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This paper investigates the hydrothermal behavior of leaky dielectric and perfect dielectric droplets impacting a superheated wall within a specific range of Weber numbers (We≤30) $(We\le 30)$ under an electric field. Through this investigation, we aim to provide a more comprehensive understanding of the dynamics involved in droplet‐superheated surface interactions under electric fields, which can be useful in various applications, such as the design of cooling systems and combustion chambers. The study utilizes the level‐set and ghost fluid techniques to capture the interface accurately. Under an electric field, different behaviors are observed during the impact process, depending on the electrical properties of the droplet. A perfect dielectric droplet experiences a reduction in spreading extent and an increase in contact time. However, no remarkable enhancement in total heat removal occurs in this case. For the leaky dielectric droplet exhibiting prolate deformation at the stationary state, increasing the electric field magnitude results in a slight decrease in the droplet spreading extent, while the droplet contact time and total heat removal from the surface increase. At an electric capillary number of 1.55E − 2 and a Weber number of 25, the enhancement in the contact time and total heat removal is about 43% and 15%, respectively. For the leaky dielectric droplet with oblate deformation at the stationary state, the spreading extent and total heat removal increase, with negligible changes in contact time. At the above‐mentioned electric capillary and Weber numbers, the enhancement in the spreading extent and total heat removal is about 7.5% and 15%, respectively. [ABSTRACT FROM AUTHOR]

The simulation of droplet-wall impact process based on computational fluid dynamics (CFD) is of great significance for understanding the dynamic behavior of droplets spreading on the solid wall, and can provide technical support for the design of superhydrophobic structures and the development of anti-icing coating. The difficulty lies in how to accurately describe the evolution process of the contact line and the dynamic contact angle in the model. Herein, 4 typical dynamic contact angle models were summarized, and their application ranges were analyzed theoretically. With the UDF function in FLUENT the dynamic contact angle model was applied to the wall boundary conditions, and the dynamic process of droplet impact on smooth wall was numerically simulated. The quantitative analysis of the changes of droplet shape parameters and the comparison with the experimental results show that, the Seebergh dynamic contact angle model is more suitable for simulating the motion of droplets with lower capillary numbers. The Kistler model and the Jiang model are more widely used and can accurately describe the motions of droplets with higher capillary numbers. Then, based on the Kistler dynamic contact angle model, the impact and spreading processes of droplets on the microstructure surface were simulated. It is found that, the application of the dynamic contact angle model will lead to the change of the internal flow fields of droplets with the surface tension playing a dominant role, and the simulated droplet contact angle value in equilibrium is close to the theoretical value. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the three-dimensional dynamics of droplet impact on a thin liquid substrate using the lattice Boltzmann method (LBM). The LBM solver is employed to simulate the fluid dynamics of the droplet-substrate interaction and the subsequent formation of waves on the substrate surface. The study specifically focuses on the influence of Weber number on the collision dynamics between the droplet and substrate, as well as the resulting wave patterns. The simulation results provide insights into the behaviour of the droplet-substrate interface, including the formation of a 'neck' region and the fluid recirculation around it. Moreover, the study examines the impact of Weber's number on the amplitude and frequency of the resulting waves. This research holds significant implications for applications in microfluidic and microelectromechanical systems (MEMS*), where a thorough understanding of droplet impact dynamics is crucial for the design and optimisation of inkjet print heads and other droplet-based systems. By offering an in-depth understanding of the droplet-substrate interaction, this study contributes to the development of more efficient and precise droplet production systems. [ABSTRACT FROM AUTHOR]

The spreading behavior of droplets impacting solid surfaces is of significant importance in numerous industrial and medical applications. This study specifically focuses on the droplet impact behaviors of shear-thinning fluids. Extensive impact experiments were conducted using high-speed visualization techniques. Image sequences in the DI water and xanthan gum solution droplets impact process are captured at a We range of around 3.2–388 and Ren of 18–8145. The morphologies of spreading droplets on hydrophilic, moderate, and hydrophobic surfaces was observed with temporal evolution. The experimental results showed that the spreading behavior of shear-thinning droplets varies with the impact velocity, surface wettability, and fluid concentration. The droplets spread more widely and rapidly at larger We with the time evolution of the spreading factor. In addition, the spreading of the droplets becomes suppressed on a hydrophobic surface or with a higher xanthan concentration. With the increase of We, the effect of fluid viscosity gradually surpasses the surface wettability. The fluid viscosity with the shear-thinning properties has more influence on the spreading behavior at high We. Furthermore, the experimental data of the maximum spreading factor could be scaled with the We and Ren at low and high We, respectively. Finally, the maximum spreading of shear-thinning droplets was correlated with a universal rescaling model. The correlation shows good accordance with the experimental results of droplet impact from this study and the literature in a wide range of impact conditions and shear-thinning properties. [ABSTRACT FROM AUTHOR]

We consider the classical problem of droplet impact and droplet spread on a smooth surface in the case of an ideal inviscid fluid. We revisit the rim–lamella model of Roisman et al. (Roisman et al. 2002 Proc. R. Soc. Lond. A458, 1411–1430 (doi:10.1098/rspa.2001.0923)). This model comprises a system of ordinary differential equations (ODEs); we present a rigorous theoretical analysis of these ODEs, and derive upper and lower bounds for the maximum spreading radius. Both bounds possess a We1/2 scaling behaviour, and by a sandwich result, the spreading radius itself also possesses this scaling. We demonstrate rigorously that the rim–lamella model is self-consistent: once a rim forms, its height will invariably exceed that of the lamella. We introduce a rational procedure to obtain initial conditions for the rim–lamella model. Our approach to solving the rim–lamella model gives predictions for the maximum droplet spread that are in close agreement with existing experimental studies and direct numerical simulations. [ABSTRACT FROM AUTHOR]

This paper proposes an improved cumulant collision model for the pseudo-potential lattice Boltzmann method (LBM) to increase the stability of multiphase flow simulations involving low viscosities. This model is based on the work of Kharmiani et al. in (J Stat Phys 175: 47, 2019), which can be extended regardless of the collision model. The original cumulant collision model (Geier et al. in Comput Math Appl 70:507, 2015) causes a non-physical shape of droplets in pseudo-potential LBM because only the first-order central moments are considered in the forcing scheme. The improved cumulant collision model proposed in this paper applies the central moment forcing scheme to the original cumulant model to cover the high-order central moments. Several numerical simulations were carried out to validate the proposed model. First, the problem of a stationary liquid layer was solved, where the proposed model was demonstrated to be thermodynamically consistent. Second, the problem of a stationary droplet was solved, where the result agreed well with Laplace’s law. Third, the problem of a droplet impact on a liquid film was solved, where the crown radius agreed well with the analytical and numerical results available. Fourth, the simulation results carried out with the raw moment, central moment, and the proposed improved cumulant collision models were compared, as the liquid and vapor viscosities were gradually lowered. With all else being equal, only the lattice Boltzmann method with the proposed improved cumulant collision model was able to successfully simulate a density ratio of 720 and a Reynolds number of 8.7 × 10 4 . [ABSTRACT FROM AUTHOR]

Droplet impact is a common occurrence in nature, agriculture, and industry. The research on the multi‐droplet impact is fundamental to understanding the tangled nature of reality. This paper numerically studies the successive oblique impact of dual droplets on the liquid film by building an effective three‐dimensional model. The leading and trailing droplets are set to pass a certain impact point with the same velocity. The main contribution of this paper is the investigation of the effects of Weber number, liquid film thickness, impact angle, and impact time interval on the interface morphology evolution after the droplet impact. Results show that splash pattern conversion of the primary or secondary crown occurs with the change of these factors. Besides, the variations of the maximum crater radius in upstream, lateral, and downstream directions with time are quantitatively analyzed. The crater radius analysis is carried out from three perspectives, the variation during the single droplet impact, the change during the dual droplets impact, and the comparison between them. It is found that the crater of dual droplets impact exhibits shape distortion in the deformation period and appears a marked dimensional increase in the secondary expansion period. [ABSTRACT FROM AUTHOR]

Recent experiments have demonstrated an extremely high output voltage of 260 V by impinging droplets on an electret surface with top and back electrodes. However, the mechanism underlying the generated high voltage remains elusive. Through high‐speed imaging of the impinging water droplet, it is found that the recorded voltage is proportional to the change rate of contact length l between the droplet and the top electrode over time, that is, dl/dt. By combining an electrostatic model with multi‐physics simulations, the time‐dependent charge distribution on the top electrode is analytically described as a function of several key factors pertaining to the spreading dynamics of droplet and the geometrical dimensions of device. These results show that the high voltage originates from the accumulation of charge in the spreading droplet and the ensuing instantaneous discharge upon high‐speed contact with the top electrode. Using the model‐optimized parameters, the experimentally generated voltage from a falling 64 µL droplet with a 2 mm salt can be boosted to 1030 V, giving rise to an energy conversion efficiency as high as 3.3%. This conversion efficiency will further increase by 14 times if the surface charge density of the electret can be enhanced to its limit using state‐of‐the‐art fabrication techniques. [ABSTRACT FROM AUTHOR]

Micro/nanostructured surfaces with extreme wettability, such as superhydrophobic surfaces, are useful for various applications. Herein, we used Ar plasma to form polydimethylsiloxane films while immobilising SiO2 nanoparticles, which were prepared on polyester fabric surfaces under mechanical stretch–release to obtain micro/nanostructures. The treated fabrics exhibited a large contact angle of 169.06° with small sliding angle of 5°. We developed corresponding micro/nanostructure models to establish the relationships between the structural parameters and the hydrostatic pressure and water-droplet bouncing. The model is useful for guiding the regulation of water pressure. [ABSTRACT FROM AUTHOR]

The spreading and splashing behavior of high-velocity metal liquid droplets upon impact with a substrate is a fundamental scientific concern in thermal spray and additive manufacturing for metal deposition. A thorough understanding of the spreading and splashing tendencies of individual liquid droplets, along with the factors influencing them, is crucial for improving the quality of sprayed coatings and additively manufactured components. In this research, high-speed photography was employed to document the propagation process, splashing events, and alterations in droplet structure following the impact of tin droplets onto stainless steel surfaces at varying velocities. The study unveiled that as the impact velocity rises, the droplet's dynamics undergo a series of transformations, encompassing retraction, spreading, and splashing, accompanied by dynamic fluctuations in the contact angle. Elevated impact velocities result in an augmentation of the droplet's maximum spreading diameter, coupled with a simultaneous reduction in spreading duration. Splashing occurs at relatively higher speeds and near the maximum forward contact angle. The paper verified a splashing prediction model based on the pressure balance of the liquid film and boundary conditions, successfully predicting the onset of droplet splashing. The results of the experiments can be applied to advanced technologies in thermal spray and additive manufacturing for metal deposition. [ABSTRACT FROM AUTHOR]

Featured Application: This work compares the impact dynamics of ethanol/propanol and ethanol/water droplets on the heated plate and a novel unique bubble shrinkage phenomenon is observed and analyzed for ethanol/water droplets. This work aims to investigate the effect of liquid physical properties on the behavior of binary droplets impact on the heated smooth aluminum alloy plate with a high-speed imaging system. Two groups of mixed solutions with similar boiling point differences are selected as the working liquid, in which the low-boiling-point components are both ethanol and the high-boiling point components are propanol and water, respectively. Compared to the ethanol/propanol binary droplets, the experimental results show that the ethanol/water binary droplets have diverse impact phenomena and significantly broad transition boiling regimes, as well as the reduced droplet residence time and increased Leidenfrost temperature point. With the decreasing ethanol content in ethanol/water binary droplets, these effects become more prominent. For secondary atomization, the ethanol/water binary droplet undergoes parent droplet breakup into fragment droplets with larger diameters (Ds > 0.3 mm). Both binary droplets produce satellite droplets with small diameters (Ds < 0.3 mm) by puffing and ejection. In terms of the ethanol/propanol binary droplet impact, the probability of puffing is higher and the satellite droplet diameters are small. In the ethanol/water binary droplet impact, the probability of ejection is higher and the satellite droplet diameter distribution is wider. When an ethanol/water binary droplet of 25 vol.% ethanol content impacts the heated wall at Ts = 120 °C, a novel large bubble shrinkage phenomenon occurs at the late stage of droplet evaporation. This phenomenon is proposed to be relevant to the increasing surface tension and saturation temperature with decreasing ethanol content, as well as the decreasing ambient temperature above the top surface of the bubble. [ABSTRACT FROM AUTHOR]

Constructing microstructures on smooth surfaces is significant for 3D printing technology and surface self-cleaning effects in engineering applications. This paper uses the lattice Boltzmann method (LBM) to study the droplet impact behavior on different microstructured surfaces: right-triangle, semicircular, and rectangular microstructures. The formulas for calculating the surface wetting characteristics of different microstructures are derived theoretically, the difference between simulation results and those from theoretical analyses is less than 5 %. The increase of the microstructure column height h may eventually lead to the disappearance of the Wenzel mode. The droplet impact on the microstructured hydrophilic surface can accelerate the droplet to the stable stage and reduce the excessive impact effect. The droplet impacts on the superhydrophobic microstructured surface can accelerate the rebound separation and shorten the time to reach the stable stage. When droplets impact the superhydrophobic surface of the right-triangle microstructure, a secondary separation phenomenon will occur, resulting in the maximum rebound height and the maximum reduction of the near-wall density. The superhydrophobic right-triangle microstructured surface can more effectively promote self-cleaning.

This study investigates the dynamic propagation of solution droplets in frost-free air source heat pump systems, focusing on calcium chloride and ethylene glycol solutions. The research examines three key stages: droplet impact on a surface, collision with stationary droplets, and impact on a stabilized liquid film. Key findings include that increased surface contact angle and solution concentration inhibit droplet spreading, while higher impact velocity enhances it. Droplet retraction is influenced by surface tension and adhesion, with calcium chloride droplets showing variable retraction and ethylene glycol droplets consistently retracting less as concentration increases. Collisions with stationary droplets lead to behaviors such as merging, spreading, and rebound, with higher velocity accelerating these processes. In the final impact stage, a stable liquid film formation results in crown sputtering and oscillation, where sputtering height is inversely related to concentration but increases with impact velocity. These insights contribute to optimizing droplet management in air treatment systems.

