Effect of a surface tension gradient on the slip flow along a superhydrophobic air-water interface

Abstract

Superhydrophobic surfaces have been shown to produce significant drag reduction in both laminar and turbulent flows by introducing an apparent slip velocity along an air-water interface trapped within the surface roughness. In the experiments presented within this study, we demonstrate the existence of a surface tension gradient associated with the resultant Marangoni flow along an air-water interface that causes the slip velocity and slip length to be significantly reduced. In this study, the slip velocity along a millimeter-sized air-water interface was investigated experimentally. This large-scale air-water interface facilitated detailed investigation of the interfacial velocity profiles as the flow rate, interfacial curvature and interface geometry were varied. For the air-water interfaces supported above continuous grooves (concentric rings within a torsional shear flow) where no surface tension gradient exists, slip velocity as high as 30% of the bulk velocity was observed. However, for the air-water interfaces supported above discontinuous grooves (rectangular channels in a Poiseuille flow), the presence of a surface tension gradient reduced the slip velocity and in some cases resulted in an interfacial velocity that was opposite to the main flow direction. The curvature of the air-water interface in the spanwise direction was found to dictate the details of the interfacial flow profile with reverse flow in the center of the interface for concave surfaces and along the outside of the interface for convex surfaces. The deflection of the air-water interface was also found to greatly affect the magnitude of the slip. Numerical simulations imposed with a relatively small surface tension gradient along air-water interface were able to predict both the reduced slip velocity and back flow along the air-water interface.

A theory for the slip and drag of superhydrophobic surfaces with surfactant

“…Assuming a strong surfactant, our model predicts that a bulk surfactant concentrationĉ 0 ∼ 10 −13 mM can reduce u I in the same extent and under the same conditions as reported by (27) for both short and long lanes; andĉ 0 ∼ 10 −15 to 10 −14 mM for the experiments reported by (28). Assuming the weak SDS surfactant, our model predictŝ c 0 ∼ 1 to 10 mM (i.e.…”

“…Similarly, (28) report: u I ≈ 8 × 10 −2 for 5 mm long lanes (see their figure 3b), which is significantly reduced compared with the theoretical (surfactant-free) prediction; and u I ≈ 8 × 10 −3 for 15 mm long lanes ( figure 5), which is practically negligible. The main difficulty in applying our theoretical model, for instance to predict the reduced slip velocities measured experimentally by (27) and (28), is that the surfactant properties and their concentrations are completely unknown in their experiments. Instead, we use our model to predict the concentration of surfactant, for three different possible surfactant types, which could lead to the measured u I reported in (27) and (28).…”

“…[16][17][18][19][20][21], as well as low-Reynolds-number, internal flows, which are the focus of the present paper (e.g. 9,11,[22][23][24][25][26][27][28]. At low Reynolds numbers, the use of SHSs has been proposed to reduce what are otherwise very large pressure differences across microchannels, as is the case in microfluidic devices or in micro-cooling applications (29)(30)(31), as well as to minimize Taylor dispersion in the chemical or biological analysis of species (10).…”

Effect of PTFE Particle Size on Superhydrophobic Coating for Supercooled Water Prevention

“…A functional chemical surface has a wide variety of applications including self-cleaning surfaces, drag reduction on ships, and anti-icing technology by repelling supercooled water droplets [1][2][3]. The most effective hydrophobic surfaces found in nature are created using a microstructure, such as that in butterfly wings or the surfaces of lotus plants, and it has been shown that varying the microstructure of such a coating can modify the hydrophobicity of a surface, as well as the wetting properties [4][5][6].…”

Anti- and De-Icing Behaviors of Superhydrophobic Fabrics

“…Superhydrophobic materials and surfaces [1][2][3][4] have attracted great interest because of their extreme water-repellent surface property for many potential applications including self-cleaning [5][6][7], hydrodynamic friction reduction [8][9][10], anti-icing [11][12][13][14], anticorrosion [15][16][17][18], biotechnology [19][20][21], thermal systems [22][23][24][25], and micro-and nanodevices [26][27][28]. In particular, previous works on the use of superhydrophobic surfaces as anti-icing coatings have demonstrated that superhydrophobic surfaces have the capability to reduce or even prevent the accumulation and formation of snow and ice on hard solid surfaces [11][12][13][14].…”