Novel method to characterize superhydrophobic coatings Mohamed A. Samaha, Hooman Vahedi Tafreshi, Mohamed Gad-el-Hak ⇑ Department of Mechanical & Nuclear Engineering, Virginia Commonwealth University, Richmond, VA 23284-3015, USA article info Article history: Received 17 November 2012 Accepted 25 December 2012 Available online 10 January 2013 Keywords: Superhydrophobic coatings Fibers Aerogel Buoyancy force Effective thickness Gas volume fraction Coating’s thickness abstract Superhydrophobic coatings possess a strong water-repellent characteristic, which, among several other potential applications, enhances the mobility of water droplets over the surface. The coating traps air within its micropores, such that a submerged moving body experiences shear-free and no-slip regions over, respectively, the air pockets and the solid surface. This, in turn, may lead to significant skin-friction reduction. The coating maintains its superhydrophobicity as long as the air remains entrapped. It is there- fore of great interest to precisely measure the amount of trapped air, which is particularly difficult to esti- mate for coatings with disordered microstructures. A novel method to measure the effective thickness and gas volume fraction of superhydrophobic coatings with either ordered or random microroughness is advanced. The technique is applied to both aerogel and electrospun fibrous coatings. The experiments utilize a sensitive weighing scale (down to 10 4 gm) and height gauge (down to 10 lm) to determine the buoyancy force on an immersed, coated glass-slide substrate. The measured force is used to calculate the volume fraction of entrapped air. The coating’s effective thickness also follows from the same calcu- lations. The sensitivity of our particular scale enables the measuring of thicknesses down to 3 lm, which is not readily possible with conventional thickness gauges. Smaller thicknesses could be measured using more sensitive scales. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction 1.1. Superhydrophobicity Superhydrophobicity could be achieved by a combination of low surface energy (chemical hydrophobicity) and micro- or nano- scale surface roughness. Chemical hydrophobicity is a material property of repelling water, which is achieved when the chemical structure of the surface is sufficiently different from that of water. This leads to strongly reducing the ability of the surface to interact with water. The superhydrophobic phenomenon is primarily man- ifested by water droplets beading on the solid surface with contact angles exceeding 150°, and the droplets readily roll off when the surface is tilted at a small angle. In nature, superhydrophobic sur- faces are exemplified by the lotus leaves, which enhance the mobility of rain drops on them, carrying dirt away, and creating a self-cleansing effect (so-called lotus effect). Neinhuis and Barth- lott [1] obtained scanning electron microscopy (SEM) images for several water-repellent plants and reported the micromorphologi- cal characteristics of 200 species. They demonstrated that the epi- dermal (i.e., outermost) cells of the lotus leaf form papillae, which act as microstructure roughness. The papillae are topped off by a very dense layer of epicuticular waxes (hydrophobic wax crystals), also referred to as hair-like structures [2] or nanostructure rough- ness [3]. Over the past two decades, biomimetic research of the lo- tus effect evoked several studies on manufacturing, characterizing, and applying superhydrophobic surfaces [1–9]. A superhydrophobic surface entraps air in its pores, and when a coated body is fully submersed in water, alternating water–solid and water–air interfaces are formed. The entrapped air is separated from water with a thin interface anchored on the solid walls and stretched due to surface tension forces. It has been observed that a moving body of water ‘‘slips’’ over the air–water interfaces, whereas the water ‘‘sticks’’ to the solid portions of the surface [7]. Therefore, if the percentage of the surface covered by air pock- ets is sufficiently high, a superhydrophobic surface can cause the so-called ‘‘slip effect,’’ resulting in a reduction in the skin-friction drag exerted on the surface [7]. As long as the air pockets exist, the surface remains superhydrophobic. In other words, the degree of hydrophobicity and the beneficial effects are diminished by the reduction of the amount of entrapped air [10–12]. The longevity of a superhydrophobic surface—how long the surface can maintain the air pockets—is critical, especially in underwater applications. An elegant experiment carried out by Sakai et al. [13] demonstrates the effect of surface microstructure on air-layer sustainability. 1.2. Fabrication Early manmade superhydrophobic surfaces were produced using the same microfabrication techniques developed for the computer industry. The coatings typically consisted of a regular ar- 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.12.066 ⇑ Corresponding author. E-mail address: gadelhak@vcu.edu (M. Gad-el-Hak). Journal of Colloid and Interface Science 395 (2013) 315–321 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis