On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part I: Sliding Length and Maximum Spreading Diameter

Abstract

Oblique water drop impacts were performed on a superhydrophobic surface at normal Weber numbers in the range of 3 < We < 80 and at angles of incidence in the range of 0 < AOI < 60°. While holding We constant, we varied the AOI to investigate how the oblique nature of the impact affects the sliding length and spreading diameter of impacting drops. Our sliding length measurements indicate that drops impacting at We < 10 retain essentially full mobility on the surface, whereas the sliding of higher- We impacts is inhibited by drag forces. We attribute this trend to increased penetration into air-trapping surface features occurring in higher- We impacts, which results in more adhesion between the liquid and solid. Regarding the spreading of drops on SHP surfaces, the dimensionless maximum spread diameter ( D ) increases not only with We but also with the angle of incidence such that more oblique drop impacts stretch to a wider maximum diameter. We attribute this behavior to adhesion forces, which act to stretch the drop as it slides tangentially across the surface in oblique impacts. On the basis of this theory, we derived a model predicting D for any We and AOI. The model's predictions are highly accurate, successfully predicting D for our entire experimental space. Finally, by placing the camera above the sample, we observed that oblique drop impacts spread into an elliptical shape, and we present a model predicting the maximum spread area.

Droplet Asymmetric Bouncing on Inclined Superhydrophobic Surfaces

“…However, the relationship between D max and We tends to slightly vary among different studies depending on the surface tested. 30,38,39 Here, we measured the maximum spreading diameter of drops impacting on the horizontal superhydrophobic surface and obtained the scaling of D max ≈ 0.79 D 0 We n 0.29 through the regression fitting, as shown in Figure 5a. Yeong et al have built a theoretical model for the maximum spreading in the tangential direction as D y max * = D max * + cWe t , where c ( c > 0) is the scaling factor and We t is the Weber number component in the tangential direction, expressed as We t = tan 2 α We n .…”

Air Cushion Storing Energy Promoting Droplets Retraction and Flow on Engineering Porous Bionic Lotus Surfaces

“…Recently researches have shown that inclined surfaces or moving horizontal surfaces are beneficial to reduce contact time [ 25–28 ] and inhibit splash, [ 39 ] and droplets show an asymmetric dynamic at the retraction stage. [ 30,31 ] Li et al. reported the rapid rebound of the droplet on inclined microflower superhydrophobic surface and ascribed it to the sudden contraction of the trailing edge.…”

“…Recently researches have shown that inclined surfaces or moving horizontal surfaces are beneficial to reduce contact time [25][26][27][28] and inhibit splash, [39] and droplets show an asymmetric dynamic at the retraction stage. [30,31] Li et al reported the rapid rebound of the droplet on inclined microflower superhydrophobic surface and ascribed it to the sudden contraction of the trailing edge. [32] LeClear et al fabricated two kinds of superhydrophobic groove surfaces with different orientations and observed droplet asymmetric retraction on 60° inclined surface.…”

Rebound Dynamics of Two Droplets Successively Impacting an Inclined Surface

“…In nature and industrial fields, the droplet encounters various solid surface, such as curve, textured, and inclined surface [24][25][26][27][28][29][30][31][32][33][34]. Compared with the flat horizontal surface, the outcomes are more complex, and the results may be different.…”

“…It is demonstrated that the droplet impacting an inclined surface behaves asymmetry between the tangential and lateral direction along the surface, which can promote the droplet to rebound from the surface. Meanwhile, due to the tangential velocity, the droplet slides along the inclined surface, away from the impact position [31][32][33]. If two droplets impact an inclined surface successively, the off-center impact should be observed, which is different from horizontal surface and still poorly understood, especially on superhydrophobic surfaces.…”