In order to study the effect of the cold surface on the anti-frosting performance of the superhydrophobic coating and the impact contact behavior of droplets, a fluorine-based self-assembled superhydrophobic coating was sprayed on the surface of carbon fibers. Subsequently, the frosting, droplet icing and droplet contact behaviors of the sample on a semiconductor refrigeration table were observed visually, and the variation mechanisms of frost formation, droplet icing and droplet impact contact behaviors over time on the surface of superhydrophobic coatings were analyzed. Results showed that unlike ordinary surfaces, condensation droplets on superhydrophobic surfaces did not freeze at-5 ℃. At temperatures of-10,-15 ℃, the frost suppression effect of superhydrophobic surfaces was significantly better than that of ordinary surfaces, with frost formation times delayed by 34 s and 22 s, respectively. Besides, the delayed freezing times of droplets on superhydrophobic surfaces at-5,-10,-15 ℃ were 16 times, 10 times and 9 times longer than those on ordinary surfaces, respectively. The maximum spreading diameter of droplets after colliding with low-temperature superhydrophobic surfaces was the same at different temperatures. The droplet partially rebounded at-5 ℃ and-10 ℃, and rebounded 11 times on the superhydrophobic surface at 25 ℃.

Spraying dust suppression liquid on the coal surface is a common dust suppression method for coal-related industrial enterprises, and the wetting process of dust suppression droplets impacting the coal surface is of great significance to the dust suppression effect. In order to study the dynamic wetting process of dust suppression droplets on the coal surface and further evaluate the wetting performance of droplets, the index of dynamic wettability during droplets impacting on the coal surface is proposed. The dynamic wetting process of droplets on the coal surface is investigated based on CLSVOF numerical simulation method. The wetting performance in the process is evaluated by using the dimensionless wetting length and the dimensionless wetting area, and the theoretical expressions of the maximum dimensionless wetting length and the maximum dimensionless wetting area are derived. At the same time, they are compared with the wettability evaluation ability of surface tension. The results show that the CLSVOF method can better simulate the impact and dynamic wetting process of droplets on the coal surface, and can be used to evaluate the dynamic wetting ability of different droplets on the coal surface under different conditions. The droplet drops on the coal surface, and first spreads as a disk. When the maximum spreading is reached, there is a velocity difference between the edge and the center of the disk droplet. When the velocity difference is small, the droplet retracts and coalesces into a sphere. When the velocity difference is large, the droplet breaks and forms many tiny droplets. And the larger the droplet size is, the earlier the breakage behavior appears and the more obvious the phenomenon is. Once the droplet breaks on the coal surface, the dimensionless wetting length and the dimensionless wetting area do not maintain the consistent trend, and the evaluation ability of the dimensionless wetting area is better. When the droplet size is the same, the larger the We number of droplet impacting the coal surface, the better the wetting ability. Increasing the droplet size at the same velocity has less effect on its wetting ability. Compared with surface tension, the dimensionless wetting length and dimensionless wetting area have better ability to evaluate wettability of droplets, which reflects the potential application value in the evaluation and optimization wetting agents.

Experimental study on impact phenomena, bouncing time, maximum spreading time, maximum spreading ratio, maximum elongation, and restitution coefficient for a water-based liquid marble impinging on hydrophilic surface. [Display omitted] Liquid marbles are droplets coated by hydrophobic particles. At low Weber numbers (We), when impacting a hydrophilic surface, the marble may bounce on the substrate repeatedly without any rupturing until the quiescence condition is achieved. The marble bouncing has gained far less attention, although its rich underlying physics is due to the interaction between liquid core, hydrophobic grain, and surrounding air. Accordingly, this research experimentally scrutinizes the marble impact and subsequent bouncing on a hydrophilic surface for the first time. Additionally, the conversion of kinetic, gravitational potential, inertial, and surface energies occurring regularly during the impact is exhaustively surveyed. Moreover, the effect of Weber and gravitational Bond numbers (Bo) on the bouncing time, maximum spreading time, maximum spreading ratio, maximum elongation ratio, and maximum restitution are investigated, which characterize the marble impact and bouncing dynamics. This study is one of the limited investigations exploring the effects of the gravitational Bond number on the results. Dimensionless correlations are proposed for the mentioned parameters based on the experimental data. Furthermore, utilizing the simplifying theoretical presumptions, correlations are suggested based on the scale analysis for the spreading time and maximum spreading ratio. The results imply that the mentioned parameters behave differently at low and moderate Weber numbers, though the distinction is more pronounced in the case of the bouncing time, maximum spreading time and maximum spreading ratio. Although increasing with the Weber number when W e < W e c r , the restitution coefficient decreases with the We number for W e > W e c r . In addition, the maximum elongation ratio linearly grows with the Weber number. [ABSTRACT FROM AUTHOR]

