Droplet impact on superhydrophobic surfaces: A review of recent developments

The development of bioinspired interfacial materials with enhanced drop mobility that mimic the innate functionalities of nature will have a significant impact on the energy, environment and global healthcare. Despite extensive progress, state of the art interfacial materials have not reached the level of maturity sufficient for industrial applications in terms of scalability, stability, and reliability. These are complicated by their operating environments and lack of facile approaches to control the local structural texture and chemical composition at multiple length scales. The recent advances in the fundamental understanding are reviewed, as well as practical applications of bioinspired interfacial materials, with an emphasis on the drop bouncing and coalescence-induced jumping behaviors. Perspectives on how to catalyze new discoveries and to foster technological adoption to move this exciting area forward are also suggested.

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

A hitherto unknown mechanism for wetting transition is reported. When a pendant drop settles upon deposition, there is a virtual "collision" where its center of gravity undergoes rapid deceleration. This induces a high water hammer-type pressure that causes wetting transition. A new phase diagram shows that both large and small droplets can transition to wetted states due to the new deceleration driven and the previously known Laplace mechanisms, respectively. It is explained how the attainment of a nonwetted Cassie-Baxter state is more restrictive than previously known.

Rapid Deceleration-Driven Wetting Transition during Pendant Drop Deposition on Superhydrophobic Surfaces

The development of bio-inspired interfacial materials with enhanced drop mobility that mimic the innate functionalities of nature will have significant impact on the energy, environment and global healthcare. In spite of extensive progress, the state of the art of interfacial materials have not reached the level of maturity sufficient for industrial applications in terms of scalability, stability and reliability, which are complicated by their operating environments and lack of facile approaches to exquisitely control the local structural texture and chemical composition at multiple length scales. In this review, we focus on the recent advances in the fundamental understanding as well as practical applications of bio-inspired interfacial materials, with an emphasis on the drop impact induced bouncing and coalescence induced jumping behaviors. We also suggest our own perspectives on how to catalyze new discoveries and to foster technological adoption to move this exciting area forward.

Bioinspired interfacial materials with enhanced drop mobility: From fundamentals to multifunctional applications

Water droplet impact on surfaces is a ubiquitous phenomenon in nature and industry, where the time of contact between droplet and surface influences the transfer of mass, momentum and energy. To manipulate and reduce the contact time of impacting droplets, previous publications report tailoring of surface microstructures that influence the droplet - surface interface. Here we show that surface elasticity also affects droplet impact, where a droplet impacting an elastic superhydrophobic surface can lead to a two-fold reduction in contact time compared to equivalent rigid surfaces. Using high speed imaging, we investigated the impact dynamics on elastic nanostructured superhydrophobic substrates having membrane and cantilever designs with stiffness 0.5-7630 N/m. Upon impact, the droplet excites the substrate to oscillate, while during liquid retraction, the substrate imparts vertical momentum back to the droplet with a springboard effect, causing early droplet lift-off with reduced contact time. Through detailed experimental and theoretical analysis, we show that this novel springboarding phenomenon is achieved for a specific range of Weber numbers (We >40) and droplet Froude numbers during spreading (Fr >1). The observation of the substrate elasticity-mediated droplet springboard effect provides new insight into droplet impact physics.

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Water droplet impact on elastic superhydrophobic surfaces.

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 behaviors occurred under a wide range of experimental conditions on our smooth surfaces; however, impacts occurring on textured surfaces were much more symmetrical, and one-sided splashing o...

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

We tested oblique drop impacts on a superhydrophobic surface at normal Weber numbers (Wen) 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 (e), we decoupled it into two separate components: a normal (en) and a tangential restitution coefficient (et). We discovered that, regardless of the impact angle, en can be accurately predicted as a function of the normal Weber number (en = 0.94Wen–1/4). We support this finding with a mathematical derivation from theory, indicating a general scaling relationship of en ∼ Wen–1/4 for the normal restitution coefficient. Likewise, the tangential restitution coefficient (et) can also be predicted as a function of Wen (et = 1.20Wen–0.12) but is much larger than en. As a result, the overall restitutio...

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On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part II: Restitution Coefficient and Contact Time

Oblique water drop impacts were performed on a superhydrophobic surface at normal Weber numbers in the range of 3 < Wen < 80 and at angles of incidence in the range of 0 < AOI < 60°. While holding Wen 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 Wen < 10 retain essentially full mobility on the surface, whereas the sliding of higher- Wen impacts is inhibited by drag forces. We attribute this trend to increased penetration into air-trapping surface features occurring in higher- Wen 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 *max) increases not only with Wen 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 *max for any Wen and AOI. The model's predictions are highly accurate, successfully predicting D *max 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.

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

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 (07 mm < D < 22 mm) and Weber numbers (10 < We < 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

Influence of liquid properties on the oblique splashing threshold of drops

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 (en) and tangential component (et), it is demonstrated that both en and et 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.

Damon G. K. Aboud

Papers

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part II: Restitution Coefficient and Contact Time

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

Influence of liquid properties on the oblique splashing threshold of drops

Influence of Microstructure Topography on the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces.