Droplet impacting on pillared hydrophobic surfaces with different solid fractions

Xia L., Chen F., Yang Z., Liu T., Tian Y., Zhang D.

Functional solid surfaces that can realise rapid shedding of liquid droplets have received significant research interest due to their close relevance to many industrial applications. Droplet impact on superhydrophobic surfaces with point-like protrusions has been demonstrated to take off as rings, which alters droplet impact dynamics and thus reduces contact time compared to flat surfaces. However, the essential role of the size ratio of protrusion-to-droplet (λ) for droplets impinging on such surface is rarely considered. Here, we numerically investigate droplet impact on superhydrophobic surfaces with convex hemispherical shapes using a phase-field model coupled with dynamic contact angles. The postimpact outcome regimes occurring for varied λ and Weber number (We) values, spanning 0 ≤ λ ≤ 1.44 and 0.69 ≤ We ≤ 33.68, are mapped on a We−λ diagram. Three distinct dynamic behaviours of droplet impact are identified: contactless bouncing, conventional bouncing, and ring bouncing. Detailed comparative analyses of these impact outcomes are also presented, including the evolution of droplet morphology, impact force, maximum impact pressure, pressure distribution, and velocity vector distribution. The results reveal a previously unknown phenomenon in contactless bouncing, where the impact force exhibits an initial increase followed by a subsequent decrease, while the maximum impact pressure remains approximately constant. Annular rotating retraction results in a longer contact time. Breakup occurs near the necked area, inducing a part of the droplet to depart from the surface as a jet. In addition, it is observed that the dimensionless maximum wetting area becomes independent of the λ and follows a scaling law of 0.67We3/5 if the We exceeds 2.75. Ring bouncing exclusively occurs within the range of 0.24 ≤ λ ≤ 0.96 and We ≥ 24.74, resulting in an approximate 50% reduction in non-dimensional contact time compared to conventional bouncing. These findings favour the understanding of the underlying mechanisms governing droplet impact and thereby provide available guidance to the design of superhydrophobic surfaces.

Peng X., Wang T., Jia F., Sun K., Li Z., Che Z.

Hypothesis: The impact of droplets is prevalent in numerous applications, and jetting during droplet impact is a critical process controlling the dispersal and transport of liquid. New jetting dynamics are expected in different conditions of droplet impact on super-hydrophobic surfaces, such as new jetting phenomena, mechanisms, and regimes. Experiments: In this experimental study of droplet impact on super-hydrophobic surfaces, the Weber number and the Ohnesorge number are varied in a wide range, and the impact process is analyzed theoretically. Findings: We identify a new type of singular jets, i.e., singular jets induced by horizontal inertia (HI singular jets), besides the previously studied singular jets induced by capillary deformation (CD singular jets). For CD singular jets, the formation of the cavity is due to the propagation of capillary waves on the droplet surface; while for HI singular jets, the cavity formation is due to the large horizontal inertia of the toroidal edge during the retraction of the droplet after the maximum spreading. Key steps of the impact process are analyzed quantitatively, including the spreading of the droplet, the formation and the collapse of the spire, the formation and retraction of the cavity, and finally the formation of singular jets. A regime map for the formation of singular jets is obtained, and scaling relationships for the transition conditions between different regimes are analyzed.

Satpathi N.S., Nampoothiri K.N., Sen A.K.

