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.

We tested oblique drop impacts on a superhydrophobic surface at normal Weber numbers ( We) in the range of 3-45, and at varying angles of incidence (AOIs), ranging from 0° (normal impact) to 60° (highly oblique). Our objective is to define the influence of the AOI on the restitution coefficient and on the contact time of rebounding droplets. To interpret the overall restitution coefficient of oblique drop rebounds (ε), we decoupled it into two separate components: a normal (ε) and a tangential restitution coefficient (ε). We discovered that, regardless of the impact angle, ε can be accurately predicted as a function of the normal Weber number (ε = 0.94 We). We support this finding with a mathematical derivation from theory, indicating a general scaling relationship of ε ∼ We for the normal restitution coefficient. Likewise, the tangential restitution coefficient (ε) can also be predicted as a function of We (ε = 1.20 We) but is much larger than ε. As a result, the overall restitution coefficient (ε) increases for more oblique impacts because most of the tangential velocity is preserved. Furthermore, using the observed correlations for ε and ε, we derived a model to predict the overall restitution coefficient of rebounding drops at any We and AOI. The model's predictions are highly accurate, lying close to our experimental observations in all cases. Regarding the contact time ( t), we found that for normal impacts, t increased slightly as We was raised. We associate this behavior with partial penetration of the liquid into the surface's pores, which results in greater solid-liquid adhesion, prolonging detachment. For highly oblique impacts (AOI = 60°), we observed the reverse trend: the drop's contact time decreases for higher- We impacts. We attribute this correlation to stretched rebounding behavior, which accelerates the rebounding of highly oblique impacts.

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.

This report investigates the influence of microstructure topography on the restitution coefficient, maximum spreading diameter, and contact time of oblique drop impacts on superhydrophobic surfaces. The five surfaces tested allow for comparison of open-versus closed-cell structures, feature size and spacing, and hierarchical versus nanoscale-only surface structures. By decoupling the restitution coefficient into a normal (ε n ) and tangential component (ε t ), it is demonstrated that both ε n and ε t are largely independent of the microstructure topography. Instead, the restitution coefficient is governed almost exclusively by the normal Weber number. Next, a new model is presented that relates the maximum spreading diameter to an adhesion coefficient that characterizes the overall adhesive properties of the superhydrophobic microstructure during drop rebounding. Through this analysis, we discovered that surface geometries with greater microstructure roughness (i.e., overall surface area) promote a higher maximum spreading diameter than flatter geometries. Furthermore, the contact time of drop impacts on flat surfaces is positively correlated with the impact velocity due to penetration of the liquid into the porous nanostructure. However, this trend reverses for oblique impacts due to the presence of stretched rebounding behavior. Finally, substrates patterned with sparse pillar microstructures can exhibit pancake bouncing behavior, resulting in extremely low contact times. This unique bouncing mechanism also significantly influences the restitution coefficient and spreading diameter of oblique impacts.

This report investigates how different splashing mechanisms affect the oblique splash threshold of drops impacting a dry solid surface. The splashing behaviors of water, ethanol, and a water/ethylene glycol solution are observed over a wide range of drop diameters (0.7 mm &lt; D &lt; 2.2 mm) and Weber numbers (10 &lt; We &lt; 1040), and several published models are tested in order to predict the thresholds between deposition, one-sided splashing, and two-sided splashing. We found that the splash threshold of liquids that exhibit the corona splashing mechanism can be readily predicted by existing models. However, for liquids such as water that exhibit prompt splashing, the oblique splash threshold is not successfully predicted by any presently established correlation. Hence, our findings identify a critical knowledge gap in the drop impact field, since the behavior of water is of fundamental importance to countless engineering problems. Finally, combining our own results with others reported in the literature, we address some contradictory reports about the influence of liquid viscosity on the splash threshold and demonstrate that the presence or lack of thin-sheet in different experiments could explain the contradictions present in the literature.