An analytical model for droplet impact onto a porous substrate is presented, based on Wagner theory. An idealised substrate boundary condition is introduced, mimicking the effect of fluid entry into a genuinely porous substrate. The asymptotic analysis yields a solution for a small porous correction with free-surfaces and pressures compared with the impermeable case. On a global scale, it is found that the impact region on the substrate grows more slowly with porosity included due to loss of mass into the substrate. The spatial distribution of liquid volume flux into the substrate is also described. Locally near the turn-over regions, the expected jetting along the surface is calculated with the same volume flux but the jet is found to be slower and thicker than for an impermeable substrate. [ABSTRACT FROM AUTHOR]

Spreading time, the time that an impacting droplet attains the maximum wetting area on a solid surface, plays a critical role in many engineering applications particularly where heat transfer or chemical reactions are involved. Although the impact dynamics of a droplet significantly differ across the different spreading regimes depending on various collision parameters, it still remains unclear how the spreading time changes for each spreading regime. In the present study, the spreading time during droplet impact on a large spherical target is systematically studied at the three different spreading regimes for a wide range of impact parameters (Weber number, equilibrium contact angle, and Ohnesorge number). The changes of spreading time depending on the impact parameters and underlying physical mechanisms are analyzed in detail at the level of three different spreading regimes. Our results show that the spreading time, proper time scales, dominant impact parameters and associated physical behaviors all significantly and non-linearly change across the three spreading regimes. An improved prediction model for the spreading time is also proposed for each regime, which is now based on only the controllable variables and has an explicit form. Finally, a data-driven prediction model is proposed to represent the complicated and non-linear nature of the spreading time broadly across the three spreading regimes. [ABSTRACT FROM AUTHOR]

In recent years, the pseudopotential multiphase lattice Boltzmann method (LBM) has been widely applied in the numerical simulations of complex multiphase flow. However, numerical stability at high Weber (We) and Reynolds (Re) numbers seriously restricts its application. In the present work, the pseudopotential multiphase LBM is developed by introducing the entropic stabilizers to the cascaded collision operator, and thus, the numerical stability is effectively enhanced. The relaxation rates for different order moments are all determined by fluid viscosities and the maximum entropy condition instead of being user-defined. Furthermore, the surface tension and contact angle can be adjusted in a wide range, which extends the application of the pseudopotential multiphase model. The accuracy and capability of the present model to simulate the complex multiphase flow are validated by reproducing the droplet impact on the flat or textured superhydrophobic surfaces. In addition, a droplet impact on the superhydrophobic surface at We = 1280 and Re = 18000 is simulated, in which the Weber and Reynolds numbers have significantly increased. This strongly demonstrates that the present model can enhance the numerical stability in the simulation of large density ratio multiphase flows at simultaneously high Weber and Reynolds numbers, which means the present model can further extend the pseudopotential multiphase LBM to more realistic scientific and technical applications. [ABSTRACT FROM AUTHOR]

Experimental research is done to determine how the lubricant-infused zinc oxide (ZnO) textures affect the splashing characteristics of droplets. The heat evaporation and photolithography methods are used to create the ZnO-textured surfaces with empty air pockets. When determining critical Weber numbers (Wecr) for droplet splashing, the surface properties of ZnO-textured surfaces and the viscosities of the lubricants are taken into account. In relation to lubricant viscosities injected on ZnO textures, the Wecr dropped. On lubricant-infused surface's (LIS's) the overall Wecr is greater than that of dry ZnO textured surfaces. In comparison to low-viscosity lubricants, the adhesive force has a significant influence in delaying the droplet splash behavior. Results indicate that droplet splashing is caused by an empty air pockets on ZnO surfaces because of the air's existence and the material's weak adherence. While LIS's prevent droplet splashing because of enclosed pockets, excellent adhesion, and the absence of an air layer. Adhesion, therefore, is crucial in minimising droplet splash on surfaces that have been impregnated with lubricant. [ABSTRACT FROM AUTHOR]

A comprehensive three-dimensional computational analysis is undertaken to track the droplet dynamics along with the heat transfer characteristics throughout the spreading and recoiling phases. Notably, the simulation results are compared within the frameworks of the static contact angle (SCA) and dynamic contact angle (DCA) models. The study uses the volume of fluid (VOF) technique within the ANSYS Fluent platform, incorporating the dynamic contact angle model. The simulation outcomes exhibit a reasonable degree of agreement with experimental results, both in quantitative and qualitative terms. The SCA model closely approximates the DCA model during the initial spreading phase, but, as the process progresses, the results based on the SCA model significantly deviate from the results of the DCA model as well as from the experimental observations. The presence of air trapped between the droplet and the solid surface acts as a barrier, impeding the heat transfer from the droplet to the surface. The heat flux attains the global maxima about the triple phase contact line region. [ABSTRACT FROM AUTHOR]