Surface acoustic waves (SAW) propagating along a solid surface can significantly affect the dynamics of droplet impact. Although droplet impact in presence of SAW has been attempted recently, here, we investigate the effects of surface wettability, droplet size, impact velocity, and SAW power on the impact and spreading dynamics along with post-impact oscillation dynamics of a drop.Here, we study droplet impact on a surface exposed to traveling SAW produced using an interdigitated electrode patterned on a piezoelectric substrate. The effects of Weber number (We), surface wettability, and SAW power on the impact and spreading dynamics and post-impact oscillation dynamics are studied.Our study unravels that the interplay between capillary and viscous forces, and inertia forces arising due to pre-impact kinetic energy and SAW-induced bulk acoustic streaming underpins the phenomena. Remarkably, we find that the effect of SAW on droplet impact dynamics is predominant in the case of a hydrophilic (HPL) substrate at a higher SAW power and smaller We and hydrophobic (HPB) substrate irrespective of SAW power. Our study reveals that the maximum droplet spreading diameter increases with SAW power at smaller We for an HPL surface whereas it is independent of SAW power at higher We. Post-impact oscillation of a droplet over an HPL surface is found to be overdamped with a smaller amplitude compared to an HPB substrate, and a faster decay in oscillation amplitude is observed in the case of an HPB surface and higher We. Our study provides an improved understanding of droplet impact on a surface exposed to SAW that may find relevance in various practical applications.

Xia L., Chen F., Chao J., Zhang D., Tian Y., Zhang D.

Membranes with super-wettability have been widely used to treat oily wastewater, but the construction of most of these membranes involves the use of corrosive and even toxic chemicals, and they often exhibit poor chemical durability. Therefore, green and durable super-wetting membranes are urgently required for remediating oily wastewater under harsh conditions. In this study, we report an eggshell membrane (ESM) with super-wettability as a green and durable candidate for efficient oil/water separation under harsh conditions. Specifically, naturally hydrophilic chicken ESMs are directly roughened and drilled using a femtosecond laser, without using any chemicals. Laser-engineered ESMs (LEESMs) possess superhydrophilicity in air and superoleophobicity under water, which renders them capable of separating immiscible oil/water mixtures after they are pre-wetted using water. Results show that the pre-wetted LEESM can cyclically treat various oil/water mixtures with a separation efficiency of >98.6% and water flux of >10000 L·m-2·h-1. Because of the inherent chemical resistance of ESM itself, the LEESMs show excellent durability towards highly corrosive aqueous solutions such as HCl (1 M), NaOH (1 M), and saturated NaCl solution (26.5 wt %). The LEESM retained its underwater superoleophobicity after being immersed in these corrosive solutions for 12 h; therefore, it could be used to separate mixtures of toluene and corrosive solutions efficiently for more than 10 cycles, demonstrating remarkable durability and recyclability in terms of oil/water separation under harsh conditions. Moreover, when the LEESM is pre-wetted or contaminated by oils, it can be easily cleaned by immersing in water, thereby restoring its underwater superoleophobicity and oil/water separation capacity, which prevents it from being fouled during long-term usage. Therefore, this green, anti-corrosive, and self-cleaning LEESM offers insights into the design and fabrication of advanced membranes for practical oily wastewater remediation, as well as the developing and utilization of bio-wastes.

Xia L., Chen F., Liu T., Zhang D., Tian Y., Zhang D.

• A phase-field model is implemented to simulate the droplet impact • The droplet's impact on the surface is divided into four categories • A change in values of dimensionless viscous extension has been reported • The droplet normalised contact time is divided into two parts according to We • The global impact outcomes are illustrated by regime maps Although the impingement of droplets on superhydrophobic surfaces has been extensively studied, the critical role of the air is typically ignored, leading to some paradoxical conclusions. In this paper, a phase-field model with dynamic contact angles is implemented to simulate the impingement of droplets on a superhydrophobic surface. The simulation results agree well with the experimental data. The droplet's impact on the surface is observed and divided into four categories: contactless bouncing, wet bouncing, dry-out bouncing and non-bouncing. The role of air was emphasised by analysing pressure distribution and velocity field corresponding to the droplet evolution, revealing the mechanism behind the droplet impact. A non-monotonically varying trend of the non-dimensional maximum wetting diameter is observed by increasing Ohnesorge number ( Oh ). The value of dimensionless viscous extension first increased and then decreased with increasing impact number, and this counter-intuitively outcome has not been reported previously. Non-dimensional contact time of a bouncing droplet decreases rapidly with increasing Weber number ( We ), but approaches a plateau value if We is greater than 1. Additionally, for liquids with different viscosities, the non-dimensional contact time of bouncing droplets is independent of Oh. Moreover, regime maps of the dynamic behaviours of a droplet impact are developed to present an intuitive interpretation of different droplet fates.

Dawson J., Coaster S., Han R., Gausden J., Liu H., McHale G., Chen J.

Droplets impacting superhydrophobic surfaces have been extensively studied due to their compelling scientific insights and important industrial applications. In these cases, the commonly reported impact regime was that of complete rebound. This impact regime strongly depends on the nature of the superhydrophobic surface. Here, we report the dynamics of droplets impacting three hydrophobic slippery surfaces, which have fundamental differences in normal liquid adhesion and lateral static and kinetic liquid friction. For an air cushion-like (super)hydrophobic solid surface (Aerogel) with low adhesion and low static and low kinetic friction, complete rebound can start at a very low Weber (We) number (∼1). For slippery liquid-infused porous (SLIP) surfaces with high adhesion and low static and low kinetic friction, complete rebound only occurs at a much higher We number (>5). For a slippery omniphobic covalently attached liquid-like (SOCAL) solid surface, with high adhesion and low static friction similar to SLIPS but higher kinetic friction, complete rebound was not observed, even for a We as high as 200. Furthermore, the droplet ejection volume after impacting the Aerogel surface is 100% across the whole range of We numbers tested compared to other surfaces. In contrast, droplet ejection for SLIPs was only observed consistently when the We was above 5–10. For SOCAL, 100% (or near 100%) ejection volume was not observed even at the highest We number tested here (∼200). This suggests that droplets impacting our (super)hydrophobic Aerogel and SLIPS lose less kinetic energy. These insights into the differences between normal adhesion and lateral friction properties can be used to inform the selection of surface properties to achieve the most desirable droplet impact characteristics to fulfill a wide range of applications, such as deicing, inkjet printing, and microelectronics.

Cai Z., Chen F., Tian Y., Zhang D., Lian Z., Cao M.

• Multi-bioinspired tridirectionally anisotropic slippery surfaces were prepared. • The pinning effect of step edge was critical to the tridirectional anisotropy. • Vertical vibration assisted programmable droplet transports were realized. • In-plane manipulation of droplet motion in more than two directions was realized. • Droplet-based chemical micro-reactors were designed. Directional droplet transport on functional surfaces with anisotropic wettability has shown great potential applications in various fields such as water harvesting, chemical micro-reaction, and biomedical analysis. However, the in-plane manipulation of the anisotropic droplet motion in more than two directions is still a challenge. Herein, through the fusion of inspirations from rice leaves, butterfly wings and Pitcher plants, we report a tridirectionally anisotropic slippery surface (TASS) with periodic step-like micro grooves for programmable droplet transport. TASS possesses a tridirectional droplet sliding behavior, i.e. , the ultra-slipperiness along the grooves with a sliding angle of ∼2°, and the bidirectionally anisotropic sliding perpendicular to the grooves with sliding angle difference up to ∼50°, which is caused by the pinning effect of the step edge. Under the assistance of periodic vertical vibration, groove-features and droplet-volume dependent unidirectional droplets transports are realized on horizontally placed TASS, based on which two micro-reactors are designed to control the sequence of droplets merging and subsequent chemical reactions. Additionally, by utilizing the slipperiness ( i.e. , ultra-low sliding angle for liquid droplet) along the grooves simultaneously, programmable droplet transport under vertical vibration is further demonstrated on a tilted TASS. This work will provide a new avenue for the understanding of anisotropic wettability on asymmetric slippery surface, and thus offer a great opportunity to develop advanced interface for multidirectional droplet transport, chemical micro-reactor, etc .

Yang K., Liu Q., Lin Z., Liang Y., Liu C.

Kingfishers stand on a branch, and raindrops tumble translationally from feathers during raining, enlightening functional surfaces design and liquid transport control. Far-ranging studies on oriented transportation are confined to vertical impacting, which is, to date, in-depth philosophy of horizontal droplets transport on motionless surface deems to be rather serviceable. This study, employed mixed-wettability surface inspired by kingfishers' feather, occurs on directional transportation issues, such as the synergies of wettability-controlled, driving force and transportation capability. Here we conduct both experimental testing and CFD-aided numerical modelling to reproduce the asymmetric bouncing and directional transport phenomena. We found that the anisotropic surface manipulates to convert normally vertical impacting to horizontal droplets transport. Law of the thrown droplet, on the other hand, is predominated by the wettability-controlled surface, while the coexistence of contact angle difference and surface offset location cooperatively dictates the intensity and patterns of the thrown droplet. Of all these factors, the post-optimized surfaces are designed first and then the regime map of transportation pattern is elaborated. Results manifest that the elements induce the maximum horizontal transport distance by up to 6.2D0, and first desorption time is only 7.8 ms. The findings shed light on engineering design principles that can pave the way for novel applications in anti-icing, lubrication, and spray cooling.

Lian Z., Cheng Y., Xu J., Xu J., Ren W., Tian Y., Yu H.

In this work, we present a simple technique for green fabrication of slippery liquid-infused surface (SLIS) with anti-friction property on various metallic substrates using wire electrical discharge machining. Micro-crater structures were successfully obtained, and the surface had excellent liquid-repellent property after modification and infusion of silicone oil. A wide range of liquids including water, juice, coffee, tea, vinegar, albumin, glycerol, and ketchup could easily slid down the surface tilted at an angle of 10° without leaving any trace. The influences of the number of cutting step on the morphology and wettability of the surface were studied comprehensively. Further, the tribological properties of the surface were analyzed and the results showed that the SLIS had a decrease of 73.2% in friction coefficient as compared to that of the smooth surface. By studying the morphology of the worn surfaces, it is found that the SLIS had slight abrasive wear behavior. To demonstrate the precision processing ability of this technology, we fabricated slippery sub-millimeter-scale asymmetric bump arrays, and the experiment results showed that the asymmetric bump arrays had excellent water harvesting ability at low temperatures. This kind of environment-friendly precision machining technology will promote the practical applications of metallic functional materials.

Chen F., Wang Y., Tian Y., Zhang D., Song J., Crick C.R., Carmalt C.J., Parkin I.P., Lu Y.

Liquid-repellent surfaces, such as superhydrophobic surfaces, superoleophobic surfaces, and slippery liquid-infused surfaces, have drawn keen research interest from the communities engaged in chemical synthesis, interfacial chemistry, surface engineering, bionic manufacturing and micro-nano machining. This is due to their great potential applications in liquid-proofing, self-cleaning, chemical resistance, anti-icing, water/oil remediation, biomedicine, etc. However, poor robustness and durability that notably hinders the real-world applications of such surfaces remains their Achilles heel. The past few years have witnessed rapidly increasing publications that address the robustness and durability of liquid-repellent surfaces, and many breakthroughs have been achieved. This review provides an overview of the recent progress made towards robust and durable liquid-repellent surfaces. First, we discuss the wetting of solid surface and its generally-adopted characterisation methods, and introduce typical liquid-repellent surfaces. Second, we focus on various evaluation methods of the robustness and durability of liquid-repellent surfaces. Third, the recent advances in design and fabrication of robust and durable liquid-repellent surfaces are reviewed in detail. Fourth, we present the applications where these surfaces have been employed in fields like chemistry, engineering, biology and in daily life. Finally, we discuss the possible research perspectives in robust and durable liquid-repellent surfaces. By presenting such state-of-the-art of this significant and fast-developing area, we believe that this review will inspire multidisciplinary scientific communities and industrial circles to develop novel liquid-repellent surfaces that can meet the requirements of various real-world applications.

Zhang B., Sanjay V., Shi S., Zhao Y., Lv C., Feng X., Lohse D.

High-speed imaging captures the impact of a water droplet on a superhydrophobic surface and uncovers the effects of a time-delayed secondary force that lifts the droplet off the surface.

Xu J., Xiu S., Lian Z., Yu H., Cao J.

Oh S., Cho J., Lee J., Han J., Kim S., Nam Y.

The lotus effect indicates that a superhydrophobic, self-cleaning surface can be obtained by roughening the topography of a hydrophobic surface. However, attaining high transmittance and clarity through a roughened surface remains challenging because of its strong scattering characteristics. Here, a haze-free, antireflective superhydrophobic surface that consists of hierarchically designed nanoparticles is demonstrated. Close-packed, deep-subwavelength-scale colloidal silica nanoparticles and their upper, chain-like fumed silica nanoparticles individually fulfill haze-free broadband antireflection and self-cleaning functions. These double-layered hierarchical surfaces are obtained via a scalable spraying process that permits precise control over the coating morphology to attain the desired optical and wetting properties. They provide a "specular" visible transmittance of >97% when double-side coated and a record-high self-cleaning capability with a near-zero sliding angle. Self-cleaning experiments on photovoltaic devices verify that the developed surfaces can significantly enhance power conversion efficiencies and aid in retaining pristine device performance in a dusty environment.

Lei W., Lu X., Liu F., Wang M.

We report non-monotonic wettability effects on displacement efficiency in heterogeneous porous structures at the post-breakthrough stage, in contrast to the monotonic ones in homogeneous porous structures. Experiments on designed microfluidic chips show that there exists a critical wettability to attain the highest efficiency of displacement in the porous matrix structure combined with a preferential flow pathway, while a stronger wettability of the displacing fluid leads to a higher displacement efficiency on the same matrix structure only. The porous structure with or without a preferential flow pathway results in totally different topological characteristics of phase distribution during displacement. Pore-scale mechanisms are identified to elucidate the formation of this non-monotonic wettability rule: cooperative pore filling under weakly water-wet conditions yields the best displacement; corner flow under strongly water-wet conditions and Haines events under strongly oil-wet conditions decrease the displacement efficiency. The pore-scale findings may provide unique insights into the joint effects of both wettability and flow heterogeneity on fluid displacement in porous media.

Xia L., Chen F., Cai Z., Chao J., Tian Y., Zhang D.

3D porous superhydrophobic materials have been widely developed for selective oil adsorption from open water surface, but it is still challenging for the oil removal from water in non-open environment. Herein, a superhydrophobic-superoleophilic, magnetic and 3D porous carbon-based foam, containing carbon-nanosheet frameworks (CNFs), iron/iron carbide nanoparticles (Fe/Fe3C NPs) and polydimethylsiloxane (PDMS) coating, is synthesized via ferric nitrate assisted chemical blowing and carbonization of polyvinylpyrrolidone (PVP), and post-treatment by PDMS. The PDMS-CNF-Fe/Fe3C foam shows low density (2700% for chloroform). The remarkable paramagnetic property of the foam enables it to be capable of removing oil selectively from water in non-open channels under assistance of a magnet bar. Additionally, the foam can be easily reused for oil adsorption by heating or releasing in other solvents with negligible adsorption capacity decline. The chemically stable carbon-based matrix and PDMS coating impart the foam with excellent high/low temperature stability and certain corrosion resistance, which facilitate effective oil removal from caustic aqueous solutions (especially in alkaline and salty conditions) or under extreme temperature environment. The 3D porous and self-similar micro structures enable the foam to be mechanical abrasion resistant, and the foam retains its water repellence and oil adsorption capacity after repeated and intensive abrasion by sandpaper. Moreover, we propose to assemble the foam with a vacuum pumping system by ring-shaped magnet to realize continuous oil removal. The mechanochemically durable PDMS-CNF-Fe/Fe3C foam with magnetic response capacity and outstanding reusability is therefore a promising candidate for selective oil removal from water in both open and non-open environments under remote control of magnet. The novel magnet assisted assembly strategy also provides a simple, reliable and automatable method to integrate magnetic 3D oil adsorbent with pumping system for large-scale oily wastewater remediation.

Yu W., Wang W., Cao D., Wang Y., Chen S., Zhao J.

Zhang H., Jia F., Wei M.

Superhydrophobic surfaces with arrayed pillar structures have huge application prospects in various industrial fields, such as self-cleaning, waterproofing, anti-corrosion, and anti-icing. The knowledge gap regarding the liquid–solid interaction between impacting droplets and microstructured surfaces must be addressed to guide the practical engineering applications more effectively. In this study, the effects of the stationary and horizontally moving superhydrophobic micro-pillar surfaces on the droplet impact dynamic behavioral characteristics are investigated numerically, focusing on the droplet morphology, spreading diameter, contact time, and energy conversion. Based on the numerical simulation results, new prediction correlations of the dimensionless maximum spreading diameter for droplets impacting stationary and horizontally moving micro-pillar surfaces are proposed. Moreover, significant rolling phenomena occur when droplets impact horizontally moving micro-pillar surfaces, which leads to an increase in viscous dissipation and forms a competitive mechanism with the asymmetric spreading–retraction process of the droplets. Two different stages are recognized according to the analysis of the contact time and velocity restitution coefficient. This study may provide new insights into understanding the dynamic behavior of droplets on microstructured surfaces.

Vishal, Rohilla L., Kumar P.

Wang G., Sohani S.M., Yang J., Lei T., Chen J., He R., Luo K.H.

Palmetshofer P., Wurst J., Geppert A.K., Schulte K., Cossali G.E., Weigand B.

Jiang B., Pan G., Zheng C., Wang X., Zhang Y., Yin J., Zhu H.

Understanding the impact process of droplets on surfaces is a crucial prerequisite for enhancing the efficiency of wetting dust removal. The collision dynamics between water droplets and quartz surfaces were investigated using a high-speed camera, revealing four distinct stages in this process. During the first three stages, there was an increase in droplet width and three-phase contact line over time, while the droplet height and contact angle decreased. These observations can be attributed to the combined effects of impulsive force and surface tension. With increasing droplet velocity, the duration of the first three stages prolonged, accompanied by an increase in both the diameter of the three-phase contact line and droplet width during the final stage, whereas there was a decrease in droplet height and contact angle. This behavior primarily arises from energy transfer involving kinetic energy converted into contact surface energy, droplet surface energy, and dissipated energy during collision events. Contact surface energy and droplet surface energy exhibited an upward trend with rising droplet velocity. Simultaneously, collision-induced dissipated energy increased proportionally with respect to droplet velocity. Notably, both the rate and ratio of dissipated energy demonstrated positive correlations with the Weber number; specifically following a linear relationship characterized by a slope value of 2.81. These findings offer valuable insights for advancing technology development related to equipment used for wetting dust removal.

Yang Y., Tian S., Zhao J., Yan M., Zhang X.

Droplet impact on rough walls is a prevalent phenomenon in engineering applications, including surface spraying and spray wetting, and understanding the morphology and wetting characteristics of such impacts is crucial for industrial processes. This study utilizes computational fluid dynamics to examine the effects of velocity, surface tension, and their interactions on the behavior of micrometer-sized droplets impacting rough walls. The findings reveal five morphological changes during droplet impact: oscillation, rebound, bubble formation/rebound, tearing/bubble formation/rebound, and rupture/localized rebound. Droplets with lower surface tension are more likely to rebound in low-speed impacts compared to those with higher surface tension. Surface tension has minimal influence on droplet spreading at the initial stage of impact but significantly affects spreading and retraction prior to the liquid reaching its residual diameter after impact. Lowering surface tension and increasing impact velocity intensify morphological changes and enhance wetting performance on rough wall surfaces. The interaction between surface tension and velocity influences the droplet's behavior, as increased surface tension reduces the enhancement of spreading caused by higher impact velocity, while higher velocity decreases the disparity in the minimum height values of droplets with varying surface tensions. This analysis of droplet morphology and wetting characteristics provides valuable insights for applications involving micrometer-sized droplets interacting with rough wall surfaces in engineering practices.

Hanosh S., George S.D.

Zhang L., Wu J., Lu Y., Yu Y.

Droplets impinging on sparse microgrooved polydimethylsiloxane (PDMS) surfaces with different solid fractions was experimentally investigated. First, wettability and stability of droplets on these surfaces was analyzed. The advancing and receding contact angles were found to have a large difference between in the longitudinal direction and in the transverse one, which could be attributed to the anisotropy of the micropatterned surfaces. The judgement of whether a droplet on a sparse microgrooved structure is collapsed or suspended is proposed, and it was found that the droplets were in the Cassie-Baxter wetting state when the actual contact line density is greater than the critical contact line density, while they were in the Wenzel wetting state otherwise. Second, for the case of droplets impacting on sparse microgrooved PDMS surfaces, it was found that droplets can bounce off the micro-patterned surface with a solid fraction of 0.158 when the impact velocity was in a certain range. The lower limit of impact velocity for bouncing droplets can be determined by balancing the kinetic energy of the droplets with the energy barrier due to contact angle hysteresis. The upper limit of impact velocity for bouncing droplets was predicted using a theoretical model taking into account the penetration of liquid into the cavities between microstripes.

Wang Q., Li C., Zheng Y., Feng L., Xiong W., Shang Y.

Syrodoy S.V., Kuznetsov G.V., Voytkova K.A., Kostoreva Z.A., Gutareva N.Y., Poznaharev A.S., Tamashevich M.S.

Bian Q., Xie Y., Zhang W., Fan X., Li W.

Dang C., Wang X., Peng Q., Sun X.

Fang K., Xiang B., Song R., Chen J., Feng X., Mao Z., Yang C.

Non-wetting surfaces have been extensively applied and studied due to their distinctive droplet impact dynamics. However, there is still a lack of studies on the droplet impact force and surface pressure distribution on the non-wetting surfaces. The impact process of a droplet on the non-wetting surface is systematically simulated using the volume-of-fluid method with a high-resolution grid, taking into account the effects of the Weber number (We) and the Ohnesorge number (Oh). The numerical results indicate that the droplet impact force exhibits a bimodal nature, which arises from the changes in the surface pressure distribution during the impact process. Meanwhile, in contrast to the bouncing process, tremendous pressure appears at the center of the droplet collision at the instant of jet generation. Most notably, the maximum pressure of the surface rapidly decays from more than ten times the dynamic pressure during the early stages of the droplet impact, while the location of the maximum pressure moves supersonically in the radial direction. These findings will deepen the understanding of droplet erosion and surface moisture resistance properties.

Xu H., Zhang B., Lv C.

Due to its scientific significance and practical applications, the common natural phenomena of drops impacting on inclined surfaces have attracted extensive attention. Previous research has primarily reported the distinct morphology and dynamic behavior of drops impacting on inclined superhydrophobic surfaces compared to the impact on the horizontal scenarios. One distinguished feature of drop impingement on inclined surfaces is the asymmetric shapes of the drop, which accounts for different underlying physics compared to the impacts on horizontal surfaces. However, the impact forces exerted by the inclined surface during impingement have remained unknown. In this study, we present a direct measurement of the normal impact force of drops on inclined superhydrophobic surfaces using a high-precision force sensor. We observe the temporal evolution of the force and identify two peak forces occurring during the spreading and retraction stages, respectively. Our findings lie on investigating the variation of these two peak forces with the normal Weber number, based on scaling arguments. We reveal that the asymmetrical morphology of the drop must be taken into account especially in the scenarios of large impact velocities and large tilt angles to revise the theoretical model of the second peak force. The physics reported in this work sheds new light on the impingement of drops.