Effects of isotropic and anisotropic slip on droplet impingement on a superhydrophobic surface

Abstract

The dynamics of single droplet impingement on micro-textured superhydrophobic surfaces with isotropic and anisotropic slip are investigated. While several analytical models exist to predict droplet impact on superhydrophobic surfaces, no previous model has rigorously considered the effect of the shear-free region above the gas cavities resulting in an apparent slip that is inherent for many of these surfaces. This paper presents a model that accounts for slip during spreading and recoiling. A broad range of Weber numbers and slip length values were investigated at low Ohnesorge numbers. The results show that surface slip exerts negligible influence throughout the impingement process for low Weber numbers but can exert significant influence for high Weber numbers (on the order of 102). When anisotropic slip prevails, the droplet exhibits an elliptical shape at the point of maximum spread, with greater eccentricity for increasing slip and increasing Weber number. Experiments were performed on isotropic and anisotropic micro-structured superhydrophobic surfaces and the agreement between the experimental results and the model is very good.

Reducing the contact time using macro anisotropic superhydrophobic surfaces — effect of parallel wire spacing on the drop impact

“…The contact time on the SHS was 11.5 ms. At the same speed, the impact on the micro-aniso-SHS was similar to that on the SHS, but the rim exhibited undulations during the spreading stage (Supplementary Figure S2c), in agreement with previous reports. 25,26 The maximum spreading factors (the ratio of the maximum spreading diameter to the initial diameter of the drop) along the x and y directions were 2.59 and 2.47, respectively (Supplementary Figure S2f and g). Therefore, the spreading shape was oblong and the momentum was slightly greater along the x direction than the y direction.…”

“…The hydrophobic silica nanoparticle decoration is shown in Figure 1c, which rendered both the stripes and the grooves water-repellent. The prepared surface had three levels (macro/micro/nano) structures and the biggest difference between the layers and those in conventional anisotropic SHSs [24][25][26][27] was the presence of the macrotextured parallel stripes. Therefore, this surface is defined as a macro-aniso-SHS in the present article.…”

“…[24][25][26][27] These impacts were characterized by elliptical spreading shapes but did not significantly reduce the contact time. The major-to-minor axis diameter ratios of the ellipses were observed to increase with the increasing cavity fraction and the surface slip had a negligible influence throughout the impingement process at low Weber numbers (W e ¼ rv 2 R=g), where v is the impacting velocity.…”

Hydrodynamics of droplet impingement on hot surfaces of varying wettability

“…Because the bottom of the droplet houses the largest velocity gradients [41], the reduction of liquid viscosity (due to increased temperature) is significant and allows for larger spreading. Second, Leidenfrost spreading encounters virtually no solid-liquid contact and thus approaches an infinite slip scenario [42], which is in contrast with spreading on the SHB surface at room temperature, which experiences no shear over cavities but frictional resistance over the pillars.…”

Droplet impact on superheated surfaces

“…Grishaev et al [276] studied the effect of adding particles on impact of water droplets on hydrophilic and hydrophobic surfaces and observed that the particles make splashing occur earlier. Clavijo et al [277] also investigated the impact on superhydrophobic surfaces considering the effects of slip (both isotropic and anisotropic) and observed that the slip can significantly affect the impact behavior at high We numbers, but its effects can be neglected at low We numbers. Wu et al [278] numerically simulated the impact of droplets on solid surface using Lattice Boltzmann method.…”