[Display omitted] Despite the flourishing studies of droplet interfacial boiling, the boiling upon vibratory surfaces, which may cause vigorous liquid–vapor-solid interactions, has rarely been investigated. Enhanced boiling normally can be gained from rapid removal of vapor and disturbance of liquid–vapor interface. We hypothesize that the vibratory surfaces enhance both effects with new intriguing phenomena and thus, attain an enhanced boiling heat transfer. We experimentally investigated the impacting fluid dynamics and coupled heat transfer patterns of multiple droplets and a single droplet impinging on still and vibratory surfaces of various materials and different wettability. The boiling under vibratory surfaces with increased vibration velocity amplitude and enhanced wettability can be enhanced by 80% in heat transfer coefficient and Nusselt number, which is attributed to several reasons: shortened bubble lifespan, thinner and smaller bubbles, and enhanced disturbances in liquid–vapor interfaces. The vibration also delays the Leidenfrost point when the droplet impacts a descending surface, which shows that the droplet impact moment (vibration phase angle) is particularly crucial. The descending surface releases the generated vapor actively and facilitates liquid–solid contact, thereby delaying the Leidenfrost. From fundamentals to application, this article strengthens our understanding of vibrated interfacial boiling in scenarios closer to multiple natural processes and practical industries. [ABSTRACT FROM AUTHOR]

[Display omitted] The solid fraction of the substrate is expected to influence the bouncing behavior of an impinging droplet, thereby affecting spreading and contact time. Hence, it should be possible to alter the velocity and pressure distribution of impacting droplet, and also affect the impact velocity for droplet penetration right upon impact. We systematically investigate the impact dynamics of water droplets on pillared hydrophobic surfaces with different solid fractions using phase-field simulations. The velocity and pressure distributions of impacting droplets on pillared hydrophobic surfaces with varied Weber numbers and solid fractions are studied. In addition, the influences of the solid fraction on the bouncing behaviors of the impinging droplet, such as the maximum wetting spreading, the maximum impacting depth, and the contact time, are also investigated to further understand the impact event. We show that a three-peak pressure profile appears on the top of the pillared hydrophobic surface during droplet impact by varying the solid fraction of the surface. The first peak is generated by the impact of the droplet itself, while the second peak arises from the droplet recoil impact associated with the dynamic properties of the jet. Moreover, we identify a hitherto unknown third pressure peak related to the hydrodynamic singularity that emerges due to the convergence of the fluid during the droplet rebound. This solid fraction-dependent impacting behavior reveals the intricate interplay between droplet dynamics and the underlying surface characteristics, providing valuable insights into the design and optimization of micro/nano structured hydrophobic surfaces for various applications. [ABSTRACT FROM AUTHOR]

Adhesion of contaminants on various polymer‐based devices during fluid‐substrate interactions is a common problem that can cause biofouling and corrosion. In this study, hierarchical structures with submicron features on polypropylene (PP), high‐density polyethylene (HDPE), and polycarbonate (PC) are fabricated by femtosecond laser ablation. The effect of the hierarchical structures on surface wettability, droplet impact, and bacterial attachment has been examined. Our results demonstrate that the structured polymeric substrates facilitate large contact angles and minimal interfacial adhesion, allowing droplets to roll off at a low angle of inclination below 5°. Further, rendering the hierarchical structures with a low‐surface‐energy coating can enable the surfaces to exhibit superamphiphobic properties. The low interfacial adhesion properties, as accounted by the large contact angles and small contact angle hysteresis, of such surfaces prevent bacterial attachment and biofilm formation. The findings provide a design principle for creating affordable biofouling resistant surfaces with a submicron topography that can be used for engineering and biomedical devices. [ABSTRACT FROM AUTHOR]

The phenomenon of droplets impacting droplets is common in many fields including the chemical, nuclear, and aerospace industries. In this paper, high-speed photography technology is used to obtain the variation law and evolution properties exhibited by droplets colliding with sessile droplets of varying sizes. We further explored how the Weber number (We) and volume ratio (Vp/Vi) influence the behavior of droplets colliding with sessile droplets. The phenomenon of droplets impacting sessile droplets of different volumes is different from that of droplets impacting liquid films. In terms of droplet spreading, compression and the non-splashing liquid crown, the phenomena and laws reported in the present study are applicable for 1 ≤ We ≤ 165 and droplet volume ratios of 1 ≤ Vp/Vi ≤ 6. With a low Weber number, the droplet compresses and deforms downward without coalescence at the initial stage of collision. A high Weber number results in a no-splashing liquid crown. These findings provide important insights into the dynamics of droplet-droplet interactions.

