Investigation of Roughness Periodicity on the Hydrophobic Properties of Surfaces

Abstract

Hydrophobic films were synthesized with a variety of silica particle sizes ranging from 58 to 1428 nm to investigate the effect of particle size on the contact and sliding angles. While the surface roughness created by varying the particle size did not appear to affect water contact angles, the sliding angles showed a significant monotonic decrease, reducing from 908 to 208. IntroductionSelf-cleaning surfaces have developed since the late 20th century and are comprised of two main types, superhydrophobic and superhydrophillic. A surface is generally considered superhydrophobic if it has a water contact angle (WCA) of greater than 1508 and a sliding angle of less than 108. Superhydrophillic surfaces, which attract water, have a WCA of less than 108, thereby resulting in the droplet spreading across the surface quite rapidly. In both cases, the water rolls or slides off the surface at low angle and high velocity, carrying with it dirt and other contaminants, which is the basis of the self-cleaning property. [1] Materials possessing these properties have significant potential from self-cleaning windows to applications on solar panels and the reflective mirrors used in concentrated thermal and photovoltaic (PV) solar plants, whereby vast arrays of mirrors are used. For example, at the Ivanpah solar tower facility in the Mojave Desert in California, 173 500 mirrors are used over an area of 1417 ha.[2] Depending on the nature, size, and density of coverage, dust accumulation can diminish transmittance and/or reflectance at a rate 7 % per month, with every percentage lost thorough scattering or absorption, thus reducing the efficiency of the plant. [3,4] The result is that these systems require almost constant cleaning in regions where water can be scarce and expensive. To be effective on mirrors, PVs, and windows, the superhydrophobic coatings must be transparent and have both thermal and mechanical durability. [5,6] Nature is the primary inspiration for this technology as researchers have investigated the ability of some plants to self-clean via their hydrophobic properties. [7,8] This is best presented by the lotus leaf, such that the self-cleaning phenomenon of superhydrophobicity is referred as the 'Lotus Effect'. [9] Hydrophobicity of a solid surface is a property of a material that is dependent on both the surface energy and surface roughness. [10][11][12] The surface energy is inherent to a material and is therefore difficult to adjust, whereas the surface roughness is much more variable. Recently, superhydrophobic materials have garnered interest not only due to their self-cleaning, but also their anti-fouling, stain-resistant, and ice-repellent properties. [13][14][15] Superhydrophobic materials are fabricated through various approaches such as chemical vapour deposition, templating, chemical etching, self-assembly, sol-gel, and electrospinning. [16][17][18][19][20][21][22] One particular method demonstrated by Tserepi et al. involved plasma treatment of polydimethylsiloxane (PDMS) in SF 6 gas to form superh...

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