthatinvolvelaser

tex-turedgroovestructureswitharegularchangeingroovespacing

[33,34].Asaresult,investigationofdirectlaserpatterningstructure

gradients(bothspatiallyandtemporally)usingnovelapproachesis

necessaryandsigniﬁcantfortakingadvantageoflasertechnology

forthecreationofsmartsurfaces.

Thisworkdemonstrates surfacegradientsobtainedbydirect

nanosecond laser texturing and their applications as

multi-sensitivitychemicalsensors.

http://dx.doi.org/10.1016/j.apsusc.2016.03.054

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584 V.D.Taetal./AppliedSurfaceScience371(2016)583–589

Fig.1.Characteristicsoffocusedlaserbeamusedforsurfacetexturing.(a)Schematicofthebeamshapeduringfabricationprocess.(b)Calculatedbeamdiameterdetermined as1/e2_of_maximum_intensity_proﬁle_and_the_{corresponding}_pulse_ﬂuence_as_function_of_the_defocus_distance_z.

2. Materialsandmethods

2.1. Materials

Alllaserprocessingwasperformedon0.6mmthickbrasssheets

(CZ121M,RSComponents).Thesampleswerecleanedwith

iso-propanolbeforeirradiatingwithlaser.

2.2. Lasersurfacetexturing

The surface morphology of the samples was modiﬁed by a

nanosecondpulsedﬁbrelaser(SPI,20WEP-S)withawavelengthof

1064nm,pulsedurationof∼220nsandrepetitionrateof25kHz.

Theﬁbrelaseroutputiscollimatedbeforedeliverytoa

galvanome-terscannerandF-Thetafocusinglens.Thelaserbeamisscanned

acrossthesamplesurfacewithanominalfocalspotsizeof28.4␮m.

Parallelmicro-groovestructureswithaﬁxedhatch(scanline

sepa-rationh)distancebetweenthemweretextured.Thescanningspeed

of75mm/sandthelaserpulseenergyof0.252mJwasﬁxedforall

fabricationprocesses.

2.3. Surfacecharacterization

Thesurfacemorphologyofthelasertexturedsampleswas

stud-iedbymeansofscanningelectronmicroscope(SEM)andoptical

microscope(LeicaDM6000M).Thearithmeticaverageofsurface

roughness(Rs)wasobtainedfromzdatameasuredwiththeLeica

microscopeonanareaof∼1mm2_.

Surfacewettabilitywascharacterizedbycontactangle()of

∼5␮Ldeionizedwaterdropletsdepositedontopofthesamples.

Thecontactangle wasdetermined byanalyzingdropletimages

(capturedbyaUnibrain1394camera)usingthesoftwareFTA32

(version2.0).

2.4. Preparationofisopropanolsolutionandsensing demonstration

Severalisopropanolsolutionswithpercentcompositions(mass

of solute/totalmass of solution) up to15.6wt% weremade by

mixingvariousamountsofisopropanol (99.5%purity)in

deion-ized water. Isopropanol solution droplets (∼5␮L) on gradient

surfaceswerecapturedandtheircontactanglesweredetermined

asdescribedinSection2.3.

3. Resultsanddiscussion

3.1. Characteristicsoffocusedlaserbeam

Forlaserprocessing,thelaserbeamisgenerallyfocusedonthe

samplesurfaceduringfabricationtooptimizelight-matter

inter-action(Fig.1a).Thebeamspotcanbecharacterizedbyadefocus

distance(z)whereapositivevalueindicatesthatthebeamfocus

isabovethesurface.Fig.1bplotscalculatedpulseﬂuencesversus

z.Itcanbeseenthatatthefocalplane(z=0mm),theﬂuenceis

thehighest,about34mJ/cm2_,_{corresponding}_to_the_smallest_beam

diameterof28.4␮m.Theﬂuencereduceswithincreasingzand

whenz=0.5mm(abeamsizeof37.1␮m),itdropsto20mJ/cm2_,

about59%oftheoriginalvalue.Asthemorphologyoflasertextured

surfacestronglyrelatestolaserpower,thegradualchangeoflaser

ﬂuencewithzopensupthepossibilityofcreatingsurfacegradients

bydirectlaserwriting.Itisdemonstratedthatbytiltingthesample

(discussedlater)lasertexturingsurfacegradientscanbefabricated.

3.2. Effectoflaserradiationonsurfacemorphology

Fig. 2a shows schematic of direct laser writing parallel

microgroovesonthebrasssurfaceusingthescanhead.Theeffectof

laserradiationonsurfacemorphologydependsonthevalueofhand

sample’spositionrelativetofocalpoint.Forﬁxedh,theeffectisthe

greatestwhentheprocessingsurfaceisatthefocalplane(z=0mm).

Asshown inFig.2b,thetexturedsampleexhibitsadarkcolour

duetoacombinationofoxideformationandnon-reﬂecting

prop-ertiescausedbyincreasedroughness.Incontrast,forz=0.45mm

(Fig.2c),thefabricatedsurfaceshowsamuchlightercolourwhich

indicatesthatthesurfacehaslessoxideandalowerroughness.

Sur-faceroughnessmeasurementswereperformedonthesesamples

andtheaverageRswasfoundtobe∼2.6␮mforz=0mm,which

istwotimeshigherthanthatof1.3␮mforz=0.45mm.Theresult

conﬁrmsthatlaserinducedroughnesscanbecontrolledby

adjust-ingsamplepositionverticallytolaserbeamdirection.Togetbetter

understandingof surfacemorphology,SEManalysiswascarried

outandisshowninFig.3.Theresultindicatescleardifferences

betweenthetwocases.Forz=0mm,correspondingtoaﬂuence

of34.3mJ/cm2_,_a_large_amount_of_material_was_evaporated_which

createdsigniﬁcantdebrisand formed obviousmicrogrooves. In

contrast,forz=0.45mmthelinestructureswerenotcompletely

formedasthelaserﬂuenceof21.9mJ/cm2_is_close_to_the_ablation

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Fig.2.Schematicoffabricationprocessandopticalimagesoflasertexturedsurfaces.(a)Thescanheaddeliversthelaserbeamoverthesamplesurfacetogenerate microstructures.(b,c)sampleimages(7mm×6mm),h=75␮m,fabricatedwhenz=0and0.45mm,respectively.

Fig.3. Lowandhighmagniﬁcationscanningelectronmicroscope(SEM)imagesofthelasertexturedsurfaceswithh=75␮m.(a,b)z=0mm.(c,d)z=0.45mm.

3.3. Timedependenceofsurfacewettability

Directly after fabrication, both surfaces (z=0 and 0.45mm)

exhibitahydrophilicpropertycharacterizedbysmallcontactangle

(<20◦)comparedwith∼70◦ oftheas-receivedsurface.The

con-tactangle of unprocessed surfacesdoes not showa signiﬁcant

changewithtimewhenexposedtoambientconditions

(temper-atureof∼22◦Candrelativehumidityof∼44%).However,under

thesameenvironment,thecontactangleforlaserprocessed

sam-plesincreasedwithtime(Fig.4).Thecontactangledevelopment

wasfasterforthesamplesfabricatedwithz=0.45mmcompared

withthosecreatedwithz=0mm.Forexample,atday4,the

con-tactangleswere99◦and49◦,forz=0.45and0mmrespectively,a

differenceof50◦.However,itwasobservedthatthedifferencein

contactanglebetweenthetwosurfacesbecomessmallerwithtime.

At14days,thedifferenceisonly10◦(151◦and141◦).From16days,

bothsurfacesexhibitsuperhydrophobicbehaviourwithsimilarand

stablecontactanglesof∼154◦.Theresultindicatesthatunderthe

sameconditions,theevolutionofcontactangleisfasterforsamples

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Fig.4.Surfacewettabilityevolution,characterizedbythecontactangleoftextured samplesovertime,h=75␮m,processedwiththedefocusdistancez=0and0.45mm. Eachdatapointpresentedisanaveragevalueoftwoindividualmeasurements.The insetshowsdropletimagesonsamplesafterexposuretoairfor9days.

reportsonfemtosecondlaserpatternedstainlesssteel[31].

Fur-thermore,thedifferenceincontactanglebetweenthetwosurfaces

suggeststhatawettabilitygradientisachievablefromsampleswith

roughnessgradients.

Thedevelopmentofcontactanglefromhydrophobicto

superhy-drophobiconlasertexturedmetallicsurfaceshasbeenpreviously

reportedandthemechanismisascribedtomodiﬁcationofsurface

chemistry[31].Theobservationofsmallcontactangledirectlyafter

fabricationcanbeprimarilyexplainedbythesurfacemorphology

describedbyWenzelequation[35]:

cosW=rcosf (1)

wherer>1istheroughnessfactor.W andf arecontactangles

onroughand ﬂat surfaces,respectively. Accordingly toEq. (1),

roughnessenhancesthehydrophilicpropertyofthesurfaceand

thehighertheroughfactor,thelowerthecontactangle.Thechange

inwettabilityis,however,duetosurfacechemistryasthereisno

changeinsurfacemorphologywithtime.Indeed,fromEq.(1),it

suggeststhatsurfacechemistry,whichinduceschangeinf(f

),wouldcontributetoacomparativevariationinW (W)as

fol-lowing[36]:

W=r

sinf/sinW

f (2)

Inthiswork,itisbelievedthattheadsorptionoforganicmatter

fromtheatmosphere[37]andpartialdeoxidationofcopperoxide

(2CuO=Cu2O+1/2O2)[38]aretwomechanismthatwere

respon-sibleforthechangeofsurfacewettability.

3.4. Surfacemorphologygradients

Fig.5aillustratesaschematicforthefabricationofroughness

gradientsonabrasssurface.Thesampleistiltedatanangleof␣

withoneendlocatedatthefocalplane.Asaresult,thelaserspot

diameter,andthuslaserﬂuence,graduallychangesacrossthe

sam-pleduringthescanningprocessresultinginstructuregradients.

Fig.5bshowsanopticalimageofatypicalfabricatedsurface

gradi-entfabricatedwith␣=1.3◦.Thesampleexhibitsacolourgradient

fromdarkyellow(zone1,highﬂuence)tolightyellow(zone3,low

ﬂuence).Thesurfacemorphologyacrossthesamplewasstudied.

Fig.5cshows theaveragesurfaceroughnessofsamplescreated

with␣=1.1◦and1.3◦.GradualvariationsofRsfromzone1tozone

3canbeseenforbothcases.Atzone1,Rswasabout2.7␮mfor

bothtiltedangles.Thesevaluesdecreasedto1.8␮mfor␣=1.1◦and

1.5␮mfor␣=1.3◦.Togetabettervisionofthesurface

morphol-ogy,SEManalysiswascarriedoutonasamplecreatedwith␣=1.3◦

andtheresultsareshowninFig.5d–f.Thedepthofthefabricated

microgroovesdecreasesfromzone1tozone3.Itisexpectedthat

otherapproachessuchasagradientﬁltercanbeusedtocontrol

thelaserpowerandsurfaceroughnessgradientscanbeobtained

bydirectlywritinglaseronﬂat(insteadofinclined)surfaces.

3.5. Surfacewettabilitygradients

Ithasbeenshownsurfacemorphologygradientsonabrass

sur-facecanbeobtainedbylaserwriting.Thesurfaceroughnessofthese

samplesisnolongerhomogenousbutafunctionofthecoordinates

r(x,y),andtherefore,theWenzelequationisexpressedinageneral

form[39]:

cosW =r(x,y)cosf (3)

AccordingtoEq.(3),agradientofroughnesswouldleadtoa

gra-dientofcontactangleorwettability.Fig.6presentsdropletimages

onthreedifferentzones(twoedgesandmiddle)ofgradient

sur-facescreatedwith_␣1=1.3◦afterexposuretoairfordifferenttime.

