Anisotropy in the hydrophobic and oleophilic characteristics of patterned surfaces

Abstract

A significant difference in the wetting angles of water and oil was observed on patterned substrates, combining interstitial spaces along with hydrophobic solid surfaces, as a function of the orientation. The difference was ascribed to a modification of the liquid–interstice interfacial surface energy due to different degrees of penetration of the liquid. A roughness metric related to the extent to which the liquid infiltrates the interstice normalized by the geometrically determined area is proposed. This study has implications in modulating surface slip behavior and would be of importance in guiding liquid droplets.

Modulation of the Streaming Potential and Slip Characteristics in Electrolyte Flow over Liquid-Filled Surfaces

“…The oil filling was found to be reliable and is stable for the LFS, e.g., compared to the AFS. For instance, the surface energy of the LFS, E LFS ∼ 0.5 × γ pary + 0.5 × γ oil ∼ 32 mJ/m 2 , is an order of magnitude smaller compared to that of the AFS, E AFS ∼ r × γ pary ∼ 290 mJ/m 2 with r as the roughness factor (=6.3, from the ratio of the total area to the projected area). The excellent liquid-interface stability was also considered previously with surface-immobilized lubricants …”

“…Due to the oleophilicity of the AFS, 32 oils immiscible with water and with low surface tension, e.g., Dupont Krytox GPL 104 (general purpose lubricant), a fluorinated synthetic oil, castor oil, and synthetic motor oils (0W-20 and 20W-50), can be filled into the troughs of the patterned surface to yield an LFS, located at the bottom of the microfluidic channel. The physical parameters for the oils used in the LFS are listed in Table 1.…”

Tensorial Modulation of Electrokinetic Streaming Potentials on Air and Liquid Filled Surfaces

“…While there seems to be an ε r related scaling for M x related to flow perpendicular to grooves, such scaling is not apparent for flows along the grooves. The difference of the wetting characteristics parallel and perpendicular to the grooves may be relevant and needs to be investigated further. Such scaling is not evident for the AFS, which consequently implies a distinct electrokinetic flow regime.…”

“…Indeed, while the LFS can yield changes of ∼30% compared to the AFS, we find that in any particular AFS or LFS the variation can be of the order of 100%, through different modes of texture. The influence of geometrical parameters of the substrate has been considered in the context of wetting, 20 e.g., with respect to anisotropy, 21 as well as its influence 22 on the slip length (b), e.g., through a relative variation for stripe and post geometries. 23 A generalization of the underlying concepts to describe surface anisotropy through slip and related interfacial electrokinetic mobility tensors 24 offers insights into the modulation of flows, such as flow reversal, 25 say, on hydrophobic/SH surfaces 26 and heterogeneous geometries.…”

A Stability Analysis for Magnetohydrodynamics Stagnation Point Flow with Zero Nanoparticles Flux Condition and Anisotropic Slip

“…Khan et al [48] also pointed up the significance of wall velocity slip for a reliable design and operation of microfluidic devicesmade of hydrophobic devices. The emerging numbers of industrial and technological applications make the study of anisotropic slip that relies upon the flow direction, which is influential for these types of surfaces: hydrophobic [49][50][51][52][53][54] and porous [55,56]. Rashad [57] investigated the coupled effects of anisotropic slip and convective condition in an unsteady nanofluid saturated with porous medium.…”