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Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellency behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired.

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The spreading of the water droplets falling on surfaces with a contact angle from 0 to 160° was investigated in this work. Superhydrophilicity of the surface is achieved by laser treatment, and hydrophobization is then achieved by applying a fluoropolymer coating of different thicknesses. The chosen approach makes it possible to obtain surfaces with different wettability, but with the same morphology. The parameter t* corresponding to the time when the capillary wave reaches the droplet apex is established. It is shown that for earlier time moments, the droplet height change does not depend on the type of used substrate. A comparison with the data of other authors is made and it is shown that the motion of the contact line on the surface weakly depends on the type of the used structure if its characteristic size is less than 10 μm. [ABSTRACT FROM AUTHOR]

The freezing of droplets upon impact with solid walls is a common phenomenon that significantly impacts various areas of social production. Therefore, it is essential to study the spreading and icing characteristics of droplets impacting solid walls. Utilizing a visualization experimental system, the effects of impact Weber number (We), wall temperature, wall wettability, and wall curvature on the dynamics of droplet impact and icing characteristics were investigated. The temporal evolution of the droplet's icing morphology and spreading coefficient are analyzed. The results indicate that low wall temperatures reduce the extent of droplet spreading and suppress, or even eliminate, retracting and oscillating phenomena. Under low-temperature conditions, the peak spreading coefficient of droplets is generally lower than that at room temperature, exhibiting a spread-freeze mode. As the We increases, the droplet spreads more vigorously, the solid-liquid contact area enlarges, the slope of the spreading coefficient increases, and the freezing rate accelerates. When the wall is hydrophobic, the spreading coefficient undergoes periodic changes, the droplet undergoes rebound behavior, the residence time and contact area decrease, delaying the freezing process. In contrast, when the wall is hydrophilic, the change in the spreading coefficient is singular, the droplet spreads more stably, and freezing is more likely to occur. Compared to horizontal surfaces (We =277), the effect of surface wettability on the morphological evolution and freezing mode of droplets impacting curved surfaces is less significant. [Display omitted] [ABSTRACT FROM AUTHOR]

Studies on droplets impacting surfaces with different wettabilities are of great significance for understanding interface phenomena and controlling droplet behaviors. In this work, the droplet impact processes on five surfaces from superhydrophilic to superhydrophobic at various Weber numbers are investigated through experimental and theoretical methods. The spreading and height factors and the oscillating cycle time and amplitude are analyzed. The results show that as the Weber number increases and the contact angle decreases, the impact droplet generally spreads faster and thus yields a larger maximum spreading factor. The maximum spreading time shortens with the increasing Weber number and contact angle. The height factors on different wettable surfaces show a similar decreasing trend in the early spreading stage. The larger the contact angle, the smaller the minimum height factor. As the Weber number increases, both the height factor and minimum height factor decrease. During the oscillating stage, the oscillation cycle time increases at a larger contact angle while it first increases and then decreases as the Weber number increases. Besides, the modified empirical formulas for predicting the maximum spreading factor and time are proposed to consider the effect of the surface contact angle following the principle of energy conservation. These findings can provide guidance for various practical applications of different wettable surfaces. [Display omitted] • Five surfaces from superhydrophilic to superhydrophobic are fabricated. • The influences of Weber number and surface wettability are investigated. • The spreading and height factors and the oscillation features are analyzed. • Modified formulas for the maximum spreading factor and time are proposed. [ABSTRACT FROM AUTHOR]

• Three sequential processes occur, including coalescence, spreading, retraction. • Spreading diameter and time are normalized by impact and stationary droplets. • Effects of Weber number, droplet volume ratio, surface wettability are clarified. • Maximum spreading coefficient elevates at larger Weber number or drop volume ratio. • An accurate theoretical model of the maximum spreading diameter is proposed. In numerous applications related to droplet impact, impacting droplets often interact with solid surfaces with adhesive droplets rather than dry surfaces, but the former is significantly less studied than the latter. Here, for the fundamental process of droplet impact on a stationary droplet on the surface, we investigate droplet dynamics and maximum spreading under various conditions. The spreading diameter and time are reasonably nondimensionalized as the spreading coefficient and dimensionless time by considering both the impacting and stationary droplets, and the complex influence of diameters of two droplets is reduced to the effect of a single droplet volume ratio. The proposed dimensionless framework provides a method to simplify influencing factors and facilitates the study of droplet dynamics under different Weber numbers, surface wettabilities, and droplet volume ratios. The maximum spreading coefficient increases with the increase of the Weber number or the droplet volume ratio, while it is nearly independent of surface wettability. A theoretical model of the maximum spreading diameter is established through energy analysis. By considering the coupled effects of various factors and modifying the viscous dissipation, the model has the merits of high prediction accuracy and broad applicability. Our findings can not only advance the basic understanding of the ubiquitous droplet impact phenomena, but also provide useful guidance for applications such as spraying cooling and inkjet printing. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Droplets impacting dry solid substrates often splash above a certain threshold impact velocity. We hypothesise that substrate curvature alters splashing thresholds due to a modification to the lift force acting on the lamella at the point of breakup. We have undertaken high-speed imaging experiments of millimetric droplets impacting convex and concave surfaces to establish splashing thresholds and dynamics across a wide range of substrate geometries and impact conditions. Our findings indicate that the tendency of droplets to splash is proportional to the reciprocal of the substrate's radius of curvature, independent of whether the substrate is convex or concave, with it being harder for droplets to splash on small spheres. Moreover, we consistently parameterise the axisymmetric splashing threshold across all curved substrate geometries via a modification to the well-known splashing ratio. Finally, the splashing dynamics resulting from initial asymmetry between the impacting droplet and curved substrate are also elucidated. [ABSTRACT FROM AUTHOR]

