다공성 나노구조 위에서의 액체 증발현상 분석

Abstract

In this thesis, we analyzed evaporation kinetics of micro sessile water/ethanol drop on nanoporous structure ？C AAO (Anodic Aluminum Oxide) microcantilever.AAO fabrication consists of electropolishing, 1st anodization, wet etching, and 2nd anodization. High purity Al sheet (99.999 %, 1-mm thickness) is electropolished in a perchloric acid/ethanol solution (1:4, volume ratio, 7 →, 20 V, 5~10 min) to reduce roughness. Unarranged AAO is fabricated by using the 1st anodization which is carried out in a 0.3 M oxalic acid solution with 40 V at 15 →. After that, AAO substrate is wet-etched to remove oxide layer in a chromic acid/phosphoric acid solution (1.8 wt% CrO3/6 wt% H3PO4) for 5 hr. Then we can get a dimple structure leaved regular patterns without AAO on Al substrate. Finally, the 2nd anodization is carried out under the same condition as the 1st anodization, and I can get well ordered nano channel array as controlling time of the 2nd anodization. Fabrication of microcantilever arrays part consists of photoresist pattern formation by lithography, transfer of the pattern to the thin Al layer as a mask, completion of microcantilever using electropolishing. A 500 nm thick layer of aluminum was deposited on the AAO by thermal evaporation, followed by the spin-coating of photoresist (AZ1512). Microstructures were patterned by UV exposure for 5.5 s, and the reacted photoresist was removed by rinsing in AZ developer CD 30. The patterns were transferred to the aluminum layer by removing the unprotected area in an aluminum etching solution (H3PO4: CH3COOH: HNO3: H2O = 80:5:5:10 by wt%) for 20 min. The AAO not covered by the photoresist was wet-etched in a 5 wt% H3PO4 solution at 20→ for 2.5 hours. Finally, electrochemical etching was carried out at the identical conditions to the initial electropolishing except for a longer time.Three different types of microcantilever were used for alaysis on evaporation kinetics of sessile drop. These cantilevers were one plain silicon cantilever, two AAO cantilevers which have different pore diameter 35 nm, 50 nm and other dimensions were same. Surface of each cantilever was treated by UV light and OTS(n-octyltrichlorosilane) to make hydrophilic and hydrophobic.At first plain silicon cantilever was used to analyze evaporation kinetics of water micro sessile drop on plain surface. As same as the results of previous paper written by E.Bonaccurso, hydrophobic Si cantilever showed CCR and CCA kinetics and hydrophilic Si cantilever showed CCR kinetics only. These phenomena were re-proved by high speed camera images. However, AAO cantilever showed some interest phenomena. First, In case of 50nm pore sized AAO cantilever with hydrophilic surface, increased deflection by water sessile drop was returned to original state rapidly. This phenomenon seemed that sessile drop on AAO surface evaporate in an instant, but resonance frequency that represents amount of mass change returned to original state slowly as same as 30nm pore sized AAO cantilever. Therefore, this rapid delfection change was not result from rapid evaporation but permeating into nano pores. Also, using images by high speed camera, we re-proved this assumption. Second, in case of every cantilever with hydrophilic surface, we discovered ‘negative inclination’ at the last phase of water evaporation. This phenomenon is caused by hydration of water. As water sessile drop evaporate, surface is hydrated and thin water film is formed on it. Thin water film applied compressive stress on cantilever structure, so cantilever bent down. Microcantilever returned to its original state as soon as all of water in thin film evaporated. As pore size of AAO cantilever is larger, this hydration effect shows larger. In case of water sessile drop on cantilevers with hydrophobic surface, they did not show any differences with silicon cantilever because water could not permeate into nano pore due to hydrophobic OTS.Ethanol is less viscous than water and more volatile than water. So deflection of cantilevers by ethanol sessile drop was smaller than that of water and evaporation time was also shorter than water. Ethanol sessile drop on cantilever with hydrophilic surface shows ‘nano pore effect’ as same as water case. However, in case of hydrophobic surface, 50nm-pore sized AAO cantilever shows strange phenomena. As same as water sessile drop on 35nm pore size AAO cantilever with hydrophilic surface, ethanol sessile drop on 35nm showed nano pore effect (negative inclination) caused by permeation and thin ethanol film. However ethanol sessile drop on 50nm-pore size AAO cantilever did not show permeation. This might be caused by increased hydrophobicity as increasing pore size, so hydrophobicity of cantilever is larger than ethanol. High hydrophobicity forbid ethanol to permeate into nanopore, so nano pore effect was not observed in this situation. To obtain precise theory, repeatable experiments are needed.