Effects of the Surface Roughness on Sliding Angles of Water Droplets on Superhydrophobic Surfaces

Abstract

Various superhydrophobic films having different surface roughnesses were prepared, and the relationships between the sliding angle, the contact angle, and the surface structure were investigated. In the highly hydrophobic region, the sliding angles of water droplets decreased with increasing contact angles. Microstructural observation revealed that surface structures that can trap air are important for the preparation of low-sliding-angle surfaces. We have also derived an equation that describes the relationship between sliding angles and contact angles on superhydrophobic surfaces with roughness. The results calculated on the basis of this equation agreed well with the experimental ones. Moreover, we have successfully prepared a transparent superhydrophobic film whose sliding angle is ∼1°for a 7 mg water droplet. On this film, there was almost no resistance to the sliding of water droplets. The film obtained satisfies the requirements of superhydrophobicity, transparency, and a low water sliding angle.

Droplet motion on inclined heterogeneous substrates

“…This formula was also derived a few years later by Frenkel (1948) through a minimization of the total energy of the system. Since then, a number of authors (see, for example Furmidge 1962;Dussan 1985;Quéré 1998;Miwa et al 2000; ElSherbini & Jacobi 2004a) proposed generalizations of (1.1) to account specifically on the steady, slow motion of rounded droplets at small inclination angles, deducing a phenomenological scaling law to explain their observations (see also Podgorski et al 2001;Le Grand et al 2005, in which both rounded and cusped droplets are investigated). To address the dynamic problem numerically, a broad range of methodologies were utilized, ranging from precursor film models (e.g.…”

“…Extrand & Kumagai (1995), who examined the validity of (1.1) for surfaces of different roughness, argued that hysteresis cannot be attributed solely to substrate roughness, but also to chemical heterogeneities, since they observed that substrates of similar roughness characteristics exhibited markedly different levels of hysteresis. Miwa et al (2000) reported experiments with hydrophobic needle-like structured substrates where they observed that the critical angle decreased with increasing contact angle. They also deduced an equation for the critical inclination angle based on energy considerations to explain qualitatively their observations, whose validity was later questioned by Roura & Fort (2002).…”

Fluid dynamics topics in bloodstain pattern analysis: Comparative review and research opportunities

“…Area of Convergence, Directionality [13] [ 5,97] Area of Origin [13,19,68 Backspatter Pattern; Forward Spatter Pattern area of origin, weapon [48] [5, 47,110,191] Blood Clot [6,162] [ 160 , 162] Bloodstain pattern area of origin, weapon [13,35,76 [ 166,170,196] Perimeter Stain time between drip and wipe-off, [155,197,198] [ 155,188] [188] [155,163,197,198] Flow Pattern [12,162,199] [12, 162,185,199] [185] [12] Impact Pattern weapon, motion; directionality [78][79][80]128] [5] [72,128,136] Insect Stain [157,200] Mist Pattern weapon, directionality [81, 154 , 186 Spatter Stain [201] [126, [15...…”

Preparation and characterization of an ultrahydrophobic surface based on a stearic acid self-assembled monolayer over polyethyleneimine thin films

“…Recently, much attention has been attracted to surfaces with ultrahydrophobic properties [1,2,[8][9][10][11][12][13][14] (Hydrophobic surfaces with water contact angles above $150°are called ''ultrahydrophobic'' [8]). Chen et al [1] reported that the plasma polymerized heptafluorobutylacrylate (HFBA) thin layer on smooth polyethyleneterephthalate (PET) film possessed advancing and receding water contact angles of 174°and 173°, respectively.…”

“…Chen et al [1] reported that the plasma polymerized heptafluorobutylacrylate (HFBA) thin layer on smooth polyethyleneterephthalate (PET) film possessed advancing and receding water contact angles of 174°and 173°, respectively. A transparent ultrahydrophobic film on Pyrex glass plates, prepared by sublimation of metal acetylacetonate and a subsequent coating of fluoroalkylsilane, was found to have contact and sliding angles about 160.9°and 1°, respectively [2,14]. Onda et al [9] prepared a fractal surface by dipping a glass plate into melted alkylketene dimmer (AKD) and achieved a contact angle about 174°.…”