The effect of hydrophobicity of micro/nanostructured-surfaces on behaviours of water spreading Hailong Zhang a , Reinhard I. Boysen a , Gemma Rius Suñé b , Xavier Borrise b , Francesco Perez- Murano b , Milton T. W. Hearn a , Dan V. Nicolau a, c∗ a ARC Special Research Centre for Green Chemistry, Monash University, Clayton, VIC 3800, Australia. b Centre Nacional de Microelectronica, CNM-IMB, CSIC, E-08193 Bellaterra, Spain; c Department of Electrical Engineering and Electronics, The University of Liverpool, Brownlow Hill, Liverpool, L69 3GJ UK ABSTRACT Atomic force microscopy (AFM) in conjunction with cross-section analysis was applied to determine the distribution, position and contact angle of spray-deposited water micro-droplets on micro-structured arrays. For this investigation, two micro-structured arrays on silicon wafers with a chessboard pattern of depressions and protrusions of various sizes were manufactured by e-beam lithography. The first array had a silicon oxide/silicon structure (hydrophilic/hydrophilic) with an elevated silicon oxide layer of 40 nm and the second had a gold/silicon structure (hydrophobic/hydrophilic) with an elevated gold layer of 35 nm. On the first array with only hydrophilic surfaces, the behavior of the water droplets was mainly affected by topography, whereby the contact angles on the structures were considerably higher than the contact angles of droplets on unstructured reference surfaces. On the second array the water micro-droplets were confined in the hydrophilic depressions by the hydrophobic boundary whereby the highest contact angle was measured on the smallest squares and the lowest contact angle was found on the largest squares, with the droplet angles of the squares of 1 and 0.5 µm exceeding the contact angles of the droplets on the unstructured reference materials. Keywords: Hydrophobicity, contact angle, micro/nanostructured-surface, atomic force microscopy 1. INTRODUCTION In the recent decade, wetting and spreading phenomena have received continued attention due to their broad applications in a wide range of research fields ranging from micro-contact printing, biofouling, DNA immobilization, cell growth, and tissue engineering. It is well known that liquid wetting on a surface depends on both surface chemistry and physical factors such as surface rough or topography. It has been reported that the micro/nano-patterned surface can greatly vary the surface wettability such that a hydrophobic surface becomes more hydrophobic and a hydrophilic surface becomes more hydrophilic [1, 2]. In the applications of nanotechnology and bio-nanofabrication, it is of fundamental importance to obtain the knowledge of the effect of surface wettability on the shape of water droplets under micro scale, whose spreading behaviour is determined by the surface chemistry and micro/nanostructures i.e., topography in the nano scale range [3, 4]. The Lotus effect, which refers to very high water repellence of the leaves of the lotus flower, Nelumbo nucifera, shows that the ever-clean leaves of the lotus flower are rough and each leaf surface is covered with tiny bumps, 5-10 µm in height and about 10-15 µm apart from each other, beyond this microstructure, the uneven surface itself is covered with waxy, hydrophobic, water repellent, crystals, measuring around 1 nm in diameter [5]. Inspired by Nature’s design, different structures have been created to produce superhydrophobic effects for the self-cleaning surfaces, in general, these rough surfaces are fabricated with a fibrillar structure attached and covered with low surface energy molecules [6- 8]. On the planar surfaces, Herminghaus et al. [9] have used AFM tapping mode imaging to examine the impact of the chemical structure on the flat surface on the shape of water drops. A strong corrugation of the contact line was observed ∗ d.nicolau@liverpool.ac.uk ; phone + 44 151 794 4506; fax: + 44 151 794 4540 Biomedical Applications of Micro- and Nanoengineering IV and Complex Systems, edited by Dan V. Nicolau, Guy Metcalfe Proc. of SPIE Vol. 7270, 727009 · © 2008 SPIE · CCC code: 1605-7422/08/$18 · doi: 10.1117/12.815259 Proc. of SPIE Vol. 7270 727009-1 Downloaded from SPIE Digital Library on 07 Oct 2009 to 138.253.168.213. Terms of Use: http://spiedl.org/terms