Wetting Behaviors of a-C:H:Si:O Film Coated Nano-scale Structured Surface Tae-Young Kim*,*** , Bialuch Ingmar **, Klaus Bewilogua **, Kyu Hwan Oh ***and Kwang-Ryeol Lee *

* Future Technology Research Division, KIST, KOREA** New Tribological Coating, Fraunhofer IST, GERMANY***Department of Materials Engineering, SNU, KOREA

Lotus SurfaceProperties of Lotus leaveWater wetting angle exceeds 150oWetting angle hysteresis is below 10o Water repellent and/or surface self cleaning effect

Lotus SurfaceSurface Material - cuticular waxSurface morphology very rough in micrometer scaleControl of surface chemical and structure enhances hydrophobic property50m20mPlanta, 202,(1998) 1

Super-hydrophobic SurfaceWater repellent surfaceSelf cleaning of surfaceSurface energy induced drop motionLow resistance coating against liquid flow

Super HydrophobicWhat is super-hydrophobic?hydrophilichydrophobicSuper hydrophobic90o150o120oFlat surfacechemical controlSurface chemical+roughnesscontrol

Just Rough Surface??Which surface is more hydrophobic?

Just Rough Surface?Water droplets behave differently on tilted surfaces because of the contact angle and contact angle hysteresis.Wetting angle hysteresis = advancing wetting angle receding wetting angle

Purpose of This WorkSuper hydrophobic surface generationHydrophobic DLC coatingSurface roughness controlled by Si etching process

GoalStatic wetting angle >150owetting angle hysteresis < 10o

Optimizing surface roughness structure Mono roughnessDouble roughness

Experimental ProcessSi waferThin metal(Cu) film deposition Metal dot formation by heat treatmentPlasma etching conditionsCF4+O2etched surface is flatCF4formation of nano post on etched surface

Plasma Si EtchingPlasma source gas : CF4Nano post formationPlasma source gas : CF4+O2Flat etched surfac

Surface Structure ManipulationCF4+O2 plasmaSi waferSi waferHydrophobic a-C:H:Si:O deposition CF4 plasmaCF4 plasmaCF4+O2 plasma

Wetting Angle AnalysisStatic wetting angle (apparent wetting angle)Water drop volume : 5LGently drop on the surface

Dynamic wetting angleWater drop size continuously changed (0.053 L/sec)Advancing angle (AA): 0 to 5 LReceding angle (RA): 5 to 0 LWetting angle hysterisis : AA-RA

Static Wetting AngleDLC coated Si(110)

Static Wetting Angle

Dynamic Wetting AngleDLC coated Si(110)

Advancing wetting angle (AA)Receding wetting angle (RA)Wetting angle hysterisis (AA-RA)148.7100.548.2

Dynamic Wetting Angle

Advancing wetting angle (AA)Receding wetting angle (RA)Wetting angle hysterisis (AA-RA)113.560.752.8

Advancing wetting angle (AA)Receding wetting angle (RA)Wetting angle hysterisis (AA-RA)165.7160.94.8

Double Rough Structure EffectDRS is more hydrophobic and suitable for moving droplet application than BP.The difference in structure is just bottom nano post in DRS DRS effect

103.8Static wetting angle159.652.8Wetting angle hysteresis4.8

SummaryWe fabricated double rough structure by nano structuring of Si.Double rough structure shows high static wetting angle and low wetting angle hysteresis.Double rough structure could be effective structure for moving droplet application. But why?

Thermodynamical CalculationSystem idealizationSurface structure : circular post typeVariables : post radius(Pr), post height(Ph), solid fractional factor (f), roughness factor (r), water drop radius(R), young contact angle(y)PrPhR

Thermodynamical CalculationLamgmuir 2004, 20, 10015Langmuir 2003, 19, 8343PrPhR

CalculationsSystem definition:Pr (205nm), Ph(413nm), f (0.5) (93o), R (5*109)

DRS calculation0

Parameters

Small Post StructureSystem definition:Pr (35nm), Ph(131nm), f (0.5) (93o), R (5*109)

Big Post StructureSystem definition:Pr (2565nm), Ph(393nm), f (0.065) (93o), R (5*109)

Double Rough StructureSystem definition:Pr (201nm), Ph(436nm), f (0.08) (93o), R (5*109) (134o)

Hysteresis and Energy Barrier (WC)

18.6828(x108)J 1.0904952.8Wetting angle hysteresis4.8

ConclusionWe fabricated double rough structure by nano structuring of Si.Double rough structure shows high static wetting angle and low wetting angle hysteresis.Double rough structure could be effective structure for moving droplet application. Low hysteresis in DRS would caused by decrement of detaching energy barrier.

Lotus leave is representative plant that shows super hydrophobic propertyIts wetting angle exceed 160o and wetting angle hysteresis is belower than 10oFor this reason, it shows water repellent and self cleaning effect.

Lotus leave . 160 water wetting angle 10 wetting angle hysteresis . super hydrophobicity , .

.To understand this phenomenon, scientist observed that lotus surface.From the observation, they find that lotus surface was covered by some kind of wax, and it has very rough surface morphology.This observation results gives some prediction which is superhydrophobic may be caused by surface roughness.And Wenzel and Cassie makes some model to show the relation surface morphology and water contact angle.

. cuticular wax , micrometer scale roughness . rough . Many scientist and engineer want to realize this surface on to real material.If we make superhydrophobic surface on to real material, we can expect water repellent surface, self cleaning surface, surface energy induced drop motion, and low resistance coating at microfludic system.In here, I hope you remember that all the candidate applications are related with moving droplet.

self cleaning effect , surface energy induced drop motion, low resistance against liquid flow .Figures shows two kinds of water drop on the rough surface. What do you think which is more hydrophobic?Maybe everybody thinks the left side is more hydrophobic..

water wetting angle ? . . .

water wetting angle ? . . .