Elsevier

Current Applied Physics

Volume 12, Issue 1, January 2012, Pages 219-224
Current Applied Physics

Complete wetting characteristics of micro/nano dual-scale surface by plasma etching to give nanohoneycomb structure

https://doi.org/10.1016/j.cap.2011.06.003Get rights and content

Abstract

A variety of flat superhydrophilic surfaces have been fabricated for biological and industrial applications. We report here the preparation of a simple and inexpensive non-polar curved superhydrophilic surface. This surface has dual-scale surface roughness, on both micro- and nanoscales. Curved surfaces with a near-zero water contact angle and ‘complete wetting’ are demonstrated. By using a conventional plasma etching process, which creates microscale irregularity on an aluminum surface, followed by an anodization process which further modifies the plasma etched surface by creating nanoscale structures, we generate a surface having irregularities on two-scales. This surface displays a semi-permanent superhydrophilic property (if the surface has no damage by the exterior failure), having a near-zero contact angle with water drops. We further report a simple and inexpensive curved (i.e., non-planar) superhydrophilic structure with a near-zero contact angle. The dual-scale character of the surface increases the capillary force effect and reduces the energy barriers of the nanostructures.

Highlights

► We report here the preparation of a non-polar curved superhydrophilic surface. ► This surface has dual-scale surface roughness. ► Plasma etching was used to generate unevenness on the microscale. ► Anodic oxidation is achieved through a nanostructure. ► Curved surfaces with a near-zero water contact angle are demonstrated.

Introduction

The wettability of a solid surface is governed by its chemical composition and geometric micro/nanostructure [1], [2], [3], [4], [5]. Wettability is important in industrial applications and remains the subject of theoretical research. Surfaces are described as super-hydrophobic if their water contact angle (CA) is greater than 150°, and superhydrophilic if the CA is less than 10° [6]. These surfaces are of special interest because they are likely to have properties such as antifogging [7], [8], antifouling [9], [10] and boiling heat transfer [11]. Super-hydrophobic surfaces can generally be obtained by controlling the roughness of hydrophobic surfaces [12], whereas superhydrophilic surfaces are generated by exploiting a capillary effect on hydrophilic surfaces [13]. These surfaces generally have a nanostructure superimposed on a micro-structure (dual-scale) and a covering of a low-or-high surface energy compound. These structures can be produced by an electric field [14], [15], lithography [16], [17], template methods [18], self-assembly of a monolayer [19] or photocatalysis [20]. These methods all involve expensive materials and complex, slow processes and they are limited to wafer size and non-curved surfaces; also, the resulting surface degrades in air with age. It is therefore important to develop a simple method for fabricating flexible large-sized areas having uniform superhydrophilicity and durability in air and water.

Below, we report a novel method that is simple and robust for fabricating curved surfaces having superhydrophilic properties.

Anodic aluminum oxide (AAO) has been proposed in the last few years as a material suitable for use in nanotechnology. In particular, porous-type AAO has been used as a “nanohoneycomb” structured template for the fabrication of nanostructures [21], [22]. AAO-based nanofabrication is of great interest because of its excellent reproducibility, and the resulting surface area can be small or large.

The present method aims to provide a surface showing ‘complete wetting’ and a contact angle of virtually zero, and also a non-planar (cylindrical) surface on the eye scale.

Our method of fabrication is based on Wenzel’s equation [23] and the capillary effect. Wenzel’s equation applies to equilibrium angles on a rough surface, for which the contact angle is normally less (i.e., greater wetting) than on a flat surface of the same material, by which the contact angle is conventionally defined. Roughness is readily generated by a plasma etching process. After the rough surface has been formed, anodization is applied so as to generate nano-hole structures on the roughened surface. The rough surface obviously has a larger surface area than a flat surface. The apparent contact is further reduced by the capillary effect of the nano-holes, which enhances the wettability of the surface.

Formation of a surface capable of ‘complete wetting’ begins with the plasma etching process. This process is used in microelectronics fabrication for pattern transfer, and there are many review articles on various aspects of plasma etching [24], [25], [26]. Plasma treatment for surface modification is used to produce hydrophobic or hydrophilic surfaces on metals, plastics, glass or polymers. The application of plasma processes to microelectronics is particularly demanding. It is often necessary to control the profile of the features that are etched. This is achieved using a combination of energetic ion bombardment and chemical reaction with species produced by the plasma. After plasma etching, the etched aluminum sheet is anodized to fabricate a dual-scale structure on its surface, by electrochemical etching in electrolyte solutions with oxalic acid. The result is a surface with superhydrophilic properties. It is also possible to generate a curved (i.e., non-planar) superhydrophilic surface with a near-zero contact angle.

Section snippets

Theoretical background

The degree of wetting of a solid by a liquid in a solid-liquid-vapor system is characterized by the thermodynamic equilibrium conditions. Young’s equation, based on a mechanistic approach, expresses the relation between the horizontal components of the three interfacial tensions of such a system when three phases are in contact ascosθ=γSVγSLγLVwhere γ denotes the interfacial tension between solid–vapor (SV), solid–liquid (SL), and liquid–vapor (LV) phases, and θ is the contact angle measured

Experimental section

We started with industrial grade aluminum (99.5%) sheets (50 mm × 40 mm × 1 mm). Plasma etching was used to generate unevenness on the microscale. Etching of the material to physically remove material may affect the chemical bonds within. To fabricate nanostructures on the plasma etched surface, we then anodized the surface. We finally obtain the superhydrophilic surface.

Fig. 2 shows the simple overall scheme used to prepare the superhydrophilic surface. The micro-structure is first fabricated

Results and discussion

Fig. 3(a–d) shows field-emission SEM images of the three different types of samples tested: normal industrial aluminum, plasma etched aluminum and the dual-scale surface. Fig. 3(a) shows the normal aluminum surface, which is a flat surface with impurities. Fig. 3(b) shows an oblique (30° angle) view of the plasma etched aluminum surface. The microscale roughness was generated by the plasma etching process. The increased surface area and structural change causes a reduction in the contact angle

Conclusion

We have fabricated a surface that undergoes complete wetting, by means of dual-scale surface modification. The first stage is plasma etching, which generates microscale unevenness on the aluminum surface. The resulting plasma etched aluminum surface was then anodized. The anodized aluminum oxide surface contains nanoscale hole structures. The resulting dual-scale surface was like the reverse side of the lotus leaf, having superhydrophilic properties and a near-zero contact angle. This is the

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0018457) and performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, funded by the Ministry of Education, Science and Technology of Korea.

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