Atday5(Fig.6a),thesurfaceshowsawettabilityfrom

hydrophilic-ity(zone1,∼40◦)tohydrophobicity(zone3,∼100◦).Contact

angleacrossthesurfaceincreasedwithtime,buttheratewas

dif-ferent.Afterninedays,thecontactanglewasaround105◦(zone1),

125◦(zone2),and140◦(zone3).Thatmeansthecontactanglegap

reducedfromabout60to35◦afterfourdays.

Fig.7plotscompletecontactangleonsurfacesfabricatedwith

␣=1.1◦and1.3◦versustime.Itcanbeclassiﬁedintwoperiodsof

time,beforeandafter17days.Before17days,thesurface

exhib-itedawettabilitygradient.For1.3◦,thevariationofroughnessis

highersotherewasaclearseparationincontactangleamongthree

zones,inwhichthecontactangleatzone3wasthehighest,and

asexpected,thelowestatzone1.For1.1◦,thecontactangleat

zone3wasstillthehighestbutthelowestmeasuredangle

ﬂuc-tuatedbetweenzone 1and 2 becauseoftheirsmallroughness

differences.From17days,bothsurfacesbecamesuperhydrophobic

withacontactangleof150–154◦.Hence,forsomeapplications,the

gradientsurfacesmayneedtobetemporallyindependent.Ithas

beendemonstratedthatstoringenvironmentscanstrongly

sup-pressthechangeofsurfacewettability[37],whichisofinterestfor

futureinvestigations.Anotherpossibilityoffreezingthese

gradi-entsmightbecoatingbyasilanizationlayer[29].However,simply

immersinginboilingwatertodeactivatetheactivesites,as

pre-viouslyreported[31],wouldnotworkforthesesubstratesasthe

surfacewilllosethehydrophobicproperty.

3.6. Applicationofsurfacegradientsasmulti-sensitivitychemical sensors

Responsivesurfacesbasedonwettabilityhavepotential

appli-cationsin microﬂuidics,smartdevices andanalysisof chemical

solutions[40].Forsurfacegradients,thewettabilityresponsesto

external environment is spatially dependent[12]. In this work,

a chemicalsolvent(isopropanol) wasusedasa stimulus.Fig.8

demonstratesthatthecontactangleofisopropanolsolutionson

thesurfacegradientsdecayexponentiallywithconcentration.The

effectisspatiallydependent.Ahigherroughnessleadstoalower

decreaseincontactangleandoldersamplesarelessresponsive.

Itiswell-knownthatsurfacetensionhasanimpactonthe

con-tactangle [41]. Thecontact angleof chemical solvents suchas

methanolandisopropanolwasfoundtobeverysmall,closetozero,

onthelaserprocessedsurfaces.Thecontactangleforwateronsuch

asurfaceishigh(∼150◦forthosehadbeenexposedtotheairfor

17daysorlongertime).Itthereforefollowsthatthecontactangle

foraliquidwhichisamixtureofthesesolventswithwatershould

dependontheconcentrationofthesolvent.Furthermore,the

con-tactanglealsodependsonsurfaceproperties.AccordingtoEq.(1),

itisproposedthatthedecreaseofcontactangleonroughsurfaces

ishigher thanthatonﬂatsurfaces,as itisampliﬁedby

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Fig.5.(a)Schematicofsurfacegradientsfabrication.Twotiltedangles(␣1,␣2)areconsidered,1.3◦₍_␣₁₎_and_1.1◦₍_␣_2)._(b)_Optical_image_of_a_surface_gradient₍₇_mm_×₂₁_mm, h=50␮m)exhibitsacolourgradientfromzone1tozone3.(c)Arithmeticaverageofsurfaceroughness(Rs)acrossthegradientsamples.(d)–(f)SEMimagesobtainedon threedifferentzonesofthegradientsamplefabricatedwhen␣1=1.3◦.

Fig.6. Behaviourofwaterdropletsongradientsurfacesatthreedifferentzones.Samplesfabricatedwith␣1=1.3◦,h=50␮mandhasbeenexposedtoairfor(a)5and(b)9 days.

surface(theleastroughness)reducedonly14◦,from69◦ to55◦,

whentheisopropanolconcentrationincreasesfrom0to15.6wt%.

Forsurfacegradient(29-dayold),thecontactangledropped84◦

(from153◦to69◦)atzone3and132◦(from153◦to21◦)atzone

1.Thatmeansatzone1(thehighestroughness),thecontactangle

reductionisoneorderofmagnitudehighercompared withthat

ofas-receivedsurface.Thedecreaseofcontactanglewith

concen-trationiswelldescribedbyanexponentialdecayfunction,which

suggeststhatthesurfacescanbeusedassolventresponsivedevices

orsensors.Ithasbeenshownthatthecontactanglechangeis

spa-tiallydependentsothesesosensorsbasedonsuchsurfacescould

havemulti-sensitivity.Usingbothzone1and3inorderto

deter-minetheconcentrationofanisopropanolsolutionwouldincrease

theaccuracy.

Fig.8bshowsthatthesurfacesbecomelessresponsivewhen

exposedtoairforalongerperiodoftime.Forasamplethatwas

39daysold,thecontactangledecreasedfrom154◦to40◦atzone1

andfrom154◦to78◦atzone3.Comparedwiththe29daysold

sam-ple,thesensitivityhasreducedabout10–14%.Thisisbelievedto

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Fig.7. Contactangleofwaterdropletsmeasuredovertimeonthesurfacegradientsatthreedifferentzones.(a,b)Thesampleswereprocessedwithh=50␮mandtilted angleof1.3◦_and_1.1◦_,_{respectively.}_Each_data_point_presented_is_an_average_value_of_two_individual_{measurements.}

Fig.8.Contactangleofisopropanol/watersolutionontheunprocessedas-receivedbrasssurfaceandlasertexturedgradientsurfaces,␣1=1.3◦,h=50␮m.Thesampleswere leftunderambientconditionsfor(a)29and(b)39daysafterfabrication.

thatanagingfactorshouldbeconsideredwhenusingthesurfaces

forsensingapplications.Inaddition,itisexpectedthesesurface

sensorscanbeappliedforsensingconcentrationofvariousorganic

solventssuchasmethanol,ethanolandacetone.Theeffectmayalso

workforotherwatersolublechemical.

4. Conclusions

Ithasbeendemonstratedthatsurfaceswithroughness

gradi-ents,andconsequentlywettabilitygradients,canbefabricatedby

directlasertexturing.Thewettabilityisboth spatiallyand

tem-porallydependent,whichisinterestingforsensingapplications.

Proof-of-conceptchemicalsensorsbasedonreductionofthe

con-tactanglewithincreasedconcentrationareshown.Inparticular,

thecontact angle of isopropanol solution decaysexponentially

withconcentration.Theeffectisspatiallydependent.Areaswith

higherroughnessexhibithighercontactangledecrease.Thatmeans

differentlocationsonthesurfacecanbeusedasmulti-channels

for accuratemeasurement of the concentration. A decrease of

132◦ in contact angle ona surface gradient when isopropanol

concentrationisincreasedfrom0to15.6wt%hasbeenobserved.It

ismuchmoresensitivecomparedto14◦forunprocessedsurfaces.

Thisworkprovidesanovelfabricationtechnologyusinglow-waste

andcost-effectivenanosecondlasersystemsanda sensing

prin-ciple for practical development of chemical sensorsand smart

surfaces.

Dataavailability

Allrelevantdatapresentinthispublicationcanbeaccessedat:

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Acknowledgements

We thank Dr. JimBuckman for helping withSEM

measure-ments.ThisworkisfundedbytheUKEngineeringandPhysical

Sciences Research Council under grants EP/L017431/1,

EP/L017350/1,EP/L016907/1andEP/L017415/1.

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