The phenomenon of droplets impacting droplets is common in many fields including the chemical, nuclear, and aerospace industries. In this paper, highspeed photography technology is used to obtain the variation law and evolution properties exhibited by droplets colliding with sessile droplets of varying sizes. We further explored how the Weber number (We) and volume ratio (Vp/Vi) influence the behavior of droplets colliding with sessile droplets. The phenomenon of droplets impacting sessile droplets of different volumes is different from that of droplets impacting liquid films. In terms of droplet spreading, compression and the non-splashing liquid crown, the phenomena and laws reported in the present study are applicable for 1 ≤ We ≤ 165 and droplet volume ratios of 1 ≤ Vp/Vi ≤ 6. With a low Weber number, the droplet compresses and deforms downward without coalescence at the initial stage of collision. A high Weber number results in a no-splashing liquid crown. These findings provide important insights into the dynamics of droplet-droplet interactions. [ABSTRACT FROM AUTHOR]

Robust superhydrophobic surfaces with excellent capacities of repelling water and anti-frosting are of importance for many mechanical components. In this work, wear-resistant superhydrophobic surfaces were fabricated by curing a mixture of polyurethane acrylate (PUA) coating and 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (HFTCS) on titanium alloy (TC4) surfaces decorated with micropillars pattern, thus, composite functional surfaces with PUA coating in the valleys around the micropillars pattern of TC4 were achieved. Apparent contact angle on fabricated surfaces could reach 167°. Influences of the geometric parameters of micropillars pattern on the apparent contact angle were investigated, and the corresponding wear-resistant property was compared. Droplet impact and anti-frosting performances on the prepared surfaces were highlighted. An optimized design of surface texture with robust superhydrophobicity, controllable droplet impact, and anti-frosting performances was proposed. This design principle is of promising prospects for fabricating superhydrophobic surfaces in traditional mechanical systems. [ABSTRACT FROM AUTHOR]

When making contact with an undercooled target, a drop freezes. The colder the target is, the more rapid the freezing is supposed to be. In this research, we explore the impact of droplets on cold granular material. As the undercooling degree increases, the bulk freezing of the droplet is delayed by at least an order of magnitude. The postponement of the overall solidification is accompanied by substantial changes in dynamics, including the spreading-retraction process, satellite drop generation, and cratering in the target. The solidification of the wetted pores in the granular target primarily causes these effects. The freezing process over the pore dimension occurs rapidly enough to match the characteristic timescales of impact dynamics at moderate undercooling degrees. As a result, the hydrophilic impact appears “hydrophobic,” and the dimension of the solidified droplet shrinks. A monolayer of cold grains on a surface can reproduce these consequences. Our research presents a potential approach to regulate solidified morphology for subfreezing drop impacts. It additionally sheds light on the impact scenario of strong coupling between the dynamics and solidification. [ABSTRACT FROM AUTHOR]

Airtanker firefighting is the most spectacular tool used to fight wildland fires. However, it employs a rudimentary large-scale spraying technology operating at a high speed and a long distance from the target. This review gives an overview of the fluid dynamics processes that govern this practice, which are characterized by rich and varied physical phenomena. The liquid column penetration in the air, its large-scale fragmentation, and an intense surface atomization give shape to the rainfall produced by the airtanker and the deposition of the final product on the ground. The cloud dynamics is controlled by droplet breakup, evaporation, and wind dispersion. The process of liquid deposition onto the forest canopy is full of open questions of great interest for rainfall retention in vegetation. Of major importance, but still requiring investigation, is the role of the complex non-Newtonian viscoelastic and shear-thinning behavior of the retardant dropped to stop the fire propagation. The review describes the need for future research devoted to the subject. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract Adhesion of contaminants on various polymer‐based devices during fluid‐substrate interactions is a common problem that can cause biofouling and corrosion. In this study, hierarchical structures with submicron features on polypropylene (PP), high‐density polyethylene (HDPE), and polycarbonate (PC) are fabricated by femtosecond laser ablation. The effect of the hierarchical structures on surface wettability, droplet impact, and bacterial attachment has been examined. Our results demonstrate that the structured polymeric substrates facilitate large contact angles and minimal interfacial adhesion, allowing droplets to roll off at a low angle of inclination below 5°. Further, rendering the hierarchical structures with a low‐surface‐energy coating can enable the surfaces to exhibit superamphiphobic properties. The low interfacial adhesion properties, as accounted by the large contact angles and small contact angle hysteresis, of such surfaces prevent bacterial attachment and biofilm formation. The findings provide a design principle for creating affordable biofouling resistant surfaces with a submicron topography that can be used for engineering and biomedical devices.

Abstract Robust superhydrophobic surfaces with excellent capacities of repelling water and anti-frosting are of importance for many mechanical components. In this work, wear-resistant superhydrophobic surfaces were fabricated by curing a mixture of polyurethane acrylate (PUA) coating and 1H,1H,2H,2H-Perfluorodecyltrichlorosilane (HFTCS) on titanium alloy (TC4) surfaces decorated with micropillars pattern, thus, composite functional surfaces with PUA coating in the valleys around the micropillars pattern of TC4 were achieved. Apparent contact angle on fabricated surfaces could reach 167°. Influences of the geometric parameters of micropillars pattern on the apparent contact angle were investigated, and the corresponding wear-resistant property was compared. Droplet impact and anti-frosting performances on the prepared surfaces were highlighted. An optimized design of surface texture with robust superhydrophobicity, controllable droplet impact, and anti-frosting performances was proposed. This design principle is of promising prospects for fabricating superhydrophobic surfaces in traditional mechanical systems.