This paper aims to develop a reliable and effective model to investigate the behavior of micron-sized biological glue droplets impacting micron-sized bioceramic powder beds applied to the 3D printing process. It also endeavours to explore the common rules of droplet impact affected by particle size and the wettability of powder, which are supposed to provide process parameters guidance for the application of new materials in 3D printing. Firstly, based on the low impulse impact model, the simplified model was proposed. Then, the observation and simulation experiments of millimeter-scale droplet impacting were carried out under the same conditions to prove the effectiveness of the model. Furthermore, the characterization of a parametric experiment of a 3D printing practice was used to verify the significance and effectiveness of the simulation study method. Lastly, the method was performed to investigate the effect of wettability and particle size of the micron powder on the micron droplet impact. The results showed that the binder powder's wettability and particle size could directly influence the droplet spreading behavior. The characterization results of samples printed in the simulation-predicted parameter showed that the amount of binder used could be reduced by 38.8~50.1%, while the green strength only lost 17.9~20%. The significance of this simulation method for prediction of 3D printing process parameters was verified. [ABSTRACT FROM AUTHOR]

Superhydrophobic functional surfaces are widely used in medical treatment, pipeline transportation, and ship corrosion protection and have great potential in the field of anti-icing. Ultrafast laser-induced plasma micromachining (LIPMM) combined with a chemical modification of a fluoroalkyl low surface-energy substance (Novec) was used to fabricate superhydrophobic surfaces with periodic layered micro-nano arrays on Ti6Al4V. The wettability, droplet collision behavior and wear resistance of surfaces with different physical and chemical properties were analyzed. In terms of static anti-icing, based on the classical nucleation theory, the icing-delay performance of different surfaces was analyzed. Ice adhesion strength and frosting properties were also studied. It was found that the micro-nano structure surface fabricated by LIPMM had strong hydrophobicity (contact angle of 164°, roll-off angle of 1.0°), high icing delay time (3072 s, 4 μL of water droplet on the surface at −10 °C), low ice adhesion strength (38.2 kPa), and better durability. Additionally, the droplets bounced off this surface after colliding at different angles (0°, 5°, 10°, and 20°). This study provides a feasible solution for fabricating anti-icing surfaces on Ti6A14V. • Micro-nanostructures machined by laser-induced plasma micromachining promote the growth and attachment of compounds. • The influence of structures and chemical elements on surface wettability is studied. • The wettability and droplet collision behavior of surfaces were analyzed. • The static anti-icing properties of the Micro-nanostructures were investigated. [ABSTRACT FROM AUTHOR]

The wetting and penetration of the coal dust layer by liquid droplet is crucial for suppressing coal dust. A combination of experimental and simulation methods was employed to investigate the impact of surfactant concentration, droplet velocity, and particle size on the penetration properties and their underlying mechanisms. Droplet impact wetting experiments were employed to evaluate their dynamic wetting behavior on the coal dust layer. The experimental results show that increasing the impact velocity of droplets is advantageous for enhancing the maximum dimensionless wetting length. Increasing the surfactant concentration can expedite the droplet reaching maximum dimensionless wetting volume in the dust layer. The larger the particle size of coal dust, the shorter time required for droplet to reach maximum wetting volume. The research results provide a theoretical basis for selecting and efficiently applying surfactants in treating coal dust. [Display omitted] • The addition of surfactants enhances water penetration into the coal dust. • L dw(max) and V dw can evaluate the dynamic wetting characteristics of droplets. • The penetration time of droplets can be divided into two components: t p1 and t p2. [ABSTRACT FROM AUTHOR]

This paper investigates the total freezing time of droplets on surfaces with various wettabilities with horizontal and inclined orientations. A two-dimensional Volume of Fluid (VOF) method was applied to capture the liquid-air interface, and an automatic localized grid treatment technique was applied to increase the accuracy, especially near the impact and spreading areas. The Kistler and Shikhumurzaev dynamic contact angle models were implemented to impose the dynamic contact angles on different surfaces. An enthalpy-porosity technique was used to predict the phase change of droplets after impact with the surface. The results of the nondimensional droplet diameter ratios and total freezing times for both dynamic contact angle models have been presented and verified with experimental data. The effects of both wetting properties and the surface inclination on the freezing time have been analyzed. The results indicate that a lower surface temperature, a decrease in static contact angle and a higher inclination will result in more rapid freezing of droplets. [ABSTRACT FROM AUTHOR]

Droplet impact on cylindrical surfaces is a widespread phenomenon observed in both nature and industrial applications. The wettability and size of the cylinders, and the initial impact velocity, are expected to influence the impact dynamics, such as generating different droplet morphology, spreading/retracting processes, and contact time, which could finally determine the anti-icing property of power transmission lines. A particle-based mesh-free numerical approach, many-body dissipative particle dynamics, is employed to study the impact dynamics. The cylinders with three different sizes and three types of wettability are modelled, and different initial impact velocities are set. The simulations show the two liquid films (or lamellae) formed after impact can coalesce at the bottom site of a hydrophilic cylinder while they keep separate on hydrophobic and superhydrophobic cylinders. The maximum spreading diameter in the axial direction increases with the increase of impact velocity and cylinder size. Besides, the distribution of droplet detachment mode shows a strong dependence on initial impact velocity and cylinder size. In bouncing mode, the contact time increases with the increase of impact velocity while in dripping/bouncing mode, it has an opposite trend. The splashing angle is also observed to be dependent on the cylinder size. [ABSTRACT FROM AUTHOR]

In this study, we investigated droplet impact dynamics offalling water drops (D0 ~ 2.3 mm) on slippery oil-infused surfaces and compared them to other features of the surfaces, to elucidate the wettability- and roughness-controlled characteristics. We prepared transparent substrates with the designed characteristics, so it would be feasible to visualize the droplet impact dynamics in detail. A wide range of impact kinetics (We ~ 800 (=ρD0Vi2/σw)) was covered, which gave rise to several types of droplet-impact: gentle spreading, wavy (undulated fingers of spreading edges), droplet break-up, and splashing with small secondary droplets. The basic parameters of the droplet-solid interactions were measured, and events were mapped with respect to the sample surface and impact kinetic conditions. We found that, generally, surface wettability has a major influence on the triple line shape and instability during the impact and retraction process, and thus determines events in of the framework of the dynamic wetting-failure model. Furthermore, while rough conditions promote instability of the impacted droplet, slippery lubricant-infused features tend to dampen perturbations of the spreading/retracting edge. [ABSTRACT FROM AUTHOR]

Controlling the impact behavior of a droplet on a superhydrophobic surface is a significant challenge for many applications. Instead of texturing the surface, this study investigated the effect of adding surfactants on the morphology of the droplet after it impacts superhydrophobic surfaces. The results showed that adding low concentrations of sodium dodecyl sulfate (SDS) surfactant prevents fragmentation and splashing. High concentrations of SDS can inhibit rebound and allow droplets to be easily deposited on surfaces after impact. When the droplet size was 3.51 mm and the SDS concentration was above the critical micelle concentration, the contact time of the solution increased by a maximum of 1.30 times compared to that of pure water. In a high-concentration SDS solution, the surfactant molecules can quickly migrate to the newly generated interface during the spreading stage, reducing the interfacial tension and causing wetting changes. The results of this study are expected to contribute toward establishing a simple method to inhibit splashing and increasing contact time on superhydrophobic surfaces. [Display omitted] [ABSTRACT FROM AUTHOR]

• Without sacrificing separation efficiency, 'Dripping mode' overcomes the water-phase obstruction problem of 'Gravity-driven mode'. • 'Dripping mode' is based on a combined mechanism of the capillary effect of liquids in pores and the impact dynamics of droplets above substrates. • Selective wettability can effectively prevent contamination of separated oil and promote expulsion of undesired water. • 'Dripping mode' can't separate heavy oil. Developing technologies of high-efficiency and low-cost oil/water separation has attracted much attention due to the environmental damage caused by oil pollution. General filtration can be coupled with selective wettability to improve oil/water separation efficiency and reduce energy consumption. However, accumulated liquid obstruction is problematic for the commonly used 'Gravity-driven mode' method, reducing permeate flux and separation efficiency. Hence, we experimentally analyze the phase-separation mechanism of an alternative ('Dripping mode') method by studying the impact dynamics of water-in-oil compound droplets on the metallic mesh with selective wettability. Firstly, the effect of selective wettability on the impact dynamics is studied. We focus on two crucial behaviors in practical application: oil/water separation and water-core expulsion. Their typical regimes are classified according to the outcomes, and theoretical models are proposed to explain the thresholds of these regimes. The max-volume quantitative prediction model of separated oil is established, and the model's error is illustrated. Additionally, the feasibility of separating heavy oil (i.e., high viscosity & density oil) is discussed. [ABSTRACT FROM AUTHOR]

• Extensive simulations were conducted on spreading process of power-law fluid droplets. • Two droplet morphologies observed at maximum spreading regardless of wettability. • β max of shear-thinning and Newtonian droplets converge to β 0 at low We. • β max ∼ ln (Re n 1/(2n+3)) proposed for shear-thinning fluids in viscous regime. • Established a rescaling law of β max for non-Newtonian shear-thinning droplets. The maximum spreading diameter of non-Newtonian fluid droplets impacting on the solid surface is a key concern in a variety of industrial and medical applications. In this work, we focus on the effect of the shear-thinning, one of the most important non-Newtonian properties, on the spreading dynamics of impacting droplets. A finite element scheme combined with a phase field method and dynamic contact angle model has been employed to perform extensive studies on the spreading process of power-law fluid droplets on solid surfaces with various rheological parameters, impact conditions and surface wettability. The simulation results show that on both hydrophilic and hydrophobic surfaces, impacting droplets exhibit two typical morphologies at the maximum spreading state: a spherical cap in the low-Weber-number range (capillary regime) and a thin-film form in the high-Weber-number range (viscous regime). The maximum spreading factor β m a x , of droplets with different degrees of shear-thinning converges to the equilibrium spreading state for a droplet with U 0 = 0 at the low-Weber-number limit. Furthermore, a theoretical relationship of β m a x ∼ W e 1 / 2 has been derived in the capillary regime. In contrast, the effect of the shear-thinning property becomes significant in the high-Weber-number regime. We discussed the influence of the power-law coefficients K and n on the spreading process and β m a x independently. Specifically, as the power-law index n decreases, the morphology of the shear-thinning droplet at the maximum spreading state tends to change from a spherical cap to a thin-film form. Considering the non-uniform distribution of shear rates in the spreading shear-thinning droplet, a new scaling relationship of β m a x ∼ l n (R e n 1 / (2 n + 3)) has been proposed based on theoretical derivation and numerical simulations. By introducing an interpolation function on the scaling relationships between the capillary and viscous regimes, we obtained a universal rescaling model that agrees well with numerical and experimental results of non-Newtonian droplets with shear-thinning fluid over a wide range of W e numbers, surface wettability and rheological parameters. [ABSTRACT FROM AUTHOR]

The dynamic interaction of droplets with slippery surfaces is essential to various industrial applications, such as anti-icing, water-repellency or water-harvesting, anti-bacterial, and phase change heat transfer. With recent progress in materials, manufacturing as well as learning from nature, the physics of droplet dynamics has been greatly enriched owing to the emergence of peculiar wetting states manifested on bio-inspired textured surfaces. This review is devoted to the discussion of the recent progress made in the authors’ understanding of the dynamic interaction of small droplets with bio-inspired surfaces. Particular attention is given to droplet impact on slippery surfaces, such as superhydrophobic surfaces characterised with air–solid–liquid triple-phase interface and slippery lubricant infused porous surfaces characterised with the liquid/liquid two-phase interface. Droplet spreading, retraction, contact time, elastodynamics as well as oblique impact are systematically reviewed. Finally, the authors offer their perspectives on this important and highly multidisciplinary research area.

