Droplet Impact Research Articles

Sound of natural and artificial rain impact on ETFE membranes

Thin membranes are known to produce a drum-like effect when subjected to rainfall. Rain noise affects acoustic comfort and might disrupt communication, speech intelligibility and cognitive task performance. Therefore, assessment of the characteristics of noise produced by impact of rain on membranes in different architectural settings is important. Given inherent disparities between natural and artificial precipitation-primarily in terms of droplet size distribution and terminal velocities-underscores the importance of conducting evaluations using natural rain noise. The focus of this study is on the comparative assessment of the sound generated by natural rainfall versus the sound generated by the impact of individual droplet in a semi-anechoic room on single Ethylene tetrafluoroethylene (ETFE) membranes. The study assesses the extent of spectral variations of the rain sound produced in both approaches and the corresponding absolute value of noise intensity.

Numerical simulations of freezing behaviors of water droplets impacting cold hydrophobic surfaces

Freezing, as a prominent phase transition phenomenon, occurs frequently in nature and industry. This study employs a combination of the Volume of Fluid(VOF) model, solidification model, and a dynamic contact angle model to simulate the dynamics and solidification process of droplets impacting supercooled surfaces. Two distinct freezing morphologies, central-concave and central-convex, are observed. These different morphologies are attributed to the competition between the time scales associated with droplet impact dynamics and solidification (retraction time tr and solidification time tf,p). Thus, we propose a scaling law to predict the occurrence of different morphologies: when tr < tf,p, the final freezing morphology exhibits a central-convex shape, while when tr > tf,p, it exhibits a central-concave shape. In order to explore effects of nucleation on solidification, the classical nucleation theory is adopted to determine the nucleation time, and thus the freezing behaviors can be well captured. Furthermore, we examine the effect of volume expansion during phase transition on the freezing behaviors, and find that the increase in freezing time due to volumetric expansion can be well predicted by the scaling law tf ∼ Δ(H2)/ΔT.

Analysis of droplet motion in cavity zone of rotating packed bed

Droplet motion in outer cavity zone of rotating packed bed is a complex phenomenon that requires analysis of liquid and gas flow to clarify influences of factors such as rotation speed, liquid flow rate, or gas flow rate. In this study, hydrodynamics is described using laser Doppler velocimetry and high-resolution visualization for droplet motion characterization. The results reveal that droplet motion initially depends on the peripheral velocity of the packing, while further from the packing, it loses kinetic energy mainly due to the drag force caused by the surrounding gas. Two-way coupling is investigated as both phases interact with each other. Rankine vortex theory is applied to simulate gas flow in the outer cavity zone. A model, based on Newton’s second law, is developed to predict droplet dynamics, including impingement velocity, trajectory length, and total residence time. Validation of the model shows that it can predict the liquid velocity in the outer cavity zone accurately. Upon impingement of droplets on the casing, part of the liquid rebounds as secondary droplets into the cavity, where they circulate with the gas flow. Overall, this study provides insights into droplet dynamics in the rotating packed bed and offers a basis for optimizing process efficiency and design in various applications, including chemical processing and environmental engineering.

Implementation of a high-frequency phosphor thermometry technique to study the heat transfer of a single droplet impingement

Contributing to a better understanding of spray cooling systems, the heat transfer process underlying the event of a droplet impinging onto a uniformly heated stainless steel surface (SS304) was experimentally investigated. Since the heat transfer process is linked to the droplet’s hydrodynamics, high-speed videos were recorded to measure the deformation of the droplet. A series of isothermal and non-isothermal impacts were performed for Weber numbers (We) within the range 17.7≤We≤58.2. A strong relationship between the maximum spreading ratio reached by the droplet and its initial kinetic energy was found. The surface temperature directly affects the droplet hydrodynamic during the impact by promoting an oscillatory behavior of the droplet after the maximum spreading is reached. Given the spatial–temporal resolution of the heat transfer process, a high-frequency phosphor thermometry technique was implemented, finding that the temperature drop upon droplet impact was independent of impact velocity. The sharp temperature drop results in an intense thermal interaction that occurred during the first 10 ms of the impact. The maximum average heat flux registered was 98.56 W/cm2 with a cooling effectiveness of 3.5%.

