Coalescence-induced Droplet Jumping Research Articles

Effect of surface nanostructure on enhanced atmospheric corrosion resistance of a superhydrophobic surface

Coalescence-induced droplet jumping behavior (CIDJB) of superhydrophobic surfaces has a potential application in atmospheric corrosion protection by spontaneously removing the corrosive water film/droplets. However, there are few studies constructed on the zinc substrate studying the CIDJB. In addition, the effects of the complex structure characteristics on the CIDJB and the subsequent atmospheric anti-corrosion performance of the superhydrophobic surface are still lacking systematic understanding. Herein, three superhydrophobic surfaces, namely, the microstructured superhydrophobic surface, the nanostructured superhydrophobic surface, and the complex structured superhydrophobic surface were rationally prepared on the zinc surface. First, the effects of the complex structure characteristics on the CIDJB and the subsequent atmospheric corrosion protection performance of the superhydrophobic surfaces were studied. Second, the corresponding mechanism of atmospheric corrosion protection based on the CIDJB of the superhydrophobic surfaces was revealed. The results suggest that the presence of the nanostructure is an important factor for CIDJB as a result of the reduced solid-liquid contact area and interfacial adhesion. Compared with Micro SS without CIDJB, both the Nano SS and complex SS with CIDJB exhibit a better anti-corrosion performance after the simulated condensation experiments due to the jumping-induced wetting transformation mechanism. This study provides criteria for designing efficient anti-corrosion materials based on the CIDJB.

How Superhydrophobic Grooves Drive Single-Droplet Jumping.

Rapid shedding of microdroplets enhances the performance of self-cleaning, anti-icing, water-harvesting, and condensation heat-transfer surfaces. Coalescence-induced droplet jumping represents one of the most efficient microdroplet shedding approaches and is fundamentally limited by weak fluid-substrate dynamics, resulting in a departure velocity smaller than 0.3u, where u is the capillary-inertia-scaled droplet velocity. Laplace pressure-driven single-droplet jumping from rationally designed superhydrophobic grooves has been shown to break conventional capillary-inertia energy transfer paradigms by squeezing and launching single droplets independent of coalescence. However, this interesting droplet shedding mechanism remains poorly understood. Here, we investigate single-droplet jumping from superhydrophobic grooves by examining its dependence upon surface and droplet configurations. Using a volume of fluid (VOF) simulation framework benchmarked with optical visualizations, we verify the Laplace pressure contrast established within the groove-confined droplet that governs single-droplet jumping. An optimal departure velocity of 1.13u is achieved, well beyond what is currently available using condensation on homogeneous or hierarchical superhydrophobic structures. We further develop a jumping/non-jumping regime map in terms of surface wettability and initial droplet volume and demonstrate directional jumping under asymmetric confinement. Our work reveals key fluid-structure interactions required for the tuning of droplet jumping dynamics and guides the design of interfaces and materials for enhanced microdroplet shedding for a plethora of applications.

Design of dual-scale composite structured superhydrophobic surfaces for atmospheric corrosion prevention based on coalescence-induced droplet jumping

BackgroundThe coalescence-induced droplet self-jumping behavior on superhydrophobic surfaces (SHS) provides a new idea for atmospheric corrosion prevention. Although the influence on droplet self-jumping on different scales in single structure has been well understood, the mechanism of composite structures on droplet self-jumping, especially the process of atmospheric corrosion prevention is still unclear. MethodsIn this work, SHS that composite structured on micron-scale and nano-scale were designed on copper substrate at given salt solution concentration using hydrothermal method; and the difference of their droplet self-jumping behavior on surface was explored in energy perspective. Significant findingsResults showed that the nano-scale composite structured SHS was more conducive to droplet self-jumping for having smaller top layer structures and interfacial adhesion (Eint). Furthermore, the nano-scale composite structured SHS exhibited superior corrosion protection performance due to the wetting transition by droplet self-jumping. This study provides theoretical guidance for the development of corrosion prevention with composite structured SHS based on coalescence-induced droplet self-jumping behavior.

How surface orientation affects coalescence-induced droplet jumping behavior and subsequent atmospheric corrosion resistance of a superhydrophobic surface?

In this study, the effect of the surface orientation on the coalescence-induced droplet jumping behavior (CIDJB) and the subsequent atmospheric corrosion resistance of the knife-like structure CuO superhydrophobic surface were studied. And the corresponding atmospheric corrosion protection mechanism on the basis of the CIDJB was revealed. The results indicate that the horizontally oriented superhydrophobic surface exhibits a bigger droplet size distribution and surface coverage during the condensation process because of the droplets falling back to the surface. And compared with the vertically oriented superhydrophobic surface, the horizontally oriented superhydrophobic surface exhibits a better corrosion resistance after condensation.

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Numerical Simulation of the Coalescence-Induced Polymeric Droplet Jumping on Superhydrophobic Surfaces

Numerical Simulation of the Coalescence-Induced Polymeric Droplet Jumping on Superhydrophobic Surfaces

Effects of the surface tension gradient and viscosity on coalescence-induced droplet jumping on superamphiphobic surfaces

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.

Fast nucleation of water by self-arrangement of hydrophilic crystals on a hierarchically structured surface promoting coalescence-induced droplet jumping

The enhancement of the condensation efficiency is required for saving energy and resources in industries exploiting condensation. In this regard, the Laplace pressure-driven spontaneous movement of condensed droplets on hierarchical superhydrophobic surfaces has been exploited to increase the condensation efficiency via the fast removal of the condensates from the surfaces. Although the moving droplets enabled the nucleation sites to be exposed to vapor before the droplets were removed from the surface, nucleation after the exposure was delayed by vapor depletion around the moving droplets. Since the heterogeneous nucleation energy barrier depends on the surface wettability, we investigated the effect of the wettability of the nucleation sites on the nucleation after the nucleation sites were exposed owing to the moving droplets. We arranged hydrophilic crystals inside the inner tips of a zigzag-structured surface and adjusted the crystal size with capillary flow-induced self-arrangement. The crystal size was highly related to the nucleation period at the inner tips because the nucleation energy barrier decreased with increasing crystal size. Consequently, the nucleation period reduced by the adequately sized crystals promoted droplet jumping, thereby increasing the frequency and cumulative volume of the jumping droplets by up to 410% and 130%, respectively. Furthermore, we verified that the increased crystal size did not affect the jumping probability on the biphilic-zigzag surfaces; this is because the wetting transition induced by the V-shapes minimized the work of adhesion. These results provide insight into the relationship between the wettability and nucleation period on hierarchically structured surfaces and highlight the synergetic effect between the hierarchical structures and heterogeneous wettability, which promotes droplet jumping.

Rational fabrication of superhydrophobic surfaces with coalescence-induced droplet jumping behavior for atmospheric corrosion protection

Coalescence-induced droplet jumping behavior on superhydrophobic surfaces has a potential application in atmospheric corrosion protection by spontaneously removing the corrosive water film/droplets. However, the design guidelines for superhydrophobic surfaces to realize coalescence-induced droplet jumping behavior remain lacking, and the antifogging ability of droplet jumping behavior for atmospheric corrosion protection is unknown. Herein, two kinds of superhydrophobic surfaces, namely, the sheet-like structure superhydrophobic surface and the cluster-like structure superhydrophobic surface, were rationally fabricated over the copper substrate by the different solution-immersion process followed by the same hydrophobized treatment. First, the correlations of the microstructure and droplet jumping behavior of the two surfaces were studied, and a droplet jumping phase map that divided the jumping and non-jumping regions was formulated from the perspective of energy. The sheet-like structure superhydrophobic surface with a higher contact angle and a lower surface roughness is more favorable to droplet jumping behavior due to a lower solid/liquid contact area and interfacial adhesion. Second, the antifogging ability of droplet jumping behavior for atmospheric corrosion protection was experimentally demonstrated. Comparatively, the sheet-like structure superhydrophobic surface presents a superior anti-corrosion performance due to droplet jumping-induced wetting transition. This work offers design guidelines for superhydrophobic surfaces to realize coalescence-induced droplet jumping behavior in self-cleaning, anti-icing/anti-frosting, and condensation heat transfer enhancement applications, and provides insights into designing efficient technologies in the application of atmospheric corrosion protection.

Enhancement and Guidance of Coalescence-Induced Jumping of Droplets on Superhydrophobic Surfaces with a U-Groove.

Coalescence-induced droplet jumping has received considerable attention owing to its potential to enhance performance in various applications. However, the energy conversion efficiency of droplet coalescence jumping is very low and the jumping direction is uncontrollable, which vastly limits the application of droplet coalescence jumping. In this work, we used superhydrophobic surfaces with a U-groove to experimentally achieve a high dimensionless jumping velocity Vj* ≈ 0.70, with an energy conversion efficiency η ≈ 43%, about a 900% increase in energy conversion efficiency compared to droplet coalescence jumping on flat superhydrophobic surfaces. Numerical simulation and experimental data indicated that a higher jumping velocity arises from the redirection of in-plane velocity vectors to out-of-plane velocity vectors, which is a joint effect resulting from the redirection of velocity vectors in the coalescence direction and the redirection of velocity vectors of the liquid bridge by limiting maximum deformation of the liquid bridge. Furthermore, the jumping direction of merged droplets could be easily controlled ranging from 17 to 90° by adjusting the opening direction of the U-groove, with a jumping velocity Vj* ≥ 0.70. When the opening direction is 60°, the jumping direction shows a deviation as low as 17° from the horizontal surface with a jumping velocity Vj* ≈ 0.73 and corresponding energy conversion efficiency η ≈ 46%. This work not only improves jumping velocity and energy conversion efficiency but also demonstrates the effect of the U-groove on coalescence dynamics and demonstrates a method to further control the droplet jumping direction for enhanced performance in applications.

Copper-Based Superhydrophobic Nanostructures for Heat Transfer in Flow Condensation

Superhydrophobic surfaces enhance the condensation heat transfer performance by facilitating removal of condensed droplets and coalescence-induced droplet jumping. During the last decade, studies on superhydrophobic surfaces have been mostly limited to working conditions, which are ideal for electron microscopy techniques. Therefore, the behavior of condensed droplets in the presence of a vapor flow with different vapor qualities and their implementation in flow condensation heat transfer enhancement have received rather little attention, which is the motivation behind this study. Thus, beside heat transfer analysis, we performed a visualization study on flow condensation in a minichannel and investigated droplet dynamics, including a histogram of droplet diameter distribution at different time intervals and stages of a condensation cycle consisting of nucleation growth and departure, droplet departure diameters, cycle time, and droplet number density. The droplet departure diameter decreases with steam mass flux, leading to a shift to smaller radii in droplet size distribution, which enhances condensation heat transfer. Enhancements up to 33% in the heat transfer coefficient were obtained at lower steam qualities for the tested superhydrophobic surface compared to the reference plain hydrophobic surface. We demonstrated that in addition to the high vapor shear stress exerted by the flow on the interface of the vapor and the condensed droplet, the advantage of an inherently unique water repellency feature of the nanostructured superhydrophobic surface can be utilized in flow condensation for enhancing condensation heat transfer.

Investigation of Coalescence-Induced Droplet Jumping on Mixed-Wettability Superhydrophobic Surfaces

Coalescence-induced droplet jumping has received more attention recently, because of its potential applications in condensation heat transfer enhancement, anti-icing and self-cleaning, etc. In this paper, the molecular dynamics simulation method is applied to study the coalescence-induced jumping of two nanodroplets with equal size on the surfaces of periodic strip-like wettability patterns. The results show that the strip width, contact angle and relative position of the center of two droplets are all related to the jumping velocity, and the jumping velocity on the mixed-wettability superhydrophobic surfaces can exceed the one on the perfect surface with a 180° contact angle on appropriately designed surfaces. Moreover, the larger both the strip width and the difference of wettability are, the higher the jumping velocity is, and when the width of the hydrophilic strip is fixed, the jumping velocity becomes larger with the increase of the width of the hydrophobic strip, which is contrary to the trend of fixing the width of the hydrophobic strip and altering the other strip width.

Coalescence-induced self-propelled jumping of three droplets on non-wetting surfaces: Droplet arrangement effects

Coalescence-induced self-propelled droplet jumping has attracted extensive attention because of its huge potential for enhancing dropwise condensation heat transfer, anti-icing, and self-cleaning. Most previous studies focus on binary droplet jumping, with little research on the more complex and realistic multi-droplet jumping. As a result, the effect of the droplet arrangement on the multi-droplet jumping phenomenon remains unclear. In this paper, the self-propelled jumping of three droplets with different arrangements (two droplets are fixed, and the location of the third one is changed) is numerically simulated, and energy conversion efficiency is studied. Based on two different forming mechanisms, region I (the coalescence between the lateral droplets forms the central liquid bridge) and region II (the changed interface curvature of central droplets turns into the central liquid bridge under surface tension) are defined in three-droplet arrangements. The liquid bridges exhibit different dynamic behaviors in two particular regions, even the jumping velocity is determined by the moving synchronicity of liquid bridges in each region. The critical distribution angle that leads to the overall nonmonotonic change of jumping velocities ranges between 110° and 120° (0.02 ≤ Oh ≤ 0.16). Compared with the symmetry of the droplet configuration, the geometry of the droplet arrangement plays a dominate role in the nonmonotonic change. The maximum energy conversion efficiency is just over 6.5% and the minimum is just under 3%. The findings of this study not only reveal how the arrangement affects ternary droplet jumping and explain the phenomenon that cannot be explained before, but deepens our understanding of multi-droplet jumping as well.

Energy-saving of air-cooling heat exchangers operating under wet conditions with the help of superhydrophobic coating

The condensate retention and bridging on the airside of the heat exchanger, such as the evaporator of the heat pump and air-conditioning system, can adversely increase the pressure drop penalty and energy consumption of the heat transfer systems. The objective of this study is to minimize the airside pressure drop of the rectangular plain heat exchangers using superhydrophobic coating which promotes continuous condensate shedding through coalescence induced droplet jumping. The experiments are carried out for different inlet air temperatures (23 °C and 27 °C), relative humidities (50%, 70%, and 90%) and the fin base temperature is fixed to be 7 °C. While the frontal air velocity is varied from 0.5 to 2.5 m/s, and the fin spacing ranges from 1 mm to 4 mm. The results showed that the heat transfer rate between untreated and superhydrophobic heat exchanger is almost similar; whereas a 30–55% increase in heat transfer is observed when inlet air temperature increased from 23 °C to 27 °C and an increase of 20–50% and 60–100% in heat transfer is noticed when relative humidity increased from 50% to 70% and 90%, respectively. Due to the effective condensate removal and the early arrival of steady state, the airside pressure drop for the superhydrophobic heat exchanger is almost two times lower than that of the untreated one. The reduction in airside pressure drop led to appreciable energy saving of the heat transfer system, and it is found that 30–60% saving can be achieved especially at higher relative humidities (70% or 90%) and frontal air velocities (more than 1 m/s).

Enhanced steam condensation heat transfer on a scalable honeycomb-like microporous superhydrophobic surface under various pressures

Steam condensation was studied experimentally over a honeycomb-like microporous superhydrophobic surface, as fabricated by electrodeposition method, under various condensing pressures. The condensing pressure was varied from 4 kPa to 13 kPa, based on the typical operating conditions of condensers in coal-fired power plants. Over this range of pressure, stable coalescence-induced condensate droplet jumping was realized on the honeycomb-like surface with hierarchical micro-/nanostructures, leading to a significantly enhanced condensation heat transfer over that on a smooth hydrophobic surface at low degrees of subcooling (<10 K). Both the frequency and amount of jumping droplets were observed to decrease at lower pressures because of the slower condensate production, and at higher degrees of subcooling as a result of the occurrence of surface flooding. However, increasing the pressure was found to lead to delayed onset of surface flooding due to the pressure-boosted droplet jumping phenomenon. The good performance and rapid, scalable fabrication of this type of modified surface on curved copper objects shed light on its great potential for applications in industrial condensation equipment operated under multiple pressures.