Bionics and dynamic interface wetting intensely appeal to many research communities due to their unique practical implications. The rose petals had a highly robust dynamic water-retaining capacity under heavy precipitation. We predicted that the roses became more "hydrophilic" at higher Weber numbers. Fresh rose petals were directly impacted by droplets, and facile artificial petal-like substrates and superhydrophobic substrates were used in the comparative analysis. The wetting dynamics of the droplet (e.g., topography, bounce dynamics, contact time, three-phase contact lines, and oscillations) were investigated when interacting with four selected target substrates. The present work first time investigated the dynamic wetting rule of the sticky superhydrophobic substrates (SSHS). Simulated and experimental investigations confirmed that the unique coupling synergy between the pinning effect and the inhomogeneous micropapillaes resulted in lopsided contact line velocities, which remarkably suppressed the lateral oscillation and rebounding. This may be a new strategy when designing dynamic water-repellent surfaces and open a promising avenue for emerging areas such as super-efficiency energy conversion and harvesting. [ABSTRACT FROM AUTHOR]

• Six typical droplet impact and boiling modes on superheated surfaces are identified. • Determine the modes in a large range of Weber numbers and surface temperatures. • Surface wettability is changed by coating and the roughness maintains very small. • The influence of wettability on the droplet impact and boiling is analyzed. In this work, single droplet impact on superheated surface with 5 to 220 Weber numbers, 110 °C to 360 °C surface temperatures and 10°, 78° and 122° surface contact angles of deionized water was experimentally investigated. The surface wettability was changed by spray-coating thin films on the aluminum plates and the surface roughness maintained very small. Six typical droplet impact and boiling modes were observed and classified. The T s - We regime map was used to illustrate the droplet impact and boiling modes. The results show that the surface temperature required for the transition from droplet boiling with atomization to droplet boiling without atomization increases with the increase of Weber number and surface wettability. The curve separating the droplet rebound with atomization mode and droplet breakup with atomization mode is not monotone on the hydrophilic surface and bare aluminum surface. On the hydrophilic surface, the recoil of the spread film is very weak at the deposition mode and droplet rebound requires higher surface temperature. Besides, the droplet breakup will not occur on the hydrophilic surface with lower surface temperature (T s ≤ 150 °C). On the hydrophobic surface, the deposition mode is absent and atomization cannot be observed for any surface temperatures when the Weber number is small (We = 5 or 20). [ABSTRACT FROM AUTHOR]

Highlights • We present three-dimensional numerical simulations of a droplet impact on a substrate featuring an occlusion. • The simulations, based on the Lattice–Boltzmann method, are validated against the Scheller and Bousfield correlation for normal droplet impact. • We derive a criterion based on an energy balance which predicts whether the occlusion will rupture the lamella or not. • Results show that rupture is more likely to appear as the diameter of the occlusion, the impact velocity, and the hydrophobicity of the substrate increase. Abstract This paper investigates how and under which conditions the lamella of an impacting droplet is punctured by the presence of a small occlusion. Better understanding the conditions which lead to the rupture of the lamella is critical to produce defect free coating layers in the context of spray coating, for example. An analytical model based on surface energy analysis is proposed to obtain the critical thickness below which the liquid layer above the occlusion is unstable and lamella rupture occurs. Furthermore, we have developed a three dimensional multiphase lattice Boltzmann code to confirm the surface energy analysis and study the influence of key parameters like size of the occlusion, impact velocity and wettability of substrate on hole formation. Results show that a hole is more likely to appear as the diameter of the occlusion, the impact velocity, and the hydrophobicity of the surface increase. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Thermally sprayed coatings are manufactured by molten or semi-molten droplets impinging onto the target substrate. The properties of coatings created (porosity, conductivity, cracks,...) depend largely on the dynamics of the droplets which is an outcome of joint aspects, such as properties of liquid droplet and surface wettability. The present work investigates the normal impact of liquid droplets (molten ceramic or metallic droplets) onto dry solid surfaces of various wetting properties. Use is made of the Volume-Of-Fluid (VOF) interface tracking method with a single set of mass and momentum conservation equations. The wettability of a solid surface, characterised by a contact angle, is taken as part of the boundary conditions in the Smooth VOF algorithm. Introducing the dimensionless time t * = tV 0 / D 0 for a droplet with the initial speed V 0 and initial diameter D 0 , its spread factor ξ (= D / D 0) is found to be proportional to the square root of t * when 0.2 < (t *)1/2 < 0.35. There exists a transitional zone where the changing rate of the spread factor decreases with the ascending contact angle. In addition, the wetting effects on the spreading can be ignored when 0.2 < (t *)1/2 < 0.35 but have to be accounted for once (t *)1/2 < 0.2. Further, the droplet jetting time is found to be inversely proportional to the impact velocity but not affected by the solid surface wettability. Finally, in the recoil stage, the influences of Weber number, Ohnesorge number and contact angle are studied; in particular, the spread factor ξ turns out to be well described by a second-order polynomial equation for t * ≥ 0.5 W e. Highlights • A transitional zone is identified in early spreading when wettability varies. • The dimensionless jetting time is inversely proportional to impact velocity and independent of wettability. • The recoil can be described by a 2nd polynomial equation of a dimensionless time. [ABSTRACT FROM AUTHOR]

