Search

Coalescence-induced droplet jumping holds great potential for applications such as water harvesting, self-cleaning, and the thermal management of electronic devices. This study investigates the impact of the droplet's initial position on coalescence-induced jumping on superhydrophobic surfaces with micropillar arrays. Numerical simulations are conducted to examine the differences in droplet jumping at various initial positions with uniform and nonuniform micropillar distributions, and the effects of the droplet's initial position on its velocity and pressure distribution are analyzed. The findings indicate that altering the initial position produces an asymmetric distribution of the high-pressure region at the droplet's base and the pressure at the solid–liquid contact surface during the contraction of the liquid bridge. This asymmetry allows the droplet to jump away from the surface with both vertical and horizontal components of velocity, and to be transported in the horizontal direction. Furthermore, numerical simulations with various initial droplet positions and radii demonstrate that the direction of the horizontal jumping velocity is influenced by both the offset distance and the direction of the initial droplet position, and that the impact of the initial position decreases as the relative size of the droplet with respect to the micropillars increases. The droplet jumping velocity, direction, and horizontal transport distance can be controlled by adjusting the initial droplet position and size. This work reveals the mechanism of coalescence-induced droplet jumping on superhydrophobic surfaces with micropillar arrays and provides an important reference for practical applications. [ABSTRACT FROM AUTHOR]

This document is a summary of a special issue on superhydrophobic surfaces published in the journal Applied Physics Letters. Superhydrophobic surfaces are engineered to repel water efficiently, and they have various applications such as anti-icing, self-cleaning, and liquid manipulation. The special issue includes research on topics such as the capture of air by superhydrophobic materials, droplet evaporation on these surfaces, the development of durable superhydrophobic surfaces, and the expansion of their functionality through macrostructures and surface wettability patterns. The issue also covers the dynamic interactions between liquid droplets and solid surfaces, the stability and durability of superhydrophobicity, and the integration of superhydrophobic properties with other effects to create multifunctional surfaces. Overall, the special issue provides a comprehensive overview of the current research and potential applications of superhydrophobic surfaces. [Extracted from the article]

Coalescence-induced droplet jumping on superhydrophobic surfaces has significant potential for various applications, such as enhanced condensation, water harvesting, self-cleaning, and thermal management of electronic devices. Existing research focuses primarily on the coalescence and jumping of two or more droplets on the horizontally placed superhydrophobic surface, but the transport behavior of condensate droplets caused by coalescence-induced jumping on the inclined superhydrophobic surface is insufficiently understood. In this work, we investigate the impact of the surface inclination angle on the dynamic behavior of condensate droplets and derive the relationship between the transport distance, transport height, jumping direction of droplets, and the surface inclination angle. Additionally, we establish a droplet motion equation to describe the trajectory of droplets after they jump off the surface. Furthermore, we demonstrate the phenomenon of multi-hop jumping on the inclined superhydrophobic surface, which can increase the droplet transport distance, and the established mathematical model can better predict the multi-hop jumping transport distance. This work exposes the effect of superhydrophobic surface inclination angle on droplet dynamics and droplet directional transport performance, which has significant implications for the application of droplet jumping. [ABSTRACT FROM AUTHOR]

Superhydrophobic surfaces decorated with macrostructures have presented remarkable potential in diverse engineering fields, such as aircraft anti-icing. Understanding the effects of the structure shape and size on droplet dynamics is crucial to the design and application of surfaces. Herein, we investigate the maximum axial spreading for droplets impacting on ridged superhydrophobic surfaces with varied ridge shapes and sizes. We propose a mathematical formula to describe the structure shape with profiles quantified by the shape factor, which is easily applied to structure-related studies. The effects of ridge shape and size on the maximum axial spreading coefficient are clarified. The axial spreading of droplets is inhibited by the ridge due to the outward flow of liquid above the ridge tip. The maximum axial spreading coefficient reduces when the ridge becomes sharper, which can be achieved by increasing the shape factor or the ridge height–width ratio. The complex effect of the ridge–droplet size ratio is divided into two regimes according to the shape factor. Furthermore, a prediction correlation of the maximum axial spreading coefficient is established, which involves the coupled effects of all parameters, agreeing well with experimental and simulation results. [ABSTRACT FROM AUTHOR]

Superhydrophobic surfaces have shown great application prospects due to their excellent water repellency in many applications involving fluid–surface interactions. As a ubiquitous fluid–surface interaction phenomenon, droplet impacting dynamics has a crucial effect on the application of superhydrophobic surfaces. In this Perspective, we summarize the basic process of droplet impacting on superhydrophobic surfaces and introduce the two most concerned parameters that describe the droplet impacting dynamics, i.e., the maximum spreading coefficient and the contact time. We then review two improvement strategies for superhydrophobic surfaces: one is to construct macrostructures and the other is to set wettability patterns on the surface. The former strategy shows great potential in reducing the droplet contact time, and the latter one can accurately regulate the behavior of impacting droplets. The motion of superhydrophobic surfaces also changes the droplet impacting dynamics due to the additional aerodynamic effect or energy input, which arouses attention recently. However, only the individual influence of each factor (e.g., macrostructures, wettability patterns, or surface motion) on the droplet impacting dynamics has been focused in literature, so we write this Perspective to emphasize the importance and urgency of studying the coupled effects of these three factors. [ABSTRACT FROM AUTHOR]

Enhancing the horizontal mobility of coalesced droplets on a plane could promote droplet jumping. Here, we achieve enhanced horizontal mobility of a coalesced jumping droplet on superhydrophobic surfaces with an asymmetric ridge and investigate the underlying mechanism through experiment and simulation. Results indicate that the coalesced droplet accelerates during the coalescence-induced jumping stage and gains horizontal velocity during the rebound stage. The nondimensional horizontal velocity can reach 0.47, which is about 2.3 times the jumping velocity on the plane. Depending on the height-to-width ratio of the asymmetric ridge, the ratio of the horizontal velocity to the fallen velocity when the fallen droplet makes contact with the ridge is 0.55–0.75. Furthermore, the coalesced droplet can still obtain considerable horizontal velocity on superhydrophobic surfaces with an asymmetric ridge when the initial droplet radius is unequal. This work provides new insights for improving droplet jumping by surface structure in related fields. [ABSTRACT FROM AUTHOR]

Droplet splitting is a fascinating interfacial phenomenon, which shows great potential in applications such as fluid dispending and liquid spraying. Splitting behaviors of droplet impact on structured superhydrophobic surfaces are highly transient and complex, but the underlying mechanism is far from clear. Here, we report the splitting dynamics on ridged superhydrophobic surfaces through experimental and theoretical investigations. As the Weber number increases, three splitting modes appear in sequence: non-splitting, departure splitting, and contact splitting. Based on the movement of the liquid film behavior on the ridge along the axial direction, the splitting time consists of durations of three stages: axial spreading, axial retraction, and oscillation retraction, and it decreases with the increasing Weber number. A theoretical model is further established to predict the splitting time, where the law of the axial spreading and retraction is revealed. Splitting dynamics can be regulated by the geometric shape of the ridge. Droplet splitting is inhibited on the rectangular ridge, while the splitting time and contact time are effectively reduced on the semi-cylindrical and triangular ridges. This work is expected to provide fundamental support for diverse applications related to droplet splitting and offer guidance for the design of superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

The energy conversion efficiency (the ratio of the maximum jumping kinetic energy to the maximum surface energy released from droplet coalescence) is an essential indicator of the self-propelled jumping of droplets, which determines its value for applications in various fields. In the practical condensation process, the initial states of the multidroplets with different sizes and distributions have a significant effect on the energy conversion efficiency, but the mechanism behind this effect remains unclear. This paper reveals the effect of the initial states of droplets on the energy conversion efficiency of multidroplet jumping (mainly three droplets) from the perspective of energy conversion and the internal flow of the merged droplets. Different initial states will lead to different flow directions of the liquid microclusters inside the merged droplets. The consistency between the flow direction and the jumping direction will affect the energy conversion efficiency. To characterize this effect quantitatively, we construct a machine learning model based on a convolutional neural network to predict the energy conversion efficiency of multidroplet jumping with different initial distribution angles and radius ratios. The input of the neural network is the images of the initial state of the droplets, and the output is the energy conversion efficiency. After training, the neural network can predict the energy conversion efficiency of multidroplet jumping with an arbitrary initial state. [ABSTRACT FROM AUTHOR]

With the development of superhydrophobic surface preparation technology, coalescence-induced droplet jumping shows broad application prospects in the fields of enhanced condensation heat transfer and self-cleaning. In this work, the coalescence-induced jumping process of heterogeneous and homogeneous droplets on superamphiphobic surfaces was studied by using glycerol–water mixtures with different glycerol volume fractions. The results showed that the surface tension gradient of heterogeneous droplets will lead to asymmetric deformation of droplets, asymmetric distribution of internal pressure of droplets, as well as decrease in the energy conversion efficiency and the vertical departure velocity. Our study also revealed that the effects of surface tension gradient and viscosity on droplet jumping are different in the two regions. When the glycerol volume fraction is less than 40%, the droplet velocity and energy conversion are dominated by the surface tension gradient, and the vertical departure velocity and the energy conversion efficiency of homogeneous droplets are larger. When the glycerol volume fraction is greater than 40%, the droplet velocity and energy conversion are dominated by the surface tension gradient and viscosity together, and the vertical departure velocity and the energy conversion efficiency of heterogeneous droplets are larger. [ABSTRACT FROM AUTHOR]

While ridged, spherical, or cone superhydrophobic surfaces have been extensively utilized to explore the droplet impact dynamics and the possibility of reducing contact time, superhydrophobic surfaces with a single small pillar have received less attention. Here, we report the rebound and splashing phenomena of impact droplets on various single-pillar superhydrophobic surfaces with the pillars having smaller or equal sizes compared to the droplets. Our results indicate that the single-pillar superhydrophobic surfaces inhibit the droplet splashing compared to the flat ones, and the rebound droplets on the former sequentially exhibit three morphologies of top, bottom, and breakup rebounds with the increasing of Weber number, while those on the latter only show the (bottom) rebound. The pillar significantly enlarges the droplet spreading factor but hardly changes the droplet width. Both the relations between the maximum spreading and width factors and the Weber number on all surfaces approximately follow a classical 1/4-power law. Reduction in the contact time is observed for the rebound droplets on the single-pillar superhydrophobic surfaces, dependent on the rebound morphology. Specially, the breakup rebound nearly shortens the contact time by more than 50% with a larger pillar-to-droplet diameter ratio yielding a greater reduction. We provide scaling analyses to demonstrate that this remarkable reduction is ascribed to the decrease in the volume of each sub-droplet after breakup. Our experimental investigation and theoretical analysis provide insight into the droplet impact dynamics on single-pillar superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

The self-propelled motion of boiling droplets has attracted strong interest, and major discoveries are concentrated in the film boiling regime, e.g., translational motion of Leidenfrost drops on ratchets, Leidenfrost wheels. However, little attention was paid to the boiling regimes below the Leidenfrost point. Here, we focus on those boiling regimes and discover a gyroscopic rotation phenomenon of boiling droplets that is ubiquitous on various types of surfaces with diverse wettability and microstructures. The occurrence of gyroscopic rotation can be attributed to the viscous stress from vapor/bubble flows in the gaps of surface microstructures, verified by the results that for the experimental surfaces, the rougher the surface structures and the larger the solid–liquid contact area, the more probable it is to generate gyroscopic rotations. A theoretical model is established to investigate the effect of substrate temperature (boiling regime) on the spinning rate of boiling droplets, and the results further approve the proposed mechanism of gyroscopic rotation. The outcomes of this work help to deepen the understanding of droplet boiling and the corresponding dynamics on surfaces with microstructures. [ABSTRACT FROM AUTHOR]

Coalescence-induced droplet jumping on superhydrophobic surfaces has good application potential in the fields of heat transfer enhancement, self-cleaning, water collection, and microfluidics. However, the direction of droplets jumping on the plane is uncontrollable and the energy conversion rate is low, which limits the application of droplets jumping. In this work, we report a flexible approach to efficiently regulate the direction and velocity of droplet jumping on superhydrophobic surfaces using superhydrophobic string. By changing the inclination angle of the string, the jumping direction of the coalescent droplet shows a maximum deviation of 68.1° from the surface normal. The maximum energy conversion rate is 32.1%, which is about 5.4 times higher than that of jumping on the plane. We analyzed the mechanism of the efficient regulation and found that the string affected the hydrodynamic characteristics of the jumping process, thus changing the jumping direction and the speed of the coalescent droplet. [ABSTRACT FROM AUTHOR]

Droplet impacts on solid surfaces are ubiquitous in nature and industry. Before impact, the droplet shape may be affected by gravity, shear flow, and the electric and magnetic fields, inducing non-spherical droplets. However, most previous studies focused on the impact dynamics of spherical droplets. In this study, we conduct experiments, simulations, and theoretical analyses to investigate the impact behaviors of ellipsoidal water droplets whose symmetry axis is perpendicular to the surface. In particular, we explore the maximum spreading and energy evolution during impact. A numerical model adopting the Volume of Fluid method and Kistler's dynamic contact angle model achieves good agreement with the experimental results for both the temporal droplet profile and spreading factor. The effects of Weber number, contact angle, and aspect ratio on the impact dynamics are systematically investigated, and the outcomes show that both the maximum spreading time and factor enlarge with the increasing aspect ratio. Their relations approximately follow the 2/3-power and 1/6-power laws, respectively. Reducing the aspect ratio enhances the viscous dissipation during impact. Based on the theoretical analyses of above results, we modify the viscous dissipation in the conventional energy balance model to include the effects of aspect ratio on the maximum spreading factor. The modified theoretical model reduces the deviations from −23%–51% to −5%–25% and elucidates the scaling law between the maximum spreading factor and aspect ratio. This work deepens our understanding of the interaction between non-spherical impact droplets and surfaces and may contribute to associated applications. [ABSTRACT FROM AUTHOR]

The impact behavior of a water droplet on small cylindrical superhydrophobic targets is studied numerically and theoretically. A numerical model using the volume of fluid method is developed to simulate the droplet impact process on small cylindrical superhydrophobic targets. The model is verified by comparing the calculated results with the experimental observations in our previous work and reference. The influences of the Weber number and the target-to-droplet diameter ratio (less than one) on the droplet impact behaviors, including the droplet profile and the deformation factor, are investigated. The results indicate that a larger Weber number accelerates the spreading and falling of the droplet and promotes the droplet breakup. An increase in the diameter ratio delays the spreading and falling of the droplet on the side of the target, thus enhancing the deformation and rebound of the droplet. Both the increases in the Weber number and the diameter ratio contribute to a larger maximum deformation factor. Furthermore, the droplet breakup criterion is analyzed theoretically based on the energy conservation. A formula describing the relationship between the critical Weber number and the diameter ratio for the droplet breakup is proposed, which shows high prediction accuracy compared with the numerical values. The critical Weber number for the droplet breakup becomes larger with the increase in the diameter ratio. The findings in this research deepen our understanding of the mechanism of droplet impact on small targets. [ABSTRACT FROM AUTHOR]

Metal nanonetworks are applied in various applications, such as biomedicine, bionic materials, optical materials, and new energy materials. Here, periodic variable-sized Pt nanonetwork gratings (PtNGs) are fabricated on the surface of a Pt/Si substrate with single pulse two-beam direct laser interference lithography. The fabricated PtNGs are transferred onto the surface of a glass substrate with polymethyl methacrylate as the transfer mediator. Exposure with different film thicknesses, contrasts, and intensity distributions of the laser interference spot is analyzed, and the formation of nanopatterns is explained. Results show that with the change in the thicknesses of the Pt film, the exposed structures present Pt nanoparticles (PtNPs), Pt gratings, and PtNGs. The morphology and the feature size of the PtNGs are influenced by intensity distributions and the contrast of the laser interference spot significantly. [ABSTRACT FROM AUTHOR]

