Droplet Bouncing Research Articles

Directional Transportation Behavior of Droplets on a Magnetically Responsive Micropillar Array Surface.

Technologies for manipulating droplets have applications in various fields, such as biomedical devices, chemical engineering, water collection, and thermal management. The morphology of magnetically responsive surfaces are modified using magnetic fields for inducing droplet directional rolling and bouncing. Herein, a study on the directional transport of droplets on magnetically responsive surfaces is reported, identifying three movement modes and analyzing the mechanisms influencing the transport behavior. The study explored working fluids with surface tension lower than that of water, achieving the directional transport of 30% ethanol droplets with a maximum transport velocity of 130 mm s-1. The practical applications of the droplets on a modified surface were analyzed, and methods were developed to accelerate droplet mixing. This study explored efficient operations and automation of complex chemical processes.

Droplet bouncing electricity generation on graphene substrate

Abstract Since the discovery of the phenomenon of droplets bouncing on hydrophobic surfaces, researchers have been dedicated to studying the underlying physical mechanisms of this phenomenon. Particularly, in recent years, researchers have focused on the observation that droplet bouncing can generate electrical signals. Inspired by this, we explore the potential applications of electrical signals generated by droplets (water droplets) bouncing in sensors and other relevant fields. In this study, we chose a well-established reduced graphene oxide (rGO) film, typical in the sensor field, as a substrate to investigate the bouncing electricity generation performance of water droplets on it. Three variable factors were considered, including droplet bouncing on rGO substrates with different inclinations, bouncing of droplets of different sizes at the same inclination on rGO substrates, and droplet dropping frequency, to investigate their effects on the electrical signals.

Sunflower-Inspired Superhydrophobic Surface with Composite Structured Microcone Array for Anisotropy Liquid/Ice Manipulation.

Precisely controlling the directional motion trajectories of droplets on anisotropic 3D functional surfaces has great application potential in self-cleaning, drug delivery, and droplet power generation, but it also faces huge challenges. Herein, inspired by the microcone structure in the heart of sunflowers, a nanoneedle-modified microcone array surface (NMAS)is reported. The surface is created using a combination of nanosecond laser direct engraving and electroforming and is subsequently fluorinated. Through programmable control of the laser spot, the geometric parameters and inclination angle of the microcone can be quickly and finely adjusted, thereby achieving precise control of the droplet bouncing trajectory. The results show that droplets can achieve programmable multiple bouncing behaviors on patterned functional surfaces, including gravity-defying hopping and directional water transport. It is worth noting that this functional surface has delayed freezing and anti-freezing effects. Furthermore, this functional surface has a wide range of potential applications, including surface self-cleaning, droplet capture, and droplet-based chemical microreactions, especially in the field of anti-icing operations. This opens up a new way for the directional transport of droplets on biomimetic functional surfaces.

Droplet Impact on Superhydrophobic Mesh Surfaces.

Reducing the contact time of droplet impacts on surfaces is crucial for various applications including corrosion prevention and anti-icing. This study aims to explore a novel strategy that greatly reduces contact time using a superhydrophobic mesh surface with multiple sets of mutually perpendicular ridges while minimizing the influence of the impacting location. The effects of the impact Weber numbers and ridge spacing on the characteristics of the impact dynamics and contact time are studied experimentally. The experimental results reveal that, for the droplet impact on mesh surfaces, ridges can segment the liquid film into independently multiple-retracting liquid subunits. The retracted subunits provide the upward driving force, which may promote the splashing or pancake bouncing of droplets. At this point, the contact time has a negligible sensitivity for the impacting position and is significantly reduced by up to 68%. Furthermore, the time, dynamic pressure, and energy criteria for triggering splashing and pancake bouncing are proposed theoretically. This work provides an understanding of the mechanism and the design guidelines for effectively reducing the contact time of the impacting droplet on superhydrophobic surfaces.

Robust Rain-Repellency and Droplet Bouncing Properties of Bauhinia Fresh and Aged Leaves Up to 6 Months.

Robust rain-repellent surfaces are useful in roofs, solar panels, windshields, etc. Herein, excellent rain-repellency and droplet bouncing properties of Bauhinia Variegata leaves are presented. They possess surface microbumps (l ∼ 13 μm, w ∼ 8 μm, h ∼ 3 μm), which in turn comprise nanoplatelets (l ∼ 741 nm, t ∼ 59 nm) and Wenzel roughness (r w) of ∼2.2. The leaf's surface energy was estimated to be 9.47 ± 0.03 mJ·m-2 by incorporating rw into the van Oss-Good-Chaudhary theory. The leaves exhibited static contact angle of 157 ± 1°, roll-off angle of 9 ± 1°, and contact angle hysteresis of 12 ± 4°, which retained as they aged up to 186 days in the natural weather and laboratory conditions. The water droplets (10 μL, 40 μL) bounced off for free-fall heights from 5 cm to ∼13 m (Weber no. 36 to ∼2990) and displayed robust rain-repellency (Weber no. ∼4500), similar to that of a lotus leaf. Also, Bauhinia leaves survived pressurized water jets (Weber no. ∼4240). Nevertheless, underwater hydrophobicity has been persistent only for up to 3 h when submerged in 20 cm (∼1.96 kPa gauge pressure) deep water, while lotus leaves retained for >7 h. Such robust Bauhinia leaf's nanoplatelets and wax chemistries can be replicated onto glass/metals for preparing rain-repellent surfaces.

Superior droplet bouncing, anti-icing/anti-frosting and self-cleaning performance of an outstanding superhydrophobic PTFE coating

The prevention of ice/frosting formation is crucial in cold region for various applications. Superhydrophobic coatings are known for their excellent anti-icing/anti-frosting properties. In this study, a durable superhydrophobic PTFE coating on a stainless steel surface was fabricated with a high water contact angle of 171.4° and a low rolling angle of 1.1°. The freezing of water droplets on the coating exhibits a significantly prolonged duration of 809 s at −10 °C, representing a 25-fold increase compared to the uncoated stainless steel surface. The ice adhesion strength is reduced by 83.7%, making ice easier to remove. Anti-frosting tests show that a thinner and lower-density layer of frost formed on the PTFE coating due to its micro-nano hierarchical structure. Furthermore, the superhydrophobic PTFE coating demonstrates excellent mechanical stability, droplet bouncing dynamics and self-cleaning properties. It is anticipated that this durable superhydrophobic PTFE coating will be a candidate for anti-icing/anti-frosting and self-cleaning applications

Understanding dilution effects on particle-containing pesticide droplets deposition on rice leaf via developing CFD-VOF-DPM model.

Pesticide dilution is one of the essential aspects of plant protection. However, the effect of dilution on the deposition characteristics of pesticide droplets containing particulate additives on crop leaf surfaces remains unclear and warrants further research. Herein, a validated computational fluid dynamics (CFD)-volume of fluid (VOF)-discrete phase model (DPM) numerical model was developed to analyze the influence of particle content on the deposition behavior of droplets on the leaf surface comparatively, taking into account the particle content of different diluted thifluzamide solutions. Additionally, the study aimed to analyze further the kinetic behavior of pesticide droplets landing on rice leaves across different dilution conditions. Pesticide droplets diluted 100-fold had a lower retraction rate during spreading than particle-free droplets, so the solution is more easily deposited in the leaves. Moreover, the low dilution (high concentration) increased the critical adhesion rate between droplets and rice leaves, inhibiting the bouncing of droplets on the leaf surface, thus promoting their effective deposition on the surface. In addition, low dilution (high concentration) is not conducive to spreading droplets when the impact velocity is high, and it also results in a large amount of pesticide use. The actual application process can be through understanding the dilution factor of the configured pesticide solution, and reasonable adjustment of the nozzle pressure can effectively improve the utilization rate of pesticides and reduce the pollution brought by pesticides to the environment. These results provide an essential reference for studying pesticide droplet deposition characteristics, including rice plant protection and spraying technology. © 2024 Society of Chemical Industry.

Excellent Foliar Deposition of Cyclodextrin‐Encapsulated Furyl/Thienyl‐Engineered Ingredients: Novel Effective Supramolecular Agrochemicals for Combating Plant Bacterial and Fungal Infections

AbstractRepeated epidemics caused by bacterial and fungal diseases pose a threat to global crop yields and food security, therefore, updating the structure of existing microbicides and improving the pesticide bioavailability are challenging tasks, perhaps planning to develop versatile supramolecular antimicrobial materials. Here, a low‐cost multifunctional supramolecular complex, named FT6@β‐CD, is constructed by host‐guest encapsulation of a thienyl‐engineered ingredient (FT6) into β‐cyclodextrin. This integrated architecture can significantly reduce the bounce and splash behaviors of droplets, improve the wetting performance, and eventually acquire excellent foliar deposition and targeted delivery. Whereafter, FT6@β‐CD can inhibit the transcription of type III secretion system (T3SS)‐associated genes, hypersensitive reaction, and final pathogenicity from a destructive pathogen Xanthomonas oryzae pv. oryzae (Xoo). Importantly, at a lower dosage of 200 µg mL−1, FT6@β‐CD presented excellent in vivo control effects (protection 48.1%, curative 50.1%) against Xoo, remarkably better than the commercial bactericidal formulation thiodiazole‐copper. Additionally, other supramolecular antifungal materials (FT24/FT25@HP‐β‐CD/β‐CD/Me‐β‐CD) constructed in this study can effectively treat Botryosphaeria dothidea and Botrytis cinerea with higher control efficiencies of 86.8% and 90.1% at 200 µg mL−1, respectively. This study provides a bright guidance for developing low‐cost, green, and eco‐friendly supramolecular microbicides for controlling agricultural diseases.

Droplet asymmetry bouncing on structured surfaces: A simulation based on SPH method

The impact of a liquid droplet onto a macro-structured super-hydrophobic surface, results in an asymmetric spreading-retraction process and a significant decrease in contact time. The process involves large deformation of droplets and evolution of surface morphology, so it is difficult to efficiently simulate using traditional grid-based methods. In this study, a numerical model is established using the mesh-free smoothed particle hydrodynamics (SPH) method to simulate the interaction of liquid droplets with solid surfaces exhibiting complex macro-textures. Leveraging the Lagrange nature of SPH, liquid droplets can be model using free surface boundary condition. The weakly compressible SPH formulation is employed to discretize the governing equations of liquid. Surface tension is captured by introducing an additional negative pressure term, inducing attractive forces among fluid particles. Kernel functions are carefully chosen to mitigate stress instability resulting from droplet spreading and retraction. The model is then applied to simulate the dynamic processes of droplets impact on flat and non-flat super-hydrophobic surfaces. The bouncing dynamics of droplets are investigated under varying conditions, including Weber number, size, and morphology of macro-scale surface structures. Our simulation results for contact time, maximum diameter, and droplet morphology align closely with experimental observations. The contact time of the droplet exhibits a dependency on its size and surface geometry, with a transition towards planar geometry diminishing asymmetric spreading effects. The findings highlight the significance of Weber number and cylinder diameter in determining contact time, providing insights for the design of optimized surface structures for specific droplet impact parameters. In addition, the use of a free-surface condition for modeling proved computationally efficient for 3D simulations. The model effectively reproduces nonlinear behaviors such as droplet spreading, splashing, and bouncing, highlighting the significant advantage of SPH in simulating such problems.

Bio-stimulant based nanodelivery system for pesticides with high adhesion and growth stimulation

Pesticides are important agricultural production materials to ensure global food security, but the low utilization rate during pesticide application has caused a large amount of waste and environmental pollution. The use of pesticide carriers to deliver pesticides can help to prevent rapid release, assist in targeted release, and improve pesticide utilization. However, after completing pesticide delivery, less part of pesticide carriers has more functions and may remain in the environment, leading to material waste and environmental burden. Here, we propose that the environment-friendly carrier materials for pesticide delivery can synergistically enhance control efficacy with pesticides. Based on the bio-stimulant polyglutamic acid, amphiphilic polymers (mPEG-b-PLG) were constructed, and they assembled into nano micelle. After encapsulating prothioconazole (PTC) into the nano micelle through hydrogen bonding, electrostatic interactions and hydrophobic interactions through co-assembly, mPEG-b-PLG-PTC nanospheres with a diameter of approximately 120 nm were prepared. The nanospheres exhibit pH responsiveness for targeted release of pesticides, as well as excellent retention capacity, droplet bounce inhibition ability, and deposition capacity. At the same time, compared with commercial formulations, it has long-term antifungal properties and lower EC50, while its acute toxicity to human cells is lower, which can reduce cell apoptosis by about 63 %. It is also gratifying that the main root length, plant height, and fresh weight of wheat seedlings treated with mPEG-b-PLG have significantly improved, effectively applying carrier materials for synergistic effects. In summary, this work proposes a promising strategy to synergize pesticide carrier materials as active ingredients, providing new ideas for the integration of pesticide and fertilizer, improving pesticide utilization efficiency, and reducing environmental pollution.