Spray coating with liquid polymers is a crucial step in producing polymer-derived ceramic (PDC) coatings before curing the precursor. However, the interactions between polymer droplets and target surfaces during the spray coating process have not been adequately characterized in existing literature. This experimental study aims to comprehensively examine the previously unknown droplet impact dynamics for a specific ceramic precursor, Polysilazane, on Inconel surfaces. The diameter of Polysilazane droplets was kept constant, while their impact velocity was varied to achieve different impact conditions. The same parameter range was also explored with water, water-glycerol, and ethanol droplets for comparison purposes. High-speed videos were captured to observe the instantaneous impact process, while low-speed videos were recorded at specific instances to document the gradual wetting process at later stages. The investigation revealed unique spreading dynamics of Polysilazane droplets, including smooth contact angle variation during the transition from impact to lateral spreading, absence of delayed wetting, no formation of wavy structures on the droplet interface during spreading, and no retraction process within the studied conditions (Weber number up to 1200). Conversely, Polysilazane displayed similarities in the normalized spreading curves with other liquids. Additionally, the maximum spread factor of Polysilazane could be well predicted using existing universal scaling models designed for different liquid types. Lastly, Polysilazane exhibited a distinctive post-impact wetting process that persisted for an extended duration. The wetting dynamics of Polysilazane could be effectively described by an empirical exponential law originally developed for partial wetting systems. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Effects of AC EWOD on impacting droplets on hydrophobic & superhydrophobic surfaces have been investigated. • Dynamics of the droplet impact with frequency- and amplitude-modulated ACEWOD have been investigated. • Droplet spreading diameter increases with amplitude, whereas the recoiling rate decreases. • At higher amplitude, partial and complete suppression of recoiling have been observed at medium frequencies. • The effects of frequency-modulated AC EWOD on droplet dynamics are more pronounced than that of amplitude-modulated AC EWOD. A droplet impacting hydrophobic/superhydrophobic surfaces undergoes spreading, recoiling, and rebounding. All these phenomena are affected by the energy exchange between the impacting droplet and the surface. In the present work, an electrowetting technique is implemented to change the droplet interaction with the surface. The droplet impact on hydrophobic and superhydrophobic surfaces combined with alternating current electrowetting on dielectric (AC EWOD) has been numerically investigated. Droplet impact characteristics have been studied for various voltage amplitudes, frequencies, and surface hydrophobicities. These factors strongly affect the droplet dynamics of the impacting droplet. An increase in voltage amplitude enhances the maximum spreading of the impacting droplet. In contrast, it reduces the recoiling rate of the droplet, resulting in partial or complete suppression of the droplet rebound. At a particular voltage amplitude, frequency and hydrophobicity of the surface affect the droplet rebound suppression. Complete rebound suppression has been observed at higher voltage amplitudes and the medium range of frequencies (25–50 Hz). The contact time of the droplet decreases with the increase in surface hydrophobicity at all frequencies, whereas it increases with the voltage amplitude. Contact line velocity and velocity inside the droplet manifest the inertia of the droplet. The droplet is unable to rebound if velocities are very low. [ABSTRACT FROM AUTHOR]

Abstract A molten droplet impacting and solidifying on a cold substrate with micro-pillars on the top is studied numerically in this paper using a newly developed multi-component, triple-phase solidification 3D lattice Boltzmann method (LBM). Simulated results confirm that droplets on a rough surface can exist in three states, depending on the static contact angle of the substrate θ : (i) Hemi-wicking Cassie-Baxter state for θ < θ c1 , (ii) Wenzel state for θ c1 < θ < θ c2 , and (iii) Cassie-Baxter state for θ > θ c2 , where θ c1 and θ c2 are two critical contact angles whose values depending on the geometry of the micro-pillars. It is shown that the air between the droplet and micro-pillars is compressed by the falling droplet before impact, and the compressed air is pushed into grooves of the surface microstructure after impact. As a result, no air bubble is formed under the droplet bottom surface after its impact and solidification on the rough surface. The solidified droplet's bottom surface is smooth for a substrate with contact angle θ > θ c2 and the bottom surface becomes rougher for θ < θ c2. Two rough substrates with wetting and non-wetting interlaced wettabilities are presented for different applications. Simulated results show that fingers are formed at the triple-phase contact line along the rim of the droplet on the rough surface with non-wetting interlaced base (with θ > θ c2). [ABSTRACT FROM AUTHOR]

This paper explores the use of time resolved infrared (IR) thermography combined with high-speed imaging to describe the liquid-surface interfacial heat transfer phenomena occurring at droplet/wall interactions. A first set of experiments is used to infer on the validation of custom made calibration and post-processing methods to evaluate the potential to obtain accurate data, useful to describe the observed phenomena. In this context, the technique showed good potential to capture very well particular details on droplet dynamics and heat transfer, allowing to identify air bubble trapping at the impact region, as well as the temperature variations at the formation of the rim. The combined analysis of droplet dynamics ( e.g. the spreading factor) with the radial temperature profiles, heat flux and cooling effectiveness allowed establishing qualitative and quantitative trends on the effect of various parameters on the heat transfer occurring at droplet/wall interactions. Particularly, details on the temperature distribution within the lamella, reflected on the surface temperature fields, which are related to the spreading dynamics, are explored and discussed. Furthermore, extreme wetting scenarios, such as superhydrophobicity are studied in detail. The analysis performed show that they limit the heat transfer between the droplet and the surface. However, the thermal analysis coupled with the spreading dynamics reveals that a major reason for this is not just related to the reduced contact time of the droplet (due to rebound) or air entrapment, but is also associated to the reduced wetted area caused by low wettability, which is not obvious from the analysis of the spreading diameter alone, but becomes evident when this is coupled with the evaluation of the temperature fields on the heated surface. [ABSTRACT FROM AUTHOR]