Spray impacts can be found in several technical applications and consist of many single droplets, which impact under different trajectories on wetted walls. This study investigates the asymmetric crown morphology resulting from an oblique impact ( α = 60°) of a single droplet on a horizontal and quiescent wall film of the same liquid. A droplet generator with an accelerated needle releases the droplets ( D = 1.5 mm) in a controlled trajectory on a thin film ( h f / D = 0.2). The impact process is recorded from two perspectives with two synchronized high-speed cameras. Varying the Weber number within the splashing regime reveals distinct crown morphologies, which are described in detail. For W e < 500, a single central finger develops at the front of the crown, with subsequent detachments of secondary droplets. At higher W e (>500), a collision of the crown with the wall film shortly after impact introduces disturbances into the rim, leading to two fingers in the middle of the front crown. A further increase in W e (>600) intensifies the crown–film interaction, resulting in an early ejection of tiny droplets and a complete breakup of the front rim. The influence of W e on the crown morphology during an oblique impact is also compared to the normal impact (90°). This study paves the way for a classification of impact regimes and a comprehensive picture of the oblique impact process, which deserve more investigation. [ABSTRACT FROM AUTHOR]

Icing detection is the premise and basis for the operation of aircraft icing protection system, and is the primary issue in flight safety assurance. At present, there is a lack of research methods and design reference for the layout optimization of ice detectors. Therefore, in order to simulate the real icing environment encountered by the aircraft more accurately, a large-scale icing wind tunnel was used to carry out experimental research on the icing characteristics of the sensor probes. A closed-loop experimental method including the typical condition selection, sensor array interference examination and ice shape repeatability verification was initially proposed. A stepwise optimization process and a sensitivity analysis on ambient conditions were combined to determine the optimal distribution for sensor installation. It is found that the water collection coefficient on the cylinder surface of the probe first increases and then decreases along the axial direction, reaching the extreme value at a certain position. The height of this extreme point will gradually increase with the development of the wall boundary layer, showing a variation range of 2~30 mm. Improper design may cause the sensor probe to fail to capture the point with the maximum icing thickness, affecting the sensitivity of icing detection. In addition, each probe position has different sensitivity to changes in flow parameters; the points with larger icing mass and lower sensitivity to changes in attack angle will have better detection effect. The measured data and analysis in the present work can provide a basis for the accurate design of icing sensor probes. [ABSTRACT FROM AUTHOR]

The investigation of the underlying physical processes involved in the impact of droplets has various practical applications in engineering and science. In this research, the spreading velocities within droplet impingement on a sapphire glass were investigated for a wide range of Weber numbers using particle image velocimetry (PIV), which involves tracking the movement of polymeric fluorescent particles (6 µm) within the droplet. The experiments were carried out at room temperature, and the droplets had impact velocities ranging from 0.41 to 2.37 m/s, which corresponded to Weber numbers of 5–183. The results showed that the radial velocity was generally linear over a wide range of spreading radius but the velocity at the exterior radial positions became nonlinear over time due to the influence of capillary and viscous forces. This nonlinearity was more pronounced for lower Weber numbers because the viscosity effects in the droplet were more significant compared to the inertia forces. As the Weber number decreases, the spreading and receding of the droplets are completed faster, leading to different trends in the radial velocity profiles. [ABSTRACT FROM AUTHOR]

An experimental visualization is undertaken to investigate the impact dynamic behaviors of water, absolute ethanol, and low surface energy droplets with different viscosities impacting on hydrophobic surfaces. Droplets' impacting behaviors, including spreading, rebounding, and oscillation retraction, are observed and quantitatively characterized by transient spreading factor and maximum spreading diameter. Effects of droplet impact velocity, surface wettability, and droplet viscosity on the impact dynamics are explored and analyzed. As the droplet impact velocity increases, the droplet kinetic energy increases, resulting in an increase in the spreading factor and spreading velocity simultaneously. Hydrophobic surfaces are not easy to be wetted by water droplets due to their low surface energy, leading to the partial rebound of water droplets when impacting on the hydrophobic surfaces. However, this phenomenon does not occur when low surface energy droplets, such as absolute ethanol and simethicone, impact on hydrophobic surfaces at the same velocity. The increasing droplet viscosity enhances the viscous dissipation, slowing down the impact process and inhibiting the droplet spreading, oscillation, and retraction behaviors. Based on the energy conservation method, a universal model for the maximum spreading factor of low surface energy droplets with different viscosities impacting on hydrophobic surface was established. According to the experimental results, a new spreading time model tm = 2D0/U0 was proposed to enhance applicability of the model for low surface energy droplets with high viscosity, reducing the calculation error to less than 10%. [ABSTRACT FROM AUTHOR]