Smart droplet bouncing on dielectric surfaces under uniform electric fields

The electric field is considered an effective stimulus to control droplet bounce or adhesion on demand on solid surfaces, which is important for various applications, including water harvesting and oil/water separation. However, it remains challenging to switch droplet bouncing/adhering on electrode surfaces smartly. Herein, we present a smart control method for droplet bouncing on dielectric surfaces by coupling charge transfer with contact electrification and a uniform electric field. Subject to electric fields, water droplets carrying the like charges in the insulating silicone oil present electric field-direction-dependent impact behaviors, bifurcating into bouncing and adhesion on hydrophobic and hydrophilic surfaces. Furthermore, oppositely charged water droplets with contact electrification led to contrary bouncing behaviors on hydrophobic and hydrophilic surfaces. The transfer charges and electric forces in the experiments are especially quantitively analyzed. By constructing dielectric pairs with hydrophobic and hydrophilic surfaces, reciprocating bouncing or selective adhesion can be modulated via switching electric field directions. This route of separately charging droplets and building electric fields facilitates droplet manipulation techniques and applications.

Preparation of super-hydrophobic BN nanotube mesh and theoretical research of wetting state

In this study, we have fabricated stainless steel mesh with columnar super-hydrophobic boron nitride nanotubes (BNNTs) through the ‘gas-liquid-solid’ mechanism annealing method. We employed iron (Fe) powder as the catalyst and utilized boron (B) and boron oxide (B2O3) as the raw materials. The synthesis took place within a tubular furnace, operating under an NH3 atmosphere at a temperature of 1250C∘. The contact angle of water droplets on the experimental surface measures approximately 150∘, while the sliding angle is approximately 3∘. In the next place, the laboratory carried out a series of experiments on the tensile properties, corrosion resistance, and erosion resistance of the prepared samples. The experimental results showed that the sample material had good mechanical properties and corrosion resistance. In addition, the erosion resistance test results showed that the sample material even at high speed (2800 rpm/min). After washing the sample for 5 min, the surface of the material still has good wettability (contact angle is 136.49∘). Finally, droplet bouncing experiments have demonstrated that the adhesion force on the surface of the boron nitride nanotube stainless steel mesh is notably minimal, indicating its substantial hydrophobic properties. Additionally, we conducted an investigation into the wetting interaction between droplets and the surface of the boron nitride nanotube stainless steel mesh. By applying the Wenzel and Cassie formulas and considering the pertinent geometric parameters of the material surface, we theoretically elucidated that, upon contact, the droplets assume a ‘Cassie’ state on the surface of the super-hydrophobic boron nitride nanotube stainless steel mesh. The research results show that the superhydrophobic boron nitride nanotube stainless steel mesh has broad application prospects in the fields of hydrogen storage, sewage treatment and oil-water separation.

New insights into head-on bouncing of unequal-size droplets on a wetting surface

The impact of a liquid droplet with another droplet or onto a solid surface are important basic processes that occur in many applications such as agricultural sprays and inkjet printing, and in nature such as pathogens transport by raindrops. We investigated the head-on collision of unequal-size droplets of the same liquid on wetting surfaces using the direct numerical simulations technique at different size ratios. The unsteady Navier–Stokes equations are solved and the liquid–gas interface is tracked using the geometric volume-of-fluid method. The numerical model is validated by comparing simulation results of two extreme cases of droplets bouncing with the experimental data from previous studies and the agreement is quite accurate. The validated model is employed to simulate droplets bouncing at several size ratios at different Weber numbers and Ohnesorge number. Two distinct regimes are identified, namely, the inertial regime, where the restitution coefficient is a constant value close to 0.3, the viscous regime, where the restitution coefficient declines. To understand the bouncing behaviour, the velocity field is analysed and an energy budget calculation is performed. The distribution of the sessile droplet energy is found to be important and the sessile droplet surface energy is calculated by its deformation characteristics such as crater depth. Finally, a scaling analysis is performed to rationalize the insensitivity of the coefficient of restitution in the inertial regime, and its decline in the viscous regime, at large size ratios.