In this paper, we experimentally investigated the impact dynamics of different viscous droplets on solid surfaces with diverse wettabilities. We show that the outcome of an impinging droplet is dependent on the physical property of the droplet and the wettability of the surface. Whereas only deposition was observed on lyophilic surfaces, more impact phenomena were identified on lyophobic and superlyophobic surfaces. It was found that none of the existing theoretical models can well describe the maximum spreading factor, revealing the complexity of the droplet impact dynamics and suggesting that more factors need to be considered in the theory. By using the modified capillary-inertial time, which considers the effects of liquid viscosity and surface wettability on droplet spreading, a universal scaling law describing the spreading time was obtained. Finally, we analyzed the post-impact droplet oscillation with the theory for damped harmonic oscillators and interpreted the effects of liquid viscosity and surface wettability on the oscillation by simple scaling analyses. [ABSTRACT FROM AUTHOR]

• The dynamic Leidenfrost phenomenon at the nanoscale is numerically investigated. • Hydrophilicity favors a lower dynamic Leidenfrost temperature. • The Leidenfrost effect can be triggered below a critical contact angle. • The critical contact angles at different surface temperatures are determined. The microscopic mechanism of the dynamic Leidenfrost phenomenon involving the impact of droplets on high-temperature surfaces remains unclear, although many efforts have been devoted to the static Leidenfrost phenomenon of evaporative sessile droplets. In this work, we study the dynamic Leidenfrost phenomenon of droplets impacting high-temperature surfaces with different wettability by using water and copper. The simulations by the molecular dynamics method show that determined by the atomic potential energy, a hydrophilic surface favors a lower dynamic Leidenfrost temperature compared to a hydrophobic surface. It is found that there is a critical contact angle that can trigger the Leidenfrost phenomenon at different surface temperatures. The critical contact angles are about 44.06°, 87.93°, and 92.63° when the surface temperatures are 998 K, 1,098 K, and 1,198 K, respectively. That the critical contact angle becomes larger with elevated surface temperature is helpful for industrial designs, in which novel insulating materials with higher Leidenfrost temperatures are required. [ABSTRACT FROM AUTHOR]

A liquid drop impacting on a solid substrate is simulated to evaluate the effect of inhomogeneities in the wetting characteristics of the surface. A high density ratio multiphase solver based on phase-field lattice Boltzmann formulation has been employed. Influence of receding contact angle and wettability gradient is investigated to analyse the coupled dynamics of the impacting droplet on the solid surface. Simulations indicate that the receding contact angle is a key wetting parameter to determine the final physical outcome of the impact process. Interplay between tangential velocity component and receding contact angle results in interesting dynamical behaviours in the evolution of the droplet shape for oblique impact scenarios. Substrate inhomogeneities in the form of wettability gradient is found to induce a directional behaviour to the motion of the impacting droplet. The droplet motion is characterised by intense recoiling along the upstream part of the droplet, followed by secondary spreading of the downstream contact line. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the influence of the surface wettability on the dynamic behavior of a water droplet impacting onto a heated surface made of stainless steel grade 304 (Sus304). The surface wettability is controlled by exposing the surfaces to plasma irradiation for different time periods (namely, 0.0, 10, 60, and 120 s). The experimental runs were carried out by spraying water droplets on the heated surface where the droplet diameter and velocity were independently controlled. The droplet behavior during the collision with the hot surface has been recorded with a high-speed video camera. By analyzing the experimental results, the effects of surface wettability, contact angle between impacting droplet and the hot surface, droplet velocity, droplet size, and surface superheat on the dynamic behavior of the water droplet impacting on the hot surface were investigated. Empirical correlations are presented describing the hydrodynamic characteristics of an individual droplet impinging onto the heated hydrophilic surfaces and concealing the affecting parameters in such process. [ABSTRACT FROM AUTHOR]

Encapsulation of active compounds into liquid marbles (LMs) represents an emerging technology with potential applications in several fields including food, cosmetics and pharmaceutics. However, existing methods for the preparation of LMs are either based on manual one-by-one fabrication or on batch processes such as high-shear granulation, which lack the required precision in LM size distribution. We present a device capable of continuous LM production with high reproducibility, control over the LM properties and production rate flexibility. The device utilizes droplet impact onto a continuously moving powder bed on a belt conveyor with subsequent product separation by sieving. The device has been applied for fabricating LMs from various combinations of coating powders and core liquids including water- and oil-based systems as well as melts. Monodisperse populations of all three LM types were obtained in the range of 2.0–3.5 mm depending on the nozzle setting, and the maximum productivity of the device was up to 50 LMs per second. The effect of process parameters was investigated and guidelines for defect-free production were obtained. We 0.5 Re 0.25 above a critical value was found to indicate the onset of undesirable satellite droplet generation or interface jamming upon the droplet impact; the critical value was also found to depend on the thickness of the moving powder bed. Surprisingly, the minimum safe interval between consecutive LMs that was necessary to avoid coalescence was found to be invariable with respect to both LM type and diameter. The device was capable of laboratory-scale LM production for a range of product testing and prototyping purposes and provided useful data for subsequent process scale-up. [Display omitted] • New device for continuous fabrication of liquid marbles was construced • Aqueous, oil- and melt-based liquid marbles were successfully produced • Feasible regions of opearing parameters for defect-free operation were determined • Criteria for trouble-shooting and scale-up were established [ABSTRACT FROM AUTHOR]

Superhydrophobic surfaces have attracted much attention due to their excellent water-repellent property. In the present study, droplets in the ideal Cassie state were focused on, and a particle-based numerical method, many-body dissipative particle dynamics, was employed to explore the mechanism of droplets impact on textured surfaces. A solid–fluid interaction with three linear weight functions was used to generate different wettability and a simple but efficient method was introduced to compute the contact angle. The simulated results show that the static contact angle is in good agreement with the Cassie–Baxter formula for smaller ∅ S and F a , but more deviation will be produced for larger ∅ S and F a , and it is related to the fact that the Cassie–Baxter theory does not consider the contact angle hysteresis effect in their formula. Furthermore, high impact velocity can induce large contact angle hysteresis on textured surfaces with larger ∅ S and F a . The typical time-based evolutions of the spreading diameter were simulated, and they were analyzed from an energy transformation viewpoint. These results also show that the dynamical properties of droplet, such as rebounding or pinning, contact time and maximum spreading diameters, largely depend on the comprehensive effects of the material wettability, fraction of the pillars and impact velocities of the droplets. [ABSTRACT FROM AUTHOR]

The numerical modeling of contact angle and wetting surface properties is investigated with volume of fluid (VOF) interface tracking method. A method is proposed to model the wetting properties with an existing continuum surface force (CSF) approach by introducing an original smooth volume of fluid function (SVOF) resulting from the solving of a penalized Helmholtz equation. For the diffusion equation proposed here, the interest of the penalty method is to account implicitly for wetting effects and, to avoid introducing sliding laws for the velocities at the wall or modifying by hand the interface shape on the solid boundaries. The SVOF model is validated and evaluated on published experiments concerning droplet impacts on flat and inclined surfaces with various wetting properties. [ABSTRACT FROM AUTHOR]

Abstract: Dynamic behavior of micrometric single water droplets impacting onto heated surfaces with and without superhydrophilic coating is investigated using a high-speed video camera in this research study. Superhydrophilic surface coating, SHS, is achieved by coating the surface with Titanium dioxide, TiO2, and by exposing the surface to ultraviolet, UV. Mirror heat transfer surfaces of different metals have been considered. The experimental runs are carried out by spraying single water droplets onto heated surfaces where, the droplet diameter and velocity were independently controlled. The droplet behavior during the collision with the hot surface has been observed with the high-speed video camera. By analyzing the experimental results and comparison between the present results and the results due to other investigators, the effects of surface wettability, thermal properties of the heat transfer surface, droplet velocity, droplet size and surface superheat on the dynamic behavior of micrometric single water droplets impacting onto the heated surfaces were investigated. Empirical correlations are presented describing the hydrodynamic characteristics of an individual droplet impinging onto the heated surfaces, and concealing the affecting parameters in such process. [Copyright &y& Elsevier]

Highlights: [•] Spreading and impact of a dielectric droplet are investigated. [•] The effects of electric field strength and surface charge are taken into account. [•] The results are reported for contact angles from 40° to 150°. [•] Droplet maximum spreading diameter increases with surface charge density. [•] The maximum wetting diameter can be conserved by increasing the surface charge. [ABSTRACT FROM AUTHOR]

Abstract: A study on the initial wetting velocity (V w0) for a water droplet impinging on glass and parafilm surfaces was performed using a fast-speed video camera system. The silhouette images were recorded sequentially from the moment the droplet made contact with the solid surface. The profile of the droplet during spreading revealed the droplet-wetting diameter (especially in the first millisecond), then the wetting velocity was determined. The relaxation of the wetting velocity, V w(t), was investigated at five different impact velocities (V i). The value of V w0 was found to be a few meters per second; V w then decreased dramatically in a few ms to approximately 1/10 of V w0. Moreover, the V w for water on parafilm became negative (i.e., retraction started) at approximately 2–3ms (dimensionless time t*∼1). A general phenomenon was observed: (i) a higher V i results in a larger V w0; (ii) at the first millisecond, the wetting velocity was dominated primarily by V i and was nearly independent of the surface hydrophobicity; (iii) for t >2ms, the V w depends mainly on the surface hydrophobicity; and (iv) for t =1–3ms, the V w for the droplet with higher V i became smaller for both solid substrates while the droplet continued spreading. [Copyright &y& Elsevier]

Abstract: A notable universal relationship has been proposed in the literature for the evolution of dimensionless droplet height and wetting diameter during the initial spreading stage of droplet impingement. In this study, this universal relationship was investigated by employing three sets of measurements. Sequential images were recorded, and the whole droplet profile ensembles were plotted to facilitate this study. These sets of experiments were designed by changing impact velocity, surface hydrophobicity, or solution property. The experimental results illustrate that the importance of parameters causing the data variation is in the order of surface hydrophobicity>initial impact velocity>surfactant on wetting diameter, and surface hydrophobicity≈initial impact velocity>surfactant on droplet height. No universal relationship was observed for dimensionless droplet height and wetting diameter. [Copyright &y& Elsevier]

The present work numerically studies the dynamics of a droplet impact on solid spheres with different sizes and surface wettability. Different initial impact velocities are also set to the droplet to consider the velocity effect. A particle-based numerical method, many-body dissipative particle dynamics (MDPD), is employed to perform the simulations. The simulation results show that the post-impact droplet morphology highly depends on the impact velocity, size and surface wettability of solid spheres. At lower impact velocities, the droplet can sit on the top of the solid surfaces and keep a semi-sphere shape; while at higher impact velocities, for smaller solid spheres, the droplet can fully wrap the spheres and even drip off. For impact on larger spheres, on the hydrophilic surfaces, the final stable liquid film thickness is highly velocity-dependent, while on the hydrophobic surfaces, the droplet always sits on top of the sphere with the same liquid thickness. [ABSTRACT FROM AUTHOR]

This study uses a two-phase finite volume method to investigate the dynamics of Newtonian and non-Newtonian droplets impacting onto hybrid surfaces with various wettabilities. Six configurations with different substrate contact angles are tested ranging from hydrophilic, hydrophobic, and superhydrophilic as well as a combination of them. The temperature-dependent properties are applied to model the Newtonian droplets, and the Arrhenius law which is a relation between viscosity and shear rate is incorporated for the non-Newtonian rheology. The results show that for a hybrid surface with linear wettabilities varying from hydrophilic to hydrophobic to superhydrophobic, the maximum spreading factor is larger for both Newtonian and non-Newtonian droplets in comparison to any other surface configurations considered in this study. However, this spreading factor is minimum when a stepwise superhydrophobic-hydrophobic-hydrophilic hybrid surface is examined. Further, the residence time of Newtonian droplet has the maximum value when collides upon a hybrid surface with linear wettability distribution ranging from hydrophilic to superhydrophobic. However, the maximum value of residence time for the non-Newtonian droplet is achieved when the stepwise pattern of hydrophilic to superhydrophobic is adopted. [ABSTRACT FROM AUTHOR]

• Experiments of the oil droplet impact on the inclined surface were performed. • Phenomenological description of droplet spreading under various inclined angle and We. • Effects of the surface roughness on the normalized droplet spreading. • The orders of the normalized droplet spreading under various parameters. This study experimentally investigated the wetting process of an oil droplet impacting on the dry surface with different surface roughness (0.3 μm, 1.6μm, 2.9μm, 5.2 μm) from the front view. Experiments were conducted under three impacting Weber ranges with the inclined angles of 37.0°, 63.0°, and 75.5°. The droplet spread as a 'pancake' on the surface when the inclination angle was 37.0°, while the shape of an 'avocado' of the flowing oil layer was observed on higher inclined angle surfaces. The upper part of the 'avocado' became narrower when the inclined angle increased from 63.0° to 75.5°, which then grew longer with the increase in the impact We. The normalized transverse and longitudinal spreading displacement β x and β y were defined, whose evolution with normalized time was then obtained. Three stages of the β x were divided and analyzed. Under the same We range and inclined angle, the largest value of the maximum normalized droplet transverse spreading displacement (β x m a x) was obtained with the smoothest surface (0.3 μm) followed by the roughest surface (5.2 μm). β x m a x was affected by the surface roughness, which also influences the contact angle of the surface. When the surface roughness and the contact angle were regarded as two separate factors, it was found that for the transverse spreading, the lower the inclined angle, the more obvious effect of the surface roughness than that of the contact angle. [ABSTRACT FROM AUTHOR]

• The full spectrum of impact outcomes of droplet-particle collisions is identified. • A total of 225 collision cases are investigated by direct numerical simulation. • Detailed interaction mechanisms of seven impact outcome scenarios are presented. • Film thickness, maximum spreading diameter, and coating time are analyzed. We use direct numerical simulation based on the level contour reconstruction method to investigate head-on droplet collisions with dry, stationary, spherical particles. Three important impact parameters are varied over broad ranges: Weber number (4 ≤ We ≤ 150), surface wettability (20° ≤ θ eqi ≤ 160°), and droplet-to-particle-size ratio (1/3 ≤ Ω ≤ 2), leading to a total of 225 collision scenarios being examined. Seven collision outcomes are identified: deposition, partial rebound, complete rebound, complete coating, gravity disintegration, momentum disintegration, and splashing, and are represented in outcome regime maps. The first six outcomes are categorized into two cases: the partial-coating case (deposition, partial rebound, and complete rebound for larger particles) and the full-coating case (complete coating, gravity disintegration, and momentum disintegration for smaller particles), with a clear boundary separating the two cases in parameter space. The partial-coating case is characterized by maximum spreading and a subsequent recoiling that depends strongly on wettability. Conversely, the full-coating case involves complete initial wetting of the particle and the following dynamics is strongly governed by impact velocity. Detailed interaction mechanisms of each outcome scenario are also presented. Furthermore, three important quantitative parameters (i.e., film thickness, maximum spreading diameter, and coating time) are analyzed in detail. [ABSTRACT FROM AUTHOR]

This study uses an efficient, continuum-level modeling method to investigate the spreading and retraction dynamics of viscous droplets impact on flat solid surfaces. The main advantage of this method is that it employs the notion of disjoining pressure to avoid the specification of contact angle boundary in traditional modeling methods. Thus, the dynamic contact angle and contact line motion are derived more physically as a result of the combined action of capillary and disjoining pressure. Based on this modeling method, we systematically analyze the influence of droplet viscosity, impact velocity and material wettability on the dynamic characteristics of droplets. Multiple general correlations are developed to predict the spreading factor β during the kinematic phase, the maximum spreading factor β max , the minimum film thickness h c,min and the retraction rate ε ˙ , which are all in good agreement with previous theoretical models. In addition, this paper also discusses the applicability and limitations of these models. Finally, we have successfully proved that this modeling method can be reliably applied to the investigations of both droplet spreading and retraction process. [ABSTRACT FROM AUTHOR]