Effects of Gravitational Force and Surface Orientation on the Jumping Velocity and Energy Conversion Efficiency of Coalesced Droplets

Due to potential applications in numerous fields (self-cleaning, anti-frosting, condensation enhancement, etc.), coalescence-induced droplet jumping has been investigated extensively by numerical simulations and experiments over the past decade. In this paper, the jumping dynamics of coalesced droplets on the microstructured superhydrophobic surfaces is simulated using the lattice Boltzmann model. Effects of gravity, inclined angle and droplet radius ratio on the jumping velocity and energy conversion efficiency are studied. The numerical results demonstrate that jumping droplets on inclined surfaces driven by the tangential gravity can successfully detach from the surface without returning to the original spot. The jumping velocity can be improved by a larger inclined angle. Coalescence-induced jumping of two-mismatched droplets on the horizontal surface can also produce a horizontal velocity that drives them detaching from the surface along x direction or coalescing with other droplets. Both jumping velocity and energy conversion efficiency are reduced by a lower radius ratio and a larger gravitational coefficient. In addition, no jumping behavior can be observed when energy conversion efficiency is less than 2%.

Laplace Pressure Driven Single-Droplet Jumping on Structured Surfaces.

Droplet transport on, and shedding from, surfaces is ubiquitous in nature and is a key phenomenon governing applications including biofluidics, self-cleaning, anti-icing, water harvesting, and electronics thermal management. Conventional methods to achieve spontaneous droplet shedding enabled by surface-droplet interactions suffer from low droplet transport velocities and energy conversion efficiencies. Here, by spatially confining the growing droplet and enabling relaxation via rationally designed grooves, we achieve single-droplet jumping of micrometer and millimeter droplets with dimensionless jumping velocities v* approaching 0.95, significantly higher than conventional passive approaches such as coalescence-induced droplet jumping (v* ≈ 0.2-0.3). The mechanisms governing single-droplet jumping are elucidated through the study of groove geometry and local pinning, providing guidelines for optimized surface design. We show that rational design of grooves enables flexible control of droplet-jumping velocity, direction, and size via tailoring of local pinning and Laplace pressure differences. We successfully exploit this previously unobserved mechanism as a means for rapid removal of droplets during steam condensation. Our study demonstrates a passive method for fast, efficient, directional, and surface-pinning-tolerant transport and shedding of droplets having micrometer to millimeter length scales.

Dynamic behavior of droplets on confined porous substrates: A many-body dissipative particle dynamics study

Droplets wetting and impacting on porous substrates play a critical role in various printing processes and industrial applications. However, due to the lack of effective observation inside the pores, the dynamic behavior of the droplet is rather unclear. Here, we used a numerical method to investigate the dynamic behavior of droplets spreading on confined porous substrates with different surface fractions. The wetting process has been divided into two stages: the inertial stage and the viscous stage. The numerical results show a power-law evolution of the contact diameter with time, and the exponent has a linear relationship with the surface wettability. The scaling laws proved to have no dependence on the porosity. The presence of confined pores causes the spreading slower and makes the droplet reach an equilibrium state more easily. Then, the impacting process was reported by changing the initial velocities of the droplets. It was found that penetration is always observed after spreading. The wetting transition was captured, and the dimensionless maximum spreading was scaled. Finally, the coalescence-induced droplet jumping has been verified on confined porous substrates with a superhydrophobicity, suggesting the potential of porous structures in designing specific droplet behaviors.

Breaking Droplet Jumping Energy Conversion Limits with Superhydrophobic Microgrooves.

Coalescence-induced droplet jumping has the potential to enhance the performance of a variety of applications including condensation heat transfer, surface self-cleaning, anti-icing, and defrosting to name a few. Here, we study droplet jumping on hierarchical microgrooved and nanostructured smooth superhydrophobic surfaces. We show that the confined microgroove structures play a key role in tailoring droplet coalescence hydrodynamics, which in turn affects the droplet jumping velocity and energy conversion efficiency. We observed self-jumping of individual deformed droplets within microgrooves having maximum surface-to-kinetic energy conversion efficiency of 8%. Furthermore, various coalescence-induced jumping modes were observed on the hierarchical microgrooved superhydrophobic surface. The microgroove structure enabled high droplet jumping velocity (≈0.74U) and energy conversion efficiency (≈46%) by enabling the coalescence of deformed droplets in microgrooves with undeformed droplets on adjacent plateaus. The jumping velocity and energy conversion efficiency enhancements are 1.93× and 6.67× higher than traditional coalescence-induced droplet jumping on smooth superhydrophobic surfaces. This work not only demonstrates high droplet jumping velocity and energy conversion efficiency but also demonstrates the key role played by macroscale structures on coalescence hydrodynamics and elucidates a method to further control droplet jumping physics for a plethora of applications.