Femtosecond laser turn droplets controllable bouncing

Droplets controllable bouncing on solid surfaces plays a vital role in microfluidic operation, liquid collection, and chemical micro reaction applications, which has attracted widespread attention. However, strategies to realize controllable bouncing of droplets on solid surface are still elusive and nanofabrication patterning methods are generally complex and time consuming. Here, we propose a simple and fast method to fabricate heterogeneously wettable surfaces on aluminum sheets by femtosecond laser processing followed by wax treatment. The surface is comprised of superhydrophobic regions with a water contact angle (WCA) of ∼ 159.5°and superhydrophilic regions with a WCA of ∼ 1.8°. The heterogeneous wettability in specific designs can control droplet bouncing motions when impacting the surface. Therefore, the liquid droplets are regulated to rotate and controllably bounce for achieving various functions, including climbing, combining, and driving small boats. This work demonstrates a facile way to fabricate patternable heterogeneous wettability surfaces for manipulating droplets and achieving multifunctional bouncing motions. Our simple, effective, and environmentally friendly method holds promise for transformative advancements in electric power generation, pharmaceutical development and manufacturing, as well as the realm of self-driving actuators.

Anti-icing application of superhydrophobic coating on glass insulator

Insulators, as important components in transmission lines, are prone to disturb the safe running of the power system by the ice accumulation on surfaces. Traditional anti-icing coatings are difficult to practically apply to insulators. Here, superhydrophobic (SHP) coatings were fabricated on the glass insulator surface by spraying. We studied microstructure, wettability, water droplet bouncing behavior, and anti-glaze icing properties of SHP coating. The results demonstrated that SHP coatings had micro-nano rough structures. The excellent superhydrophobicity (contact angle of 165.2° and sliding angle of 3.7°) was achieved. The water droplet was easily adhered to the surface of the glass insulators. At the same time, individual water droplets could bounce away from the surface after impacting the SHP coating. In the glaze environment, water droplets sprayed onto the SHP coating merged with each other and slid off the surface. These significantly reduce the likelihood of freezing. Furthermore, the SHP coating could dramatically delay the glaze icing and decrease the icing area. The icing weight and icicle length were smaller than glass insulators. The SHP coatings prepared in this work display great potential for the anti-icing of glass insulators.

Characteristics and mechanism of droplet bouncing on cross-ridge superhydrophobic surfaces: Simulations and theory

The behaviors of droplets impinging on superhydrophobic surfaces have received much attention from industry and academia due to potential applications such as anti-icing, spray cooling, and self-cleaning. Previous studies have shown that the superhydrophobic surfaces can significantly reduce contact time and thus effectively suppress surface icing and condensation phenomena. In this paper, the bouncing behaviors of droplets on superhydrophobic surfaces decorated with cross ridges were investigated using numerical simulations and theoretical analysis. The effect of cross-ridge structures with different pinch angles and droplets with different Weber numbers on droplet bouncing behaviors was investigated using the volume-of-fluid method. The results showed that the solid–liquid contact time was shortest when the angle between the two ridges was 75°. Compared to droplet bouncing on a smooth surface, the contact time was reduced by up to 30% for complete bouncing behaviors and up to 68% for broken bouncing behaviors. As the angle decreased, the ratio of spreading areas between child droplet sizes increased exponentially. The momentum of the smaller child droplet decreased until it was no longer generated. A theoretical analysis based on energy conservation was also performed. A theoretical equation was proposed to predict the maximum spreading coefficient. The maximum error was less than 10% for the experimental and numerical results compared to the predicted results.

Oriented bouncing of droplets with a small Weber number on inclined one-dimensional nanoforests.

Asymmetric superhydrophobic structures with anisotropic wettability can achieve directional bouncing of droplets and thus can have applications in directional self-cleaning, liquid transportation, and heat transfer. To achieve convenient large-scale preparation of asymmetric superhydrophobic surfaces, inclined nanoforests are prepared in this work using a technique of competitive ablation polymerization, which allows the control of the inclined angles, diameters, and heights of the nanostructures. In this study, such asymmetric structures with the smallest dimension (230 nm diameter) known are achieved by a simple etching method to guide droplet unidirectional bouncing. With such nanoforests, the mechanism of droplet bouncing on their surface is investigated, and controllable droplet bouncing over a long distance is achieved using droplets with a low Weber number. The proposed structure has a promising future in directional self-cleaning, liquid transportation and heat transfer.

Dynamic behaviors and heat transfer characteristics of impacting droplets on heated superhydrophobic surfaces with randomly distributed rough structures: Numerical simulation and theoretical analysis

Due to the extensive usage of superhydrophobic surfaces, numerous studies have investigated the dynamics and heat transfer of a droplet impacting on superhydrophobic surfaces. However, previous studies did not consider the actual superhydrophobic surfaces with randomly distributed rough structures, especially so in heat transfer of droplet impacting on superhydrophobic surfaces with random rough structures. To address this issue, a multiple distribution function phase-field lattice Boltzmann model was developed to model the droplet impacting on randomly rough surfaces. After experimental validation, this model was used in the analysis of the impacting dynamics and heat transfer characteristics. The differences between actual randomly rough surface and ideal smooth surface were revealed. Herein, random rough structures significantly influence the wetting state and heat transfer of impacting droplet on solid surface. Moreover, the effects of roughness on droplet bouncing ability and the total transferred heat were evaluated. The results show that the random rough surface with smaller skewness, a kurtosis of 3.0, and a standard deviation of 0.3 μm could simultaneously promote impacting droplet to rebound from the surface and to reduce the total transferred heat. Subsequently, the predictive correlation for total transferred heat was proposed based on the roughness parameters. This work provides novel insights into the design of rough structures on functional superhydrophobic surfaces.

Visualization of Charge Dynamics when Water Droplets Bounce on a Hydrophobic Surface.

Visualizing the motion of water droplets and understanding their electrification behavior holds significance for applications related to droplet transport, self-cleaning, and anti-icing/deicing and for providing a comprehensive explanation of the solid-liquid triboelectrification mechanism. Here, by constructing microcolumnar structures on the polytetrafluoroethylene surface, a water droplet-based single electrode triboelectric nanogenerator was fabricated for visualizing charge dynamics when a water droplet bounces on a hydrophobic surface. The motion state of the water droplet is closely linked to its electrification behavior through the integration of a high-speed camera and an ammeter. The electrification behavior stemming from the bounce of the water droplet is dynamically captured in real-time. The results show that the magnitude and polarity of the electrical signal have strong dependence on the motion state of the water droplet. For instance, when a water droplet approaches or moves away from the substrate in a single direction, a unipolar electrical signal is generated. However, when the water droplet reaches its limit in the initial motion direction, it signifies a static equilibrium state, resulting in the electrical signal being at zero. Furthermore, we examine the impact of factors such as impact speed, drop contact area, contact line spreading/retraction speed, and impact angle on electrification. Finally, based on the close relationship between poly(ethylene oxide) (PEO) droplet bounce dynamics and electrical signals, the bouncing details of PEO droplets with different concentrations are tracked by electrical signals. This study digitally presents the whole process of droplet bounce in situ and provides a means for monitoring and tracking droplet movement.

Magneto-responsive wettability-switchable surfaces for in-situ large distance droplets capturing-releasing-bouncing

In this paper, superhydrophobic magnetically responsive micro-plate arrays (SMMAs) with reverse switching wettability were studied for in-situ capturing, releasing, and directional bouncing of droplets over a large distance. A novel method for producing superhydrophobic pattern on one side surface of the micro-plates by laser texturing at 90 degree bending state, created water contact angle of 151°, a rolling angle of < 10°, and a maximum surface adhesion force of 35.5 μN. Reversible switching between droplets (volume 5–40 μL) capturing with high adhesion force (high surface energy) and droplet releasing with low adhesion force were achieved spontaneously. The instantaneous response of the SMMAs to magnetic field provided a guarantee for the directional bouncing of droplets in long distance. Droplets transportation up to 40 μL, directional bouncing of 5 μL droplets with a vertical bounce height of 4.5 mm and a horizontal bounce distance of 24.7 mm have been achieved. This work provides a novel approach for flexible droplets manipulation and multifunctional operation of droplets for non-contact, remote-driven and non-destructive reagent transfer applications.