The conventional coalescence-induced jumping of droplets has been extensively studied, but the energy conversion efficiency of conventional droplet jumping is low and the jumping direction is uncontrollable, which greatly limits the application of droplet jumping. In this work, we report an enhanced and guided self-propelled jumping on a superhydrophobic surface with macrotexture and analyze the mechanism of enhancement and guidance. The interaction between the liquid bridge and macrotexture changes the direction of the jumping droplet and enhances the energy conversion efficiency. The direction of droplet jumping can be controlled at about 45°, and the energy conversion efficiency can be increased by about 2.7 times. [ABSTRACT FROM AUTHOR]

Water icing is a natural phase change phenomenon which happens frequently in nature and industry and has negative effects on a variety of applications. Deicing is essential for iced surfaces, but even for a nanoengineered superhydrophobic surface, deicing may be incomplete with many adherent unmelted ice droplets which have potential for reicing. Here, we focused on the reicing characteristics of droplets on a solid superhydrophobic surface, which has lacked attention in previous studies. Our results show that the nucleation and ice crystal growth characteristics of a reicing droplet are quite different from those of a first-time icing droplet. During reicing, secondary nucleation due to fluid shear always occurs first on the edges of unmelted ice, accompanied by fast-growing ice crystals that can trigger heterogeneous nucleation when in contact with the solid surface. The reicing takes place under very small supercooling (less than 0.5 °C), and the superhydrophobic surface does not play a key role, meaning that any current icephobic surfaces lose their features, which poses great challenges for anti-icing. In addition, because of the small supercooling, no recalescence phenomenon appears during reicing and the droplet remains transparent instead of clouding. Owing to the unmelted ice floating on the top of the droplet, the droplet shape after reicing is also distinguishing from that after normal icing, but the pointy tip formation during reicing and normal icing shows a uniformity. These results shall deepen the understanding of the anti-icing and deicing physics. [ABSTRACT FROM AUTHOR]

Droplet impact and droplet coalescence are two very common phenomena. When these two processes occur on a superhydrophobic surface in an appropriate sequence, an interesting but little-noticed phenomenon will occur with rotation of the rebounding-coalescing droplet. When a droplet impacts another stationary droplet on a superhydrophobic surface with an appropriate velocity and position, the reaction force produced by the impact and the moment arm formed by the liquid bridge produces a reversed torque. This reverse torque causes the droplet to rotate after rebounding. The liquid bridge in the early stage of the coalescence process is the key to the rotation, and the relative development speed of coalescence and rebound determines whether rotation can occur and its relative strength. [ABSTRACT FROM AUTHOR]

Fabrication of metal oxide thin film transistor (MOTFT) arrays using the inkjet printing process has caused tremendous interest for low-cost and large-area flexible electronic devices. However, the inkjet-printed MOTFT arrays usually exhibited a non-uniform geometry due to the coffee ring effect, which restricted their commercial application. Therefore, in this work, a strategy is reported to control the geometry and enhance device performance of inkjet-printed MOTFT arrays by the addition of an insulating polymer to the precursor solution prior to film deposition. Moreover, the impact of the polymer molecular weight (MW) on the geometry, chemical constitution, crystallization, and MOTFT properties of inkjet-printed metal oxide depositions was investigated. The results demonstrated that with an increase of MW of polystyrene (PS) from 2000 to 200 000, the coffee ring was gradually faded and the coffee ring effect was completely eliminated when MW reached 200 000, which is associated with the enhanced viscosity with the insulating polymer, providing a high resistance to the outward capillary flow, which facilitated the depinning of the contact line, leading to the elimination of the coffee ring. More importantly, the carrier mobility increased significantly from 4.2 cm2 V-1 s-1 up to 13.7 cm2 V-1 s-1 as PS MW increased from 2000 to 200 000, which was about 3 times that of the pristine In2O3 TFTs. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy results indicated that PS doping of In2O3 films not only frustrated crystallization but also altered chemical constitution by enhancing the formation of the M-O structure, both of which facilitated the carrier transport. These results demonstrated that the simple polymer additive process provides a promising method that can efficiently control the geometry of MO arrays during inkjet printing and maximize the device performance of MOTFT arrays, which showed great potential for the application in next generation printed displays and integrated circuits. [ABSTRACT FROM AUTHOR]