Droplet Spreading Research Articles

How tank-mix adjuvant type and concentration influence the contact angle on wheat leaf surface.

Currently, the utilization of unmanned aerial vehicles (UAVs) for spraying pesticides is a prevalent issue in Asian countries. Improving the pesticide efficiency of UAV spraying is a major challenge for researchers. One of the factors that affect the efficiency is the wetting property of the spraying solutions on crop leaves. Tank-mix adjuvants, which can modify the wetting ability of the solutions, are often used for foliar application. However, different types and concentrations of tank-mix adjuvants may have different impacts on the wetting properties of droplets. In this article, we investigated the effects of four tank-mix adjuvants, Beidatong (BDT), Velezia Pro (VP), Nongjianfei (NJF), and Lieying (LY), on the dynamic contact angle (CA) values of droplets on the adaxial surface of wheat leaves. We measured the dynamic CA values of various concentrations of each adjuvant solution and determined the optimal concentrations based on the CA values, droplet spreading time, and cost. The results showed that adding any of the four adjuvants decreased the CA values, but the patterns of decrease varied among them. The CAs of BDT and VP solutions decreased slowly during the observation time (0-8.13 s), while those of NJF and LY solutions decreased rapidly throughout the observation period. According to the dynamic CA values of different concentrations, the optimal concentrations of BDT, VP, NJF, and LY for wheat field application were 12%, 16%, 6‰, and 0.3‰, respectively. Alkoxy-modified polytrisiloxane adjuvant (LY) could be recommended as an appropriate tank-mix adjuvant for wheat field application, considering spreading efficiency and cost. This study provides theoretical and practical guidance for selecting and optimizing tank-mix adjuvants for UAV spraying.

Coupled lattice Boltzmann method–discrete element method model for gas–liquid–solid interaction problems

In this paper, we propose a numerical model to simulate gas–liquid–solid interaction problems, coupling the lattice Boltzmann method and discrete element method (LBM–DEM). A cascaded LBM is used to simulate the liquid–gas flow field using a pseudopotential interaction model for describing the liquid–gas multiphase behaviour. A classical DEM resorting to fictitious overlaps between the particles is used to simulate the multiple-solid-particle system. A multiphase fluid–solid two-way coupling algorithm between LBM and DEM is constructed. The model is validated by four benchmarks: (i) single disc sedimentation, (ii) single floating particle on a liquid–gas interface, (iii) sinking of a horizontal cylinder and (iv) self-assembly of three particles on a liquid–gas interface. Our simulations agree well with the numerical results reported in the literature. Our proposed model is further applied to simulate droplet impact on deformable granular porous media at pore scale. The dynamic droplet spreading process, the deformation of the porous media (composed of up to 1277 solid particles) as well as the invasion of the liquid into the pores are well captured, within a wide range of impact Weber number. The droplet spreading dynamics on particles is analysed based on the energy budget, which reveals mechanisms at play, showing the evolution of particle energy, surface energy and viscous dissipation energy. A scaling relation based on the impact Weber number is proposed to describe the maximum spreading ratio.

Experimental study on impact patterns and dynamics characteristics of NaCl water droplet with different concentrations on heated surface

This paper reports the experiments for NaCl water droplet with various concentrations impact on heated surface (50 ∼ 400 °C) at Weber numbers from 67 to 101. The impact patterns, spread diameters, residual energy and rebound times were studied in detail. The results show that the patterns can be divided into deposition, partial rebound, rebound, breakup and jet ejection for droplet with different concentrations impact on heated surface, and mapped the distribution of patterns based on the Tw/TsatlgWe and Tw/TsatlgRe parameters. In the deposition region, stable spreading diameter decreases with increasing concentration. At high surface temperatures, high concentration droplets boil more violently due to the intense Marangoni effect and the fluid heat transfer coefficient increases. Moreover, the models of the relationships between the dimensionless maximum spreading factor and Weber number and Reynolds number were established, which optimizes the prediction of spreading of droplets with different concentrations. Based on the energy conservation equation, residual energy was found to decrease with increasing concentration. Rebound time decreases with increasing concentration. Moreover, droplet breakup is resulted from boiling bubbles and lateral splashing of atomized droplets. Besides, the NaCl water droplet is more likely to breakup and jet ejection compared to pure water droplet.

Experimental investigation on the dynamics of a single water droplet impacting wood surface

The dynamic characteristics of water mist droplets hitting the wood surface is one of the important factors affecting the fire extinguishing performance. However, the spreading dynamics between water droplets and wood surfaces during the impacting process have not been adequately characterized in existing literature. This study experimentally investigates the dynamic process of a single water droplet with changing Weber numbers from 66.7 to 1268.5 impacting different solid wood surface from the perspective of fire suppression. Typical impacting phenomena, the phenomena distribution, the droplet spreading dynamics and the maximum spread factor have been analyzed and discussed. Typical phenomena including no splash, transition from no splash to prompt splash, and prompt splash with varying Weber number are observed for the wood surface. The typical phenomena distribution can be distinguished by lgWe and lgRe and the result shows that lgWe < 2.11, 2.11 < lgWe < 2.42, and lgWe > 2.42 correspond to the phenomena of no splash, transition from no splash to prompt splash and prompt splash respectively. The spreading time can be predicted by the expression of ts/(ρRmax3/σ)0.5=6.52∗We-0.92. By analyzing the maximum spreading factor, the model βmax∝(We/Oh)0.198 is found to present good predictions for wood surfaces.

Wetting and spreading of AgCuTi on Fe substrate at high temperatures: A molecular dynamics study

In this study, the wetting and spreading behavior of AgCuTi ternary alloys on Fe substrate was investigate using a molecular dynamics (MD) modeling method. Modeling indicates that precursor films consisted of single-layer atoms are formed and that the wetting and spreading morphologies of the molten droplets on various crystal surfaces are distinct. Diffusive processes play a key role in the formation of the precursor film, and the droplet spreading morphology is correlated critically with the atomic arrangement of the substrate. The droplet spreading process can be divided into two stages of rapid and slow spreading, and there is an obvious interdiffusion phenomenon close to the solid-liquid contact line. The rate at which droplets spread quickens as the temperature rises. Furthermore, the spreading and diffusion capacities of various elements vary, and these properties are closely related to their chemical reactivity.

Review: the risks of spray adjuvants to honey bees.

Pesticide applications are often made as tank mixes containing multiple pesticide products and may include spray adjuvants to enhance pesticidal activities. The primary aim of adjuvant products is to increase the spreading and sticking of spray droplets and to increase the penetration of active ingredients through the cuticles of leaves or targeted pests, which can reduce the amount of active ingredient needed for effective pest control. Adjuvants are made up of compounds drawn from the "inert ingredient" list maintained by EPA but are identified as "principal functioning agents" when used in adjuvant products. These inert compounds do not undergo the same testing and risk assessment process that is required of pesticide active ingredients and generally have no mitigation measures that prevent application onto crops during bloom at times of day when bees are foraging. Honey bees (Apis mellifera;Hymenoptera:Apidae) are at an increased risk of exposure to adjuvant tank mixtures while providing agricultural pollination services. Colony losses attributed to pesticide applications thought to have low risk to honey bees have been reported, highlighting the need to better understand the toxicity of adjuvants included in pesticide tank mixtures. This review summarizes current literature on the risks posed to honey bees by agricultural adjuvants and tank mix combinations of adjuvants with pesticides. Based on the current state of knowledge, we make recommendations to pesticide applicators, product manufacturers, regulatory agencies, and researchers regarding adjuvant toxicity to honey bees with the goal of reducing risks that adjuvants pose to honey bees and other beneficial insects.

Investigation of optimized spraying process for directly coated electrode in polymer electrolyte membrane fuel cell

Membrane electrode assembly (MEA) is a core component in polymer electrolyte membrane fuel cells (PEMFCs). A well-constructed catalyst layer is essential for achieving high-performance and reliable fuel cells. Among MEA fabrication processes, direct spray coating has advantages regarding cost-efficiency, processability, and controllability compared to direct sputtering and decal transfer method. In the spray process, the morphology of the catalyst layer depends on conditions in which ink droplets are sprayed and evaporated. Although quantitatively optimizing the spray process is crucial, there is still a lack of well-defined research for optimized spray process. Herein, we propose k-value that can indicate uniform coverage, expressed as a ratio of the area where ink droplets spread compared to the sprayed area. The k-1.0 MEA is fabricated under k-value conditions of one by adjusting nozzle moving speed, which is expected to have the most uniform coverage. Compared to k-0.3 MEA and k-3.0 MEA, the power density improves by 22.3 % and 11.1 %, respectively. This is attributed to the reduced ion transport resistance of the catalyst layer being reduced by 23.7 % and 8.5 % and the increased electrochemical active surface area by 16.1 % and 9.4 %, respectively. Furthermore, the morphology of catalyst layers and substrate temperature effect are investigated.

Fluid Mechanics of Droplet Spreading of Chitosan/PVA-Based Spray Coating Solution on Banana Peels with Different Wettability.

The spreading behavior of a coating solution is an important factor in determining the effectiveness of spraying applications. It determines how evenly the droplets spread on the substrate surface and how quickly they form a uniform film. Fluid mechanics principles govern it, including surface tension, viscosity, and the interaction between the liquid and the solid surface. In our previous work, chitosan (CS) film properties were successfully modified by blending with polyvinyl alcohol (PVA). It was shown that the mechanical strength of the composite film was significantly improved compared to the virgin CS. Here we propose to study the spreading behavior of CS/PVA solution on fresh bananas. The events upon droplet impact were captured using a high-speed camera, allowing the identification of outcomes as a function of velocity at different surface wettabilities (wetting and non-wetting) on the banana peels. The mathematical model to predict the maximum spreading factor, βmax, was governed by scaling law analysis using fitting experimental data to identify patterns, trends, and relationships between βmax and the independent variables, Weber (We) numbers, and Reynolds (Re) numbers. The results indicate that liquid viscosity and surface properties affect the droplet's impact and spreading behavior. The Ohnesorge (Oh) numbers significantly influenced the spreading dynamics, while the banana's surface wettability minimally influenced spreading. The prediction model reasonably agrees with all the data in the literature since the R2 = 0.958 is a powerful goodness-of-fit indicator for predicting the spreading factor. It scaled with βmax=a+0.04We.Re1/3, where the "a" constants depend on Oh numbers.

Corrosion-driven droplet wetting on iron nanolayers

The classical Evans’ drop describes a drop of aqueous salt solution, placed on a bulk metal surface where it displays a corrosion pit that grows over time producing further oxide deposits from the metal dissolution. We focus here on the corrosion-induced droplet spreading using iron nanolayers whose semi-transparency allowed us to monitor both iron corrosion propagation and electrolyte droplet behavior by simple optical means. We thus observed that pits grow under the droplet and merge into a corrosion front. This front reached the triple contact line and drove a non radial spreading, until it propagated outside the immobile droplet. Such chemically-active wetting is only observed in the presence of a conductive substrate that provides strong adhesion of the iron nanofilm to the substrate. By revisiting the classic Evan’s drop experiment on thick iron film, a weaker corrosion-driven droplet spreading is also identified. These results require further investigations, but they clearly open up new perspectives on substrate wetting by corrosion-like electrochemical reactions at the nanometer scale.

Droplet absorption and spreading into thin layers of polymer hydrogels

From biological tissues to microactuators and absorption of solvents into layers of paint, macroscopically non-porous materials with the capacity to swell when in contact with a solvent are ubiquitous. In these systems, owing to strong solid–fluid interactions, chemically driven flows can yield large geometric changes. We study experimentally and theoretically the canonical problem of the swelling of a thin hydrogel layer by a single water drop. Using a bespoke experimental set-up, we observe fast absorption leading to a radially spreading axisymmetric blister. We use a fully three-dimensional linear poroelastic framework with nonlinear kinematic equations to obtain governing equations, which we then reduce with thin-layer scalings to a one-dimensional nonlinear diffusion equation for the evolution of the blister geometry. In the limits of large and small deformations, the evolution of the blister characteristic height and radius are self-similar, following power laws in time. Our experimental measurements show that the evolution of the blister is broadly within the theoretical predictions in the large and small deformation regimes. In the general intermediate deformation regime, the measurements are well captured by our reduced one-dimensional diffusion model, which does not require the sophisticated and computationally expensive numerical approaches necessary for the original two-dimensional nonlinear coupled transport problem. By adapting modelling techniques from the fluid dynamics of thin porous elastic layers to a polymer swelling problem, our modelling framework extends the range of polymer swelling problems that can be treated with semi-analytical methods. Moreover, our detailed experimental data can serve as a test case for future nonlinear poroelastic frameworks of swelling polymer materials.

Ecofriendly Nanoemulsion to Promote the Wetting and Spreading of Droplet on Soybean Leaves for Regulating the Sheath Blight

Ecofriendly Nanoemulsion to Promote the Wetting and Spreading of Droplet on Soybean Leaves for Regulating the Sheath Blight

Sliding Behavior of Droplets on a Tilted Substrate with a Chemical Step.

Controlling and predicting the motion of droplets on a heterogeneous substrate have received widespread attention. In this paper, we numerically simulate the droplet sliding through a "chemical step", that is, different wetting properties at two sides of the step, on a tilted substrate by the multiphase lattice Boltzmann method (LBM). Three kinds of equilibrium statuses are reproduced by observing the deformation of the droplet and the velocities of the front contact line. This study shows the droplet obtains a driving force to break through the step by deformation in the initial stage that the droplet is blocked. The droplet spreads to two sides along the step when the front end is blocked and is stretched after the front end is passed over the step. The lengths of the lateral spreading and the longitudinal stretching and the time required to pass over the step depend on the strength of the step. In the sliding process, the kinetic energy is converted into surface energy as the droplet is blocked, and the gravitational potential energy is converted into surface and kinetic energy following the droplet passes over the step. If the droplet can slide through the step, the more strength in the step, the more the gravitational potential energy is converted, and the more the surface energy increases. When the strength of the step is small, unbalanced Young's force hinders the contact line moving forward after the central part of the front end of the droplet breaks through the step. While the velocity of droplet sliding slows down with the increasing strength of the step, the unbalanced Young's force pushes the contact line forward against the resistance. These observations throw insight into the dynamics of the droplets sliding on a heterogeneous surface, which may facilitate potential applications like microfluidics and liquid transportation.

Virus spreading in cruiser cabin

This paper concerns virus droplet simulations in a typical cruiser's cabin. Effects of ventilation rates and positions of the coughing person were investigated. The study also emphasizes the importance of including evaporation models to simulate the process accurately. A higher ventilation rate is not always the best strategy to avoid the spread of airborne diseases, as saliva droplets can spread further at high ventilation rates. Regardless of the ventilation strategy, they evaporate faster than the room's air renewal. One should aim for minimum droplet spreading inside the cabin and different ventilation strategies for occupied cabins. The authors propose using ventilation systems at medium flow rates of around 120 m3/h or three air changes per hour when a cabin is occupied. This value is also close to the recommended value of 108 m3/h from the latest standard by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The suggested value minimizes droplet spreading while maintaining good ventilation, comfort, and energy consumption.

Wetting Behavior of CO2 Droplets on Smooth Solid Surface: Molecular Simulation Perspective

The wettability of droplets on solid surfaces is important for accurately revealing the microscopic mechanisms of gas condensation nucleation and droplet growth. During the contact condensation of CO2 gas on the heat exchanger surface in the pressurized liquefied natural gas technology, the wettability of CO2 droplets on the heat exchanger surface directly affects the heat transfer thermal resistance of the heat exchanger, which then affects the heat transfer efficiency of methane and ethane in the heat exchanger. Therefore, molecular dynamics simulations were used to study the spreading process and wetting patterns of nanoscale CO2 droplets on different energy surfaces. The results show that as the potential well depth ε of the wall atoms increases, the intensity of the solid-liquid interaction increases and the corresponding surface energy increases accordingly, showing different droplet spreading rates and wetting characteristics. Unlike the interfacial characteristics of macroscopic droplets, there are significant fluctuations at the gas-liquid interface of droplets on the molecular scale, but microdroplets can still form a specific contact angle after spreading on different energy surfaces in a statistical sense, and this contact angle decreases with increasing intensity of solid-liquid interaction. The low-energy surface at potential well depths ε less than 266 J·mol-1 exhibits a CO2-phobicity, and the surface becomes CO2-philic as the potential well depth continues to increase. The trend of the contact angle of CO2 droplets affected by temperature is the same as that of the center-of-mass height, which characterizes the spreading morphology of CO2 droplets. As the temperature increases, the contact angle decreases due to the further spreading and wetting of droplets on different energy surfaces. As the CO2-philicity of the surface gets higher, the contact angle decreases to a greater extent.

Dilute oil-in-water emulsion initiates multiple holes formation during bubble bursting

The air bubble bursting dynamics involve intricate interfacial phenomena of millisecond timescale, which are of interest in many natural and industrial processes. The current study concerns bubble containing oil droplets with size comparable to bubble cap thickness. The assumption is that the dynamics of bubble bursting and aerosol formation will be modified as oil droplets interact with the air-water interface during drainage. Indeed, the experiments show that with the increase of the droplet spreading coefficient (S), the average film retraction speed decreases and the average film thickness increases at breakup. Notably, as S reaches 3.6 mN/m, a distinct multiple hole formation phenomenon has been observed for the first time during bubble bursting. The frequency of multiple hole formation increases with increasing S, and is as high as 22 holes per bursting event observed when S = 21.3 mN/m. The multiple holes evolve with time and form convoluted ligament structures that finally break up into much more aerosolized droplets. However, after normalization, all droplet size distribution is well predicated by a Gamma distribution.The results highlight the importance of the interfacial physiochemistry for the bubble cap rupture, regarding the wetting state between the oil droplets, air, water phase.

Understanding under-liquid drop spreading using dynamic contact angle modeling

We present numerical modeling and experimental data to investigate droplet spreading dynamics on a substrate submerged in another liquid. Dynamic contact angle equations are formulated and used to simulate the spreading of different oil drops in the presence of water. Drop spreading in the presence of water starts with a viscosity-dominated regime where a scaling law of r∼t is observed. For low viscosity drops, an intermediate inertial regime with scaling r∼t12 is observed after the viscous regime. For high viscosity drops, such an intermediate regime diminishes. At later stages near equilibrium, the spreading dynamics follow Tanner's law for all drops.

Dynamic droplet morphology measurement method based on digital image correlation and refraction effect

The measurement and analysis of the droplet morphology on a solid surface play a crucial role in investigating solid–liquid interactions and characterizing the properties of surfaces. The contact angle is a widely used parameter in characterizing the droplet, but it cannot well describe the irregular shapes, which can be easily found in the investigations of the spreading and evaporation of the droplet. In this study, a novel method for three-dimensional measurement of the transparent droplet morphology is developed by combining droplet-induced image displacement with the principles of refraction imaging. First, a full-field optical method is employed to measure the image displacement of the solid surface caused by the refraction of the droplet. Second, a physical model is established based on refraction imaging, so the relationship between image displacement and the morphology of the droplet is formulated. Finally, the three-dimensional surface of the droplet is reconstructed by resolving the physical formulations. Experimental results demonstrate that the proposed method can accurately measure the three-dimensional morphology of droplets on solid surfaces and is applicable for irregular liquid surfaces. This method provides important technical support for studying droplet spreading, evaporation, and other related behaviors.

Effect of attack angles on droplet impact on textured metal surfaces: Droplet morphisms, surface tilt angles, droplet deformation

The experimental results of the water droplet impact on the tilted metal substrates are given. To establish the effect of the texture on the spreading of droplets, parallel-directed scratches were applied using a grinding machine. To analyze the shape of water droplets after impingement, topological-based data analysis (TDA) was used. The TDA showed that the droplet could be represented as a group of trivial topological structures: spheres, discs, and hemispheres. The latter creates the prerequisites for the development of relatively simple mathematical models that describe the complex processes of liquid droplets impacting on tilted, textured substrates. Moreover, the time dependence of the droplet contact line (L) and the left and right contact angles (LCA and RCA) on a textured surface were established. It was shown that anisotropic modification of the substrate has a significant effect on the characteristics and conditions of the spreading of the coolant droplets.

Prince Rupert's Drop bouncing on high-speed moving superhydrophobic surfaces

Superhydrophobic surfaces have shown huge potential in diverse fields. However, the hydrodynamics of droplet impacting on moving superhydrophobic surfaces are largely unexplored, especially for surfaces with a high moving velocity. The knowledge gap in liquid-solid interaction between impacting droplets and high-speed moving superhydrophobic surfaces should be filled to better guide practical engineering applications. Here, a superhydrophobic surface is fabricated as the experimental surface by a chemical deposition-etching method and droplet impacting experiments on horizontally moving superhydrophobic surfaces (realized by impacting the edge of a rotating surface) are systematically conducted. The moving velocity of superhydrophobic surfaces reaches nearly 10.0 m/s, which is approximately three times the velocity in previous relevant studies. The results show that the droplet spreading is greatly enhanced when the surface velocity exceeds a critical value, and it is independent of the droplet Weber number (We). The droplet detaches from the surface in the shape of “Prince Rupert's Drop”, reducing the contact time by over 40%. The critical surface velocity is characterized by We/Wes = 0.17, where Wes is the Weber number of the moving surface. To reveal the dynamics of Prince Rupert's Drop bouncing, scaling laws of the maximum spreading coefficient and the contact time are developed, which agree well with experimental results.

Spreading of a thin droplet on a soft substrate

A thin liquid droplet spreads on a soft viscoelastic substrate with arbitrary rheology. Lubrication theory is applied to the governing field equations in the liquid and solid domains, which are coupled through the free boundary at the solid–liquid interface, to derive a set of reduced equations that describe the spreading dynamics. Fourier transform techniques and the finite difference method are used to construct a solution for the dynamic liquid–gas and solid–liquid interface shapes, as well as the macroscopic contact angle. Substrate properties affect the spreading dynamics through the contact angle and internal droplet flow fields, and these mechanisms are revealed. Increased substrate softness increases the spreading rate, whereas increased viscoelasticity decreases the spreading rate. For the case of a purely elastic substrate, the spreading power-law exponent recovers Tanner's law in the rigid limit and increases with substrate softness.