Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Abstract

Oblique drop impacts were performed at high speeds (up to 27 m/s, We>9000) with millimetric water droplets, and a linear model was applied to define the oblique splashing threshold. Six different sample surfaces were tested: two substrate materials of different inherent surface wettability (PTFE and aluminum), each prepared with three different surface finishes (smooth, rough, and textured to support superhydrophobicity). Our choice of surfaces has allowed us to make several novel comparisons. Considering the inherent surface wettability, we discovered that PTFE, as the more hydrophobic surface, exhibits lower splashing thresholds than the hydrophilic surface of aluminum of comparable roughness. Furthermore, comparing oblique impacts on smooth and textured surfaces, we found that asymmetrical spreading and splashing behaviours occurred under a wide range of experimental conditions on our smooth surfaces, however impacts 2 occurring on textured surfaces were much more symmetrical, and one-sided splashing occurred only under very specific conditions. We attribute this difference to the air-trapping nature of textured superhydrophobic surfaces, which lowers the drag between the spreading lamella and the surface. The reduced drag affects oblique drop impacts by diminishing the effect of the tangential component of the impact velocity, causing the impact behaviour to be governed almost exclusively by the normal velocity. Finally, by comparing oblique impacts on superhydrophobic surfaces at different impact angles, we discovered that although the pinning transition between rebounding and partial rebounding is governed primarily by the normal impact velocity, there is also a weak dependence on the tangential velocity. As a result, pinning is inhibited in oblique impacts. This led to the observation of a new behaviour in highly oblique impacts on our superhydrophobic surfaces which we named the stretched rebound, where the droplet is extended into an elongated pancake shape and rebounds while still outstretched, without exhibiting a recession phase.

Steric‐Structure‐Dependent Gel Formation, Hierarchical Structures, Rheological Behavior, and Surface Wettability

“…Functional surfaces with different wettability have come to the forefront of research for their great potential in biological and industrial applications . Due to the differences in the molecular steric structures of GC , GN , and GO , their self‐assembly process and morphology also showed large differences.…”

“…Functional surfaces with different wettability have come to the forefront of research for their great potential in biological and industrial applications. [89][90][91][92][93] Due to the differences in the molecular steric structures of GC, GN,and GO,t heir self-assembly processa nd morphology also showed large differences. Therefore, the wettability of the xerogel surfaces from these gelators self-assembled in different solvents wase xaminedb y using the contact-angle test ( Figure 6).…”

Bioinspired Interfacial Materials with Enhanced Drop Mobility: From Fundamentals to Multifunctional Applications

“…With a larger apex angle of the tapered post arrays, the pancake bouncing can occur with smaller critical We and wider We range. Interestingly, elongated pancake bouncing also occurs on the conventional SHS in the condition of a highly oblique impact …”

“…Interestingly, elongated pancake bouncing also occurs on the conventional SHS in the condition of a highly oblique impact. [ 89 ]…”

“…Note that in practical applications, most surfaces are obliquely placed and thus the study of oblique impact has received increasing attention . It is reported that the contact time in the case of the oblique impact is basically the same as that of the vertical impact .…”