Droplet Impact Research Articles

Development and assessment of a three-field subchannel code for thermo-hydraulic transient analysis under reflood conditions

A three-field twelve-equation subchannel code was developed that contains separate energy equations for the continuous liquid and droplet phases. Several physical models were modified and optimized to accommodate the code and improve predictions of reflooding. Eight RBHT tests were used to assess the code. Comparisons show that the predictions of the code are in good agreement with experimental data on the quench front propagation, peak cladding temperature, and temperatures of the cladding, vapor and spacer grid. Moreover, the code can capture trends in droplet size and velocity and obtain the continuous liquid-droplet temperature difference. The impact of new interfacial heat transfer model, droplet impingement heat transfer model and energy equations on the reflooding calculation are analyzed. Overall, the code is applicable to the simulation of the reflooding phenomenon, and the results can provide guidance for model modification. Meanwhile, the pre-assessment of droplet temperature provides new perspectives for the code development.

Regulation of droplet dynamic behavior after droplet impact on superhydrophobic surfaces

Controlling the impact behavior of a droplet on a superhydrophobic surface is a significant challenge for many applications. Instead of texturing the surface, this study investigated the effect of adding surfactants on the morphology of the droplet after it impacts superhydrophobic surfaces. The results showed that adding low concentrations of sodium dodecyl sulfate (SDS) surfactant prevents fragmentation and splashing. High concentrations of SDS can inhibit rebound and allow droplets to be easily deposited on surfaces after impact. When the droplet size was 3.51 mm and the SDS concentration was above the critical micelle concentration, the contact time of the solution increased by a maximum of 1.30 times compared to that of pure water. In a high-concentration SDS solution, the surfactant molecules can quickly migrate to the newly generated interface during the spreading stage, reducing the interfacial tension and causing wetting changes. The results of this study are expected to contribute toward establishing a simple method to inhibit splashing and increasing contact time on superhydrophobic surfaces.

Thermocapillary central lamella recess during droplet impacts onto a heated wall

We experimentally observe a new phenomenon, the formation of a toroidal region of lower film thickness in the center of the lamella formed during high Weber number water droplet impacts onto smooth heated walls. This region forms around the air bubble, which is entrapped during the initial impact phase at the impact center. Our study encompasses a variation of the droplet size, impact velocity, surface wettability and temperature. We show how this phenomenon can be explained considering a two-step process involving thermocapillary convection in two separate regions: The temperature gradient along the surface of the entrapped air bubble caused by heat conduction induces flow that pumps warmer liquid to the lamella-ambient interface due to the Marangoni effect. The non-uniform temperature distribution along it then causes fluid acceleration in the radial direction, depleting the fluid volume around the bubble in a self-amplifying manner. We use direct numerical simulations of a stagnant liquid film with an enclosed bubble at the wall to confirm this theory.

Numerical simulation of droplet impingement and film flow for three-dimensional wings

Abstract In order to investigate the three-dimensional effects on the flow characteristics of the thin water film for the three-dimensional wings, the numerical simulation of the droplet impingement and film flow on the MS-0317 wing is implemented based on the open-source package OpenFOAM. The simulation focuses on the effects of the angle-of-attack and the angle of sweepback. The movement and impingement of the droplets are calculated using the Lagrangian method, and the film flow is simulated using the thin film assumption and the finite area method. The simulation of the water film flow of the three-dimensional MS-0317 wing shows that there is a spanwise flow of the water film due to the three-dimensional effects. This suggests that more research should be conducted on the warm glaze ice with surface water film of three-dimensional ice accretion on three-dimensional geometries.

Submerged pulsating water jet erosion of ductile material

This article deals with the manifestations of erosion of statically acting concentrated multiple droplet impingements on materials over different lengths of time in order to assess the erosion evolution when the materials are submerged, as compared with the action of droplets on the materials under atmospheric conditions. The study aims to determine the extent of the erosive effects of droplets in underwater conditions. Experiments were conducted in a plastic pool in which the water level was varied from h = 80 to h = 120 mm to identify the effect of hydrostatic pressure at p120 = 0.1025 MPa and p80 = 0.1021 MPa. The results were compared with that of a control group of samples obtained under atmospheric air conditions. To observe the erosive damage evolution, 15 (n = 5) sites on the ductile materials EN AW-Al 99.5 and CW004A were exposed to high-intensity droplet impingement at defined exposure times. The exposure time varied from 0.125 s to 1.875 s, with increments of 0.125 s. As a droplet generator, an ultrasonic pulsating water jet with a frequency f = 20 kHz, a pressure p = 30 MPa and a nozzle diameter d = 0.4 mm was used to achieve the theoretical subsonic speed of the droplets. The results exceeded the assumptions regarding the possible attenuation of water pulses. It was found that under the theoretical speed of the jet vw = 225 m/s, the submerged condition causes attenuation in its erosive action. The incident area in the case of submerged treatment was more symmetrical. The erosion shift in term of prolonging incubation erosion stage was found to be a result of the increasing hydrostatic pressure. The results suggest that this method can also be used under submerged conditions for treatment or material drilling.

Cold granular targets slow the bulk freezing of an impacting droplet.

When making contact with an undercooled target, a drop freezes. The colder the target is, the more rapid the freezing is supposed to be. In this research, we explore the impact of droplets on cold granular material. As the undercooling degree increases, the bulk freezing of the droplet is delayed by at least an order of magnitude. The postponement of the overall solidification is accompanied by substantial changes in dynamics, including the spreading-retraction process, satellite drop generation, and cratering in the target. The solidification of the wetted pores in the granular target primarily causes these effects. The freezing process over the pore dimension occurs rapidly enough to match the characteristic timescales of impact dynamics at moderate undercooling degrees. As a result, the hydrophilic impact appears "hydrophobic," and the dimension of the solidified droplet shrinks. A monolayer of cold grains on a surface can reproduce these consequences. Our research presents a potential approach to regulate solidified morphology for subfreezing drop impacts. It additionally sheds light on the impact scenario of strong coupling between the dynamics and solidification.

Elimination of gas entrapment in droplet-based 3D printing by induced electric-field

Gas entrapment during droplet deposition may induce pinhole defects in droplet-based 3D printing. However, achieving effective suppression of gas entrapment is challenging given that gas entrapment behavior is the physical essence of droplet impact. Inspired by that charged droplets will be deformed in the electric field, a method of suppressing gas entrapment by weakly charging the droplet was proposed. Here, a numerical model is developed to systematically investigate the gas entrapment behaviors during neutral droplet and charged droplet deposition. Results show that the electric field will be induced before the instant of droplet impact on the substrate, which fundamentally changes the gas entrapment behavior. Moreover, the competition mechanism between Maxwell stress and lubrication pressure during droplet deposition is revealed, and the pressure inside and outside the droplet bottom profile is quantified. Compared with the neutral droplet, the additional electric stresses act against the lubrication pressure, which increases sharply before the droplet touches the substrate. When the charge level reaches a critical value, the electric stresses are slightly stronger than the lubrication pressure at the bottom center, and the decrease rate of electric stresses along the horizontal distance r is higher than that of gas film pressure, leading to a central touchdown with residue concentric circle gas rings. Furthermore, the evolution of phase fraction, velocity field, and pressure field during droplet deposition with different charge level is investigated comprehensively. Based on this, the dimensionless scale of critical charge level to fundamentally eliminate gas entrapment is obtained. This work may provide new insights into various applications involving droplet deposition, such as eliminating pinhole defects in droplet-based additive manufacturing.

Regulating droplet impact dynamics of nanoparticle suspension: Phenomena, mechanisms, and implications

Droplet impact on solid substrates is a ubiquitous phenomenon in nature, agriculture, and industrial processes, playing a crucial role in numerous applications including self-cleaning, pesticide utilization, and inkjet printing. As a promising technique, adding nanoparticles into simple fluids to form nanofluids can effectively manipulate droplet impact behaviors. However, a comprehensive understanding of how nanoparticles modify the droplet impact dynamics, especially on the nanoscale, is still far from being fully explored. Hence, in this work, through the combined effort of molecular dynamics simulations and theoretical analysis, we elaborate on the influences of nanoparticles on droplet impact process. Using simple droplets as a control, we summarize four typical droplet impact modes and reveal how nanoparticles alter the impact behaviors of droplets, taking into account the key parameters including substrate wettability, impact velocity, volume fraction, and mass fraction of nanoparticles. We also demonstrate that with appropriate modifications, the theoretical/empirical models to predict the maximum contact diameter and the occurrence of breakup for simple droplets still hold to predict those of nanofluid droplets. Our findings and results enhance the understanding of the impact of nanoparticles on the droplet impact dynamics, with promising possibilities for various applications where regulating droplet impact behaviors is desired.

Simulation and analytical modeling of high-speed droplet impact onto a surface

The fluid dynamics of liquid droplet impact on surfaces hold significant relevance to various industrial applications. However, high impact velocities introduce compressible effects, leading to material erosion. A gap in understanding and modeling these effects has motivated this study. We simulated droplet impacts on solid surfaces and proposed a new analytical model for impact pressure and droplet turning line, targeting at predictions for enhanced cavitation. The highly compressed liquid behind the droplet expands sideways, causing lateral jetting. As the droplet encounters a shock wave, it reflects as a rarefaction wave, leading to low-pressure zones within the droplet. These zones converge at the droplet's center, causing cavitation, which, upon collapse, induces another shock wave, contributing to erosion. Using the well-established model for the low-velocity impact shows a significant discrepancy. Hence, an analytical model for the turning line radius is introduced, incorporating the lateral jetting's characteristic length scale. Comparing our model with existing ones, our new model exhibits superior predictive accuracy.

A droplet‐based electricity generator incorporating Kelvin water dropper with ultrahigh instantaneous power density

AbstractHarvesting renewable water energy in various formats such as raindrops, waves, and evaporation is one of the key strategies for achieving global carbon neutrality. The recent decade has witnessed rapid advancement of the droplet‐based electricity generator (DEG) with a continuous leap in the instantaneous output power density from 50 W/m2 to several kW/m2. Despite this, further pushing the upper limit of the output performance of DEG is still constrained by low surface charge density and long precharging time. Here, we report a DEG incorporating the Kelvin water dropper (K‐DEG) that can generate an ultrahigh instantaneous power density of 105 W/m2 upon one droplet impinging. In this design, the Kelvin water dropper continuously replenishes the high density of surface charges on DEG, while DEG fully releases these surface charges into electric output. K‐DEG with such a high output can directly light five 6‐W commercial lamps and even charge a cellphone by using falling droplets.

A computational study of air film evolution during droplet impact on a flat solid surface

A computational study of air film evolution during droplet impact on a flat solid surface

Analyzing the early impact dynamics of single droplets impacting onto wetted surfaces

Single droplet impacts onto thin wall-films are a common phenomenon in many applications. For sufficiently high impact velocities, the droplet impact process consists of three phases, i.e., initial contact stage, droplet deformation with radial momentum transfer inducing an upward rising lamella, and crown propagation. Here, we present the results of a combined numerical and experimental study focusing on the early dynamics of the impact process. Specifically, the effects of the initial droplet shape, wall-film thickness, and contact line motion are analyzed. Prior to impact, an oblate spheroidal droplet shape was observed. Using direct numerical simulation, we show that the droplet shape affects the impact dynamics only during the first two phases, as it is one of the key parameter influencing the correct prediction of the impact zone. The contact line propagation is described by a square-root-time dependence R¯CL=ατ for both, dry and wetted surfaces. On dry surfaces, the advancement of the contact line is determined by the rolling motion of the truncated droplet. On wetted surfaces, the value of the α-parameter is controlled by two concurrent effects, namely, rolling motion and wall-film inertia. For impact onto thin films, the rolling motion prevails. With increasing wall-film height, the droplet penetrates into the soft substrates and wall-film inertia becomes the controlling factor. These insights into the early impact dynamics on wetted surface are important for the formulation of a unified modeling approach.

Impingement of a water droplet onto a rotational surface

In the present study, the impact of a water droplet onto a rotational surface was experimentally studied. The effects of the impact velocity, the rotational speed, and the impact radial position on the impact phenomena were carefully investigated. The phenomena of asymmetric spreading, finger formation, and secondary droplets were observed. Moreover, experiments of a water droplet impacting onto a stationary surface with different impact velocities were also conducted for comparisons. The results showed that, at the same impact velocity, the increase in the rotational speed or the impact radial position could result in the increase in the wetting length in the angular direction and the wetting area. In addition, at the same impact velocity, the dimensionless wetting length in the radial direction on the rotational surface was generally lower than the spreading factor on the stationary surface. In addition, new correlations for the spreading ratio, the dimensionless excess spread area, the dimensionless maximum wetting area, and the number of fingers appearing at the angle of 50° after impact were proposed. The increase in the impact velocity and the surface linear velocity could both promote the formation of the fingers at the angle of 50° after impact. In addition, the dimensionless horizontal displacement of the secondary droplet was found to be mainly influenced by the impact velocity, while the surface linear velocity only had a minor influence.

Bacteria-surface interactions: role of impacting bacteria-laden droplets.

Understanding the interactions of pathogenic droplets with surfaces is crucial to biomedical applications. In this study, using E. coli as the model microbe, we investigate the impact of a bacteria-laden droplet on different substrates, both bare and antimicrobial. In doing so, we unveil the significance of kinetic energy and spreading parameters of the impacting droplet in determining the microbes' proliferation capabilities. Our results indicate an inverse relationship between the impact Weber number and the bacterial ability to proliferate. We reveal that the mechanical stress generated during impact impedes the capabilities of microbes present inside the droplet to create their progeny. Following an order analysis of the mechanical stress generated, we argue that the impact does not induce lysis-driven cell death of the bacteria; rather, it promotes a stress-driven transition of viable bacteria to a viable-but-non-culturable (VBNC) state. Furthermore, variations in the concentration of particles on the antimicrobial surfaces revealed the role of the post-impact spreading behaviour in dictating bacterial proliferation capabilities. Contrary to the conventional notion, we demonstrate that during the early stages of interaction, a bare substrate may outperform an antibacterial substrate in the inactivation of the bacterial load. Finally, we present an interaction map illustrating the complex relationship between bacterial colony-forming units, bactericide concentration on the surface, and the impact Weber number. We believe that the inferences of the study, highlighting the effect of mechanical stresses on the soft cell wall of microbes, could be a useful design consideration for the development of antimicrobial surfaces.

Visualisation of Droplet Flow Induced by Ultrasonic Dental Cleaning

IntroductionDuring dental treatment procedures ultrasonic scalers generate droplets containing microorganisms such as bacteria and viruses. Hence, it is necessary to study the dynamic properties of generated droplets in order to investigate the risks associated with the spread of infection. The aim of this study was to visualise the flow state of droplets and to evaluate the impact of droplets generated during the use of an ultrasonic scaler during an oral surgical procedure. MethodsWe studied the spatial flow of liquid droplets through a combination of imaging and numeric simulation of a simulated dental treatment processes. First, we photographed the real time images of the ultrasonic scaler and evaluated the images using image-processing software Image J to visualise the flow of liquid droplets. Finally we simulated the flow process of liquid droplets by using the initial velocity of droplet splashing and the angle of the obtained information using computerised fluid dynamics technology. ResultsUnder different working conditions, the droplet particle splashing velocity, maximum height, and spray angle varied, but the particle trajectory was generally parabolic. The maximum droplet velocity varied between 3.56 and 8.56 m/s, and the splashing height was between 40 and 110 mm. ConclusionsDuring risk assessment of an ultrasonic scaler usage, difficulties arise due to the insufficient research on droplet velocity and distribution. This study aims to address this gap by visualising the flow trajectories of droplets generated by ultrasonic scalers. The obtained data will assist in developing more effective interventions based on spatial and temporal distribution of droplets. This provides a new approach for droplet particle research and offers new strategies for public health prevention and control.

OFFSET COALESCENCE BEHAVIOR OF IMPACTING LOW-SURFACE TENSION DROPLET ON HIGH-SURFACE-TENSION DROPLET

The impact of droplets of varying surface tension and subsequent spreading over a solid surface are inherent features in printing applications. In this regard, an experimental study of the impact of two drops of varied surface tension is carried out where the sessile water droplet on a hydrophilic substrate is impacted upon by another droplet of sequentially lowered surface tension. The impacts are studied for different impact velocities and offsets with respect to the mid-plane of the two colliding droplets. Sodium dodecyl sulfate is used to: (i) alter the surface tension without altering the viscosity, (ii) study the various parameters affecting the spreading length viz. the surface tension, (iii) offset between the drops, and (iv) impact velocity. The spreading lengths are obtained through image processing of the captured footage of the impact dynamics by a high-speed camera. It is found out that upon lowering the surface tension, the maximum and equilibrium spreading length varies to a significant extent, and the nature of the spreading dynamics changes. Both side- and top-view imaging are performed to understand the overall hydrodynamics. There is also a substantial change in "drawback" when dissimilarity in surface tension between the impacting droplets exists. Finally, a fit model is obtained to predict the maximum spread length of the various cases.

Development of technical solutions for environmentally safe irrigation

The article presents technical solutions to reduce the negative impact on soil. Scientists have developed and continue to study ways to eliminate the negative effects of sprinkling irrigation occurring due to the mechanical impact of droplets on the soil leading to its compaction and the formation of soil crust depending on the type of soil. The useful model refers to the technique of sprinkler irrigation and can be used in mobile and stationary sprinkler systems to produce rain with droplet sizes acceptable for irrigation of a wide range of cultivated crops. The claimed design provides reduction of dynamic impact of drops on the soil, as a result of changes in the deflector, due to the installation of wedge-shaped flow dissectors on the surface of the flutes.

Dynamics of individual inkjet printed picoliter droplet elucidated by high speed laser speckle imaging.

Inkjet printing is a ubiquitous consumer and industrial process that involves concomitant processes of droplet impact, wetting, evaporation, and imbibement into a substrate as well as consequential substrate rearrangements and remodeling. In this work, we perform a study on the interaction between ink dispersions of different composition on substrates of increasing complexity to disentangle the motion of the liquid from the dynamic response of the substrate. We print three variations of pigmented inks and follow the ensuing dynamics at millisecond and micron time and length scales until complete drying using a multiple scattering technique, laser speckle imaging (LSI). Measurements of the photon transport mean free path, l*, for the printed inks and substrates show that the spatial region of information capture is the entire droplet volume and a depth within the substrate of a few μm beneath the droplet. Within this spatial confinement, LSI is an ideal approach for studying the solid-liquid transition at these small length and time scales by obtaining valid g2 and d2 autocorrelation functions and interpreting these dynamic changes under through kