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 Surface Properties of Lotus leave Water wetting angle exceeds 150o Wetting angle hysteresis is below 10o Water repellent and/or surface self cleaning effect Lotus leave is representative plant that shows super hydrophobic property It”s wetting angle exceed 160o and wetting angle hysteresis is belower than 10o For this reason, it shows water repellent and self cleaning effect. Lotus leave 은 초 척수성을 보이는 자연계 대표적인 식물입니다. 그 특성을 정리하면 160 도 이상의 높은 water wetting angle 과 10도 미만의 낮은 wetting angle hysteresis 를 가지는 것을 특징으로 들 수 있습니다. 이러한 특성을 흔히들 super hydrophobicity 라고 말하는데, 이러한 특징으로 인하여 연 잎 표면의 방수 특성과 또 자기 청결 효과등이 일어나는 것으로 알려져 있습니다. 이에 많은 연구자들이 연 잎을 재료 표면에 모사함으로 다양한 응용을 시도하려 하고 있습니다.

Lotus Surface 20μm 50μm Planta, 202,(1998) 1 Surface Material - cuticular wax Surface morphology – very rough in micrometer scale 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. 자 그럼 표면의 초척수성을 구현하기 위한 방법을 알기 위하여 연 잎을 관찰해 보면 다음과 같은 사실을 알 수 있습니다. 우선 연잎의 표면은 척수성을 보이는 cuticular wax 로 구성되어져 있으며, 또한 표면의 형상은 micrometer scale 의 돌기들이 분포한 매우 높은 roughness 를 보이는 것을 알 수 있습니다. 이상의 관찰결과들로부터 초 척수성의 재현을 위해선 rough 한 척수성 표면을 만드는 것이 기본 조건임을 알 수 있습니다. Control of surface chemical and structure enhances hydrophobic property

Super-hydrophobic Surface Water repellent surface Self cleaning of surface Surface energy induced drop motion Low resistance coating against liquid flow 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. 현재까지 기대되는 초 척수 표면의 응용처로는 연잎에서 관찰되는 표면 방수 효과나 self cleaning effect 뿐만 아니라, surface energy induced drop motion, low resistance against liquid flow 등을 기대 할 수 있습니다.

Surface chemical+roughness Super Hydrophobic What is super-hydrophobic? Super hydrophobic hydrophilic hydrophobic 90o 150o Flat surface chemical control Surface chemical+roughness control 120o

Which surface is more hydrophobic? Just Rough Surface?? 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.. 높은 water wetting angle 을 가지는 것이 초 척수성이라고 말 할 수 있을까? 보시는 그림에서 어떤 표면이 더 척수성을 보이는 것인지 대답해 보십시요. 물론 왼쪽 그림이 훨씬 척수성을 보인다고 모두들 대답하실 것입니다. 그러면 다음그림을 보시도록 하겠습니다. Which surface is more hydrophobic?

Just Rough Surface? 다시말해 단지 높은 water wetting angle 을 가지는 것이 초 척수성이라고 말 할 수 있을까? 보시는 그림에서 어떤 표면이 더 척수성을 보이는 것인지 대답해 보십시요. 물론 왼쪽 그림이 훨씬 척수성을 보인다고 모두들 대답하실 것입니다. 그러면 다음그림을 보시도록 하겠습니다. 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 Work Super hydrophobic surface generation Goal Hydrophobic DLC coating Surface roughness controlled by Si etching process Goal Static wetting angle >150o wetting angle hysteresis < 10o Optimizing surface roughness structure Mono roughness Double roughness

Thin metal(Cu) film deposition Experimental Process Thin metal(Cu) film deposition Metal dot formation by heat treatment Si wafer Plasma etching conditions CF4+O2etched surface is flat CF4formation of nano post on etched surface

Plasma source gas : CF4+O2 Plasma Si Etching Plasma source gas : CF4 Nano post formation Plasma source gas : CF4+O2 Flat etched surfac

Surface Structure Manipulation CF4+O2 plasma Si wafer CF4+O2 plasma CF4 plasma CF4 plasma Si wafer Si wafer Si wafer Si wafer Hydrophobic a-C:H:Si:O deposition

Wetting Angle Analysis Static wetting angle (apparent wetting angle) Water drop volume : 5μL Gently drop on the surface Dynamic wetting angle Water drop size continuously changed (0.053 μL/sec) Advancing angle (AA): 0 to 5 μL Receding angle (RA): 5 to 0 μL Wetting angle hysterisis : AA-RA

Static Wetting Angle DLC coated Si(110) DLC coated nano post

Static Wetting Angle DLC coated big post DLC coated DRS

Dynamic Wetting Angle Advancing wetting angle (AA) Receding wetting angle (RA) Wetting angle hysterisis (AA-RA) 94.0 74.7 15.3 DLC coated Si(110) Advancing wetting angle (AA) Receding wetting angle (RA) Wetting angle hysterisis (AA-RA) 148.7 100.5 48.2 DLC coated nano post

Dynamic Wetting Angle Advancing wetting angle (AA) Receding wetting angle (RA) Wetting angle hysterisis (AA-RA) 113.5 60.7 52.8 DLC coated big post Advancing wetting angle (AA) Receding wetting angle (RA) Wetting angle hysterisis (AA-RA) 165.7 160.9 4.8 DLC coated DRS

Double Rough Structure Effect BP DRS 103.8 Static wetting angle 159.6 52.8 Wetting angle hysteresis 4.8 DRS is more hydrophobic and suitable for moving droplet application than BP. The difference in structure is just bottom nano post in DRS DRS effect

Summary We 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 Calculation System idealization Surface structure : circular post type Variables : post radius(Pr), post height(Ph), solid fractional factor (f), roughness factor (r), water drop radius(R), young contact angle(θy) R Pr Ph

Thermodynamical Calculation Pr Ph Lamgmuir 2004, 20, 10015 Langmuir 2003, 19, 8343

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

DRS calculation 0<z<h: calculation results same with BP

Parameters

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

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

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

Hysteresis and Energy Barrier (WC) BP DRS 18.6828 (x108)J 1.09049 52.8 Wetting angle hysteresis 4.8

Conclusion We 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.