Investigation into the mechanism of ignition of magnesium alloy plates by high-temperature molten droplet

The ignition and spread of magnesium alloy fires are mainly attributed to the ignition of high-temperature molten metal droplets, but the underlying ignition mechanism remains unclear. This study addresses this knowledge gap by employing metallic aluminum and copper particles heated to temperatures over 1200 °C. Systematically, these heated particles were released at a fixed rate onto a magnesium alloy plate located inside a controlled heating furnace that maintained a constant temperature of 500 °C. Experimental results show that whether there are irregular pores and cracks in the resulting molten product can be used as a criterion for judging whether the magnesium alloy sample ignites. Furthermore, it is worth noting that molten copper droplets of the same size exhibit a higher heat-carrying capacity than their aluminum counterparts when both are heated to the same temperature. In addition, this study expanded the research scope through numerical simulation and analysis, using the PATO (Porous-Material Analysis Toolbox) solver to clarify the transient heat conduction process between the high-temperature droplet and the magnesium alloy plate. Simulations show that heat transfer upon droplet impact is primarily longitudinal, directed toward the base of the material. This results in a significantly higher temperature in the center of the affected magnesium alloy plate than at its periphery. In summary, this study helps to improve the understanding of the dangers of high-temperature molten droplets, and it is of great significance to investigate fire accidents caused by high-temperature molten droplets.

Numerical and theoretical modeling of water droplet impact on hydrophilic and superhydrophobic cones

The phenomenon of droplet impingement on solid surfaces is prevalent in various natural and industrial contexts. Research on impact dynamics on conical surfaces keeps emerging, with superhydrophobic cones receiving more attention than hydrophilic ones. This study systematically investigates water droplet impact dynamics on both hydrophilic and superhydrophobic cones using a two-phase numerical solver under different Weber numbers (We) and cone angles (φ). Three distinct phases are identified in the We–φ map to describe the different outcomes on each type of cones. Generally, deposition occurs ultimately on hydrophilic cones, whereas rebounding is observed on superhydrophobic ones. The maximum spreading area βAmax on hydrophilic cones depends only slightly on φ but consistently increases with We, following a scaling law of We0.5 at higher We. In contrast, on superhydrophobic cones, βAmax increases significantly with both We and φ, and the exponent in the scaling laws of βAmax with respect to We increases notably as φ increases. Three characteristic times are defined to describe important motion states on both types of cones. Corresponding scaling laws for each time with We are established. Two theoretical models are developed to predict the maximum spreading position for droplets on hydrophilic cones and the rebound position on superhydrophobic cones, respectively. Gravitational potential energy is included in the energy budget for both models, and an auxiliary viscous dissipation due to spontaneous spreading is accounted for the hydrophilic case. Satisfactory agreement between the theoretical and numerical results is achieved.

Investigation of droplet splashing behavior during oblique jet impact onto a wall

For the issue of jet impingement on the wall in industrial cooling processes, an experimental setup based on high-speed photography for oblique jet impingement onto the wall was constructed. The experimental focus was on the study of liquid droplet splashing behavior after oblique jet impingement on the wall, discussing the liquid droplet splashing behavior under three jet impingement modes: Rayleigh regime, first wind-induced regime, and secondary wind-induced regime. By employing methods such as trajectory imaging and particle image velocity for liquid droplet parameter image measurement, obtaining the particle size, velocity, and distribution of splashed droplets after oblique jet impact on walls under different working conditions. The impact of jet impingement velocity and angle on droplet splashing parameters was analyzed. The results showed that when the impingement point is before the breakup length, with increasing flow velocity, the surface wave of the liquid column, and the spreading liquid film became more pronounced, but the loss of liquid-phase components due to splashing was relatively small. When the impingement point is after the breakup length, the secondary breakup resulting in a “crown”-shaped liquid film after droplet impingement leads to a significant loss of liquid-phase components through splashing. As the inlet velocity of the jet increases, there is a decreasing trend in droplet size and an increasing trend in droplet velocity. With an increase in jet angle, there is a decreasing trend in droplet size and velocity. Based on the concentration, size, and velocity distribution characteristics of splashing droplets, the area after oblique jet impingement on the wall can be divided into the impingement zone, low-concentration low-velocity zone, high-concentration high-velocity zone, and lateral splashing zone. This has significant implications for understanding the splashing mechanism after oblique jet impingement on the wall and optimizing operating conditions.

Numerical simulation of a droplet impact onto a curved liquid film

Numerical simulation of a droplet impact onto a curved liquid film

Direct numerical simulation of droplet impact onto dry stationary and moving walls at low to high Weber numbers

The accurate prediction of the drop-wall impact dynamics and regime are of interest in many engineering applications. Considerable progress has been made in different elements of Direct Numerical Simulation (DNS) approaches. However, identifying the optimal combination of improved submodels that perform well under a wide range of operating conditions, covering low to high Weber numbers with varying surface wettabilities and wall motions, still requires further investigation. Here, we conduct a comparative study on the performance of key DNS elements, including the Interface Tracking (IT) algorithm for the widely-used volume-of-fluid approach, Contact Angle (CA), Contact-Line Velocity (CLV), Instability Actuation (IA), and numerical discretization. The study spans a broad range of Weber numbers, encompassing various impact phenomena such as drop oscillation, partial rebound, fingering, and splash. It is concluded that the Kistler CA model provides the most accurate predictions among the models considered here. In terms of IT, the Multidimensional Universal Limiter with Explicit Solution (MULES) algorithm is the most efficient one, especially for moderate to high Weber numbers. For high Weber numbers, involving the fingering instability and splash events, while applying the IA mechanism slightly improves the results, using a high-quality fine-enough grid and appropriate numerical discretization scheme to control the dispersion and dissipation errors are more important ingredients. It is shown that the recommended model combination is also able to predict the important features of drop impact on moving walls with reasonable accuracy.

Experimental study on the influence of surface properties on droplet collision dynamics, from adhesion to rebound to breakup

Liquid droplet impact on dry surfaces often results in bouncing or breakup beyond a certain threshold. Surface contact angles, especially dynamic ones present during impact, significantly affect this process. Our experimental study underscores that advancing and receding contact angles influence droplet behaviors like rebounding and different types of breakup. This discovery provides new insights and criteria for understanding liquid droplet impact on surfaces. Special characteristics were found in the breakup on microstructured surfaces: the size of fractured droplets notably decreases, and the spreading–breakup occurs more easily and earlier. Additionally, microstructured surfaces reduce contact time to some extent. Furthermore, the uniqueness of oblique impacts is mainly reflected in how they lower the threshold of the receding contact angle for rebound. Studying the correlations and differences in droplet rebound and breakup related to these surface characteristics will contribute to improving research on liquid–solid interactions and the design of hydrophobic surfaces, including microstructured surfaces.

Study of the Impact of Droplets on Superhydrophobic Surfaces

The potential applications of superhydrophobicity for self-cleaning, microfluidic systems, and so on, inspired by the “lotus leaf effect”, have recently attracted great attention from academia and industry. To date, however, neither experimental nor theoretical studies have reached the scalable application of superhydrophobic surfaces, and no simple methods have been developed to produce durable superhydrophobic surfaces. In this project, conical, cylindrical, and cube microstructures with different shapes and dimensions were prepared by 3D printing. Among these structures, the conical microstructures without further modification exhibited the best hydrophobic performance with a contact angle of 120°. However, the hydrophobic performance of the cube microstructures was significantly improved with the addition of an FAS coating to the surface, reaching 131°. This indicates that, with the help of the coating, the 3D printed materials with special features can be transformed into hydrophobic surfaces. The outcome of this project will be useful in promoting the efficient and low-cost preparation of superhydrophobic functional surfaces and their engineering applications.

Dynamics of impact of polymer droplets on viscoelastic surfaces

Dynamics of impact of polymer droplets on viscoelastic surfaces

Influence of hollow droplet impact and spreading characteristics on the cylindrical lateral surface

In this study, 3D model of a hollow glycerin droplet impacting on a cylindrical cross-section was numerically analyzed to coat the lateral surface of the cylinder. The hollow droplets had exterior diameters of 5.5 mm, 5.25 mm, and 5 mm, and the impact velocity was 1 m/s. To model, the influence of droplets on OpenFoam software, the Volume Of Fluid technique was utilized (VOF). The Newtonian, incompressible, and laminar fluid phase of a glycerin droplet was investigated using air with a diameter of 4 mm as the gas phase. The hydrodynamic behavior of droplet collisions as well as fluid properties were investigated. As a result, when can be observed in the simple cylinder, as the outer diameter increased, the spread diameter shrank. The least amount distributed diameter in Case 3 was 1.404, whereas the highest amount in Case 4 was 1.625. The situations with a simple lateral surface, uniform spread occurred, while in cases with a spiral lateral surface, non-uniform spread occurred. Observed the maximum length in Case 3 was 4.12 mm and the minimum was 1.83 mm in Case 4.

Spreading of droplet impact on ribbed superhydrophobic surfaces with varying structure height

Hypothesis:Quantifying droplet spreading characteristics on ribbed superhydrophobic surfaces is challenging due to the complex fluid dynamics, limiting the practical applications. Droplet spreading dynamics are significantly influenced by the diameter ratio (rib diameter to droplet diameter) and height ratio (structure height to rib diameter) during impact. Experiment:Ribbed superhydrophobic surfaces with diameter ratios (0.18 to 0.75) and height ratios (0.125 to 0.5) underwent impact experiments with droplets at varied Weber numbers. Through simulations and analysis, we summarized the patterns of the maximum ribwise spreading coefficient, elucidated the droplet spreading mechanism on ribbed superhydrophobic surfaces. Findings:The findings revealed asymmetry in impact dynamics on ribbed superhydrophobic surfaces in the ribwise and radial directions. The maximum ribwise spreading coefficient initially decreases, followed by increase with increasing diameter ratio, decreasing in a slowing down trend with increasing height ratio. These observations indicated that maximum ribwise spreading coefficient can be minimize with critical ratios. Comparatively, the maximum spreading coefficient at critical height ratios is 72.7% to 92.3% of that on flat surfaces. To precisely forecast maximum ribwise spreading coefficient, two models are introduced for diameter ratios below and above critical range when height ratios below the critical range.

Visualization study on the dynamic behavior of shear thinning droplets impacting superhydrophobic spheres

The surface and droplet characteristics significantly affect the dynamic of droplet impact on solid surfaces. In present work, a platform is utilized to experimentally study the dynamic behavior of shear thinning fluid droplets impacting superhydrophobic spheres, where the effects of different concentrations, Weber numbers (We), particle size ratios on droplets impact are studied. The results showed that on the superhydrophobic sphere, as the shear thinning droplet concentration increases, the diffusion diameter increases, and the contact time decreases. For the same curvature, the contact time between the droplet and the sphere decreases with the increase of the We. When the We reaches 75 or above, the contact time remains constant with an increase in the We. For various curvature conditions, the contact time increases as the curvature increasing. When the diameter ratio of the sphere to the droplet is D∗≥6, the maximum diffusion coefficient remains almost constant. However, when D∗<6, the maximum diffusion dimensionless diameter significantly increases as the particle size ratio decreases. More importantly, a theoretical model considering gravity effect is presented to predict the maximum dimensionless diameter of shear thinning droplets on superhydrophobic spheres according to energy conservation. The predicted results are rationally agreement with experiments with the deviation within ±10 %.

Effect of the Surface Peak-Valley Features on Droplet Impact Dynamics under Leidenfrost Temperature.

This study explores the kinetic behavior of droplets impacting microtextured surfaces under a Leidenfrost temperature, employing high-speed photography and picosecond laser micromachining techniques. The investigation focuses on two types of microtextured surfaces with totally different surface peak-valley features: a negatively skewed surface with micropit arrays (Ssk < 0) and a positively skewed surface with micropillar arrays (Ssk > 0). The results indicate that both microtextured surfaces contribute to a higher Leidenfrost temperature compared with the original smooth surface, which is consistent with previous studies. However, it is worth noting that the Leidenfrost points of the micropit and micropillar surfaces showed opposite trends with the microtexture area occupancy. Specifically, the Leidenfrost temperature on micropit surfaces increases with greater micropit area occupancy, while it decreases on micropillar surfaces under similar conditions, which is mainly attributed to the differential impact of area occupancy on droplet heat transfer efficiency. When the microtexture area occupancy is 50%, it is worth noting that the micropit and micropillar surfaces have nearly same roughness (Sa), but the Leidenfrost temperature was notably higher on the micropit surface with negative skewness (Ssk < 0), which was related to differences in vapor flow dynamics. We further find that the Weber number (We) significantly influences the Leidenfrost point, with the droplet impact wall behavior going through the states of film bounce back, ejecting tiny droplets and bounce back, and ultimately droplet breakup as the We increases. The dynamic Leidenfrost point was found to be generally higher than the static point and increases with the We. Finally, we compare the cooling efficiency of these surfaces, and it is found that the micropit surfaces with a negative skewness exhibit superior heat dissipation performance under the same conditions, which proved that the negatively skewed surface may have great potential in high-density heat dissipation technology.

A durable superhydrophobic surface with bud-particle structure prepared by one-step spray method

Superhydrophobic surfaces play an essential role in numerous applications such as anti-fouling, waterproofing, and drag reduction due to their water-repellent. However, for most fabrication methods, there are high requirements for the preparation process and conditions. Meanwhile, the superhydrophobic surfaces generally have the problem of insufficient durability. In this study, a one-step method to fabricate a stable superhydrophobic surface by spraying is proposed. A new bud-particle structure is obtained by modifying micron and nano Aluminium hydroxide(Al(OH)3) particles, which helps the coated surface to demonstrate excellent superhydrophobicity with the water contact angle ∼157.0° and rolling angle ∼2.0°, and it shows great repellency to water droplet impact. Due to the compatibility of Al(OH)3with the pretreatment substrate and the reinforcing effect of nanoparticles, the coating maintained the superhydrophobicity well in the sandpaper wear and the ultrasonic damage test, which reflects the outstanding robustness of the coating.Moreover,benefiting from the control of the wetting state by micronand nanoparticles doping,the coating showed an excellent underwater drag reduction effect in the rheometer experiment, with a drag reduction rate of ∼67.7 %. The one-step spraying method proposed is a simple and convenient way to create the surface with excellent superhydrophobicity and durability, which has great potential applications in improving metal surface oxidation resistance and underwater reducing drag.

Hydrogel‐Based Droplet Electricity Generators: Intrinsically Stretchable and Transparent for Seamless Integration in Diverse Environments

AbstractDroplet‐based electricity generators (DEGs) have emerged as innovative devices capable of transducing the kinetic energy of falling water droplets into electrical power, thereby presenting a promising solution for ambient energy harvesting. For effective application, DEGs must exhibit transparency and flexibility to facilitate aesthetic integration and optical/mechanical adaptability for utilization on natural objects and wearable devices. In this study, a hydrogel‐based DEG (Hy‐DEG) designed is proposed to be seamlessly integrated into everyday environments without compromising design or functionality. With the intrinsic characteristics of hydrogel, the Hy‐DEG exhibits superior optical transmittance (≈99%) and stretchability (≈70% of its original length), generating the electrical output performance of ≈45 V of voltage, 15 µA of current, and 42 µW of maximum peak power, with single droplet impact. This capability of the Hy‐DEG can operate electronic devices for environmental monitoring while maintaining seamless integration with commonly encountered surfaces. In this context, this study elucidates the characteristics of hydrogel bestowing novel functionalities upon the DEG, expanding the potential for energy harvesting in residential and commercial contexts. The implications of this research extend beyond conventional energy generation, fostering the development of self‐powered, transparent, and flexible devices.

Pressure and wall shear stress from high-speed droplet impact

We report on experimental and numerical results of the impact of millimeter sized droplet with high-speed projectile. The impact velocity Vimp ranges between 70m/s and 245m/s and is sufficiently high that the compressibility of the liquid becomes important. High-speed images reveal non-axisymmetric lamella spreading, hydrodynamic and secondary cavitation and the ejection of a thin jet from the distal side of the droplet. The spreading velocity is supersonic for the impact velocities tested. Secondary cavitation at the distal side of the droplet is found for Vimp≳ 120m/s. The experiments are compared and analyzed further with a CFD model based on a compressible volume of fluid (VoF) method. Excellent agreement of the early droplet and lamella dynamics is obtained with the experimental data. Additionally, we provide quantitative data for the pressure loading and the shear stress on the projectile using spatio-temporal maps.

SEV-mediated lipid droplets transferred from bone marrow adipocytes promote ferroptosis and impair osteoblast function

Osteoporosis is linked to increased bone marrow adipocyte (BMAd) proliferation, which displaces bone-forming cells and alters the local environment. The impact of BMAd lipid droplets on bone health and osteoblast function remains unclear. This study investigates the interplay between BMAd-derived lipid droplets and osteoblast functionality, focusing on ferroptosis pathways. Osteoblast cultures were treated with conditioned media from adipocytes to simulate in vivo conditions. High-throughput mRNA sequencing and Western blot analysis were used to profile changes in gene expression and protein levels related to ferroptosis, oxidative phosphorylation, and osteogenic markers. Cellular assays assessed the direct impact of lipid droplets on osteoblast activity. Results showed that osteoblasts exposed to adipocyte-conditioned media had increased intracellular lipid droplet accumulation, upregulation of ferroptosis-related genes and proteins, and downregulation of oxidative phosphorylation and osteoblast differentiation markers. Treatment with ferroptosis inhibitors reversed the detrimental effects on osteoblasts, indicating the functional relevance of this pathway in osteoporosis. BMAd-derived lipid droplets contribute to osteoblast dysfunction through ferroptosis induction. Inhibiting ferroptosis could preserve osteoblast function and combat osteoporosis-related bone issues, suggesting that modulating lipid metabolism and redox balance in bone cells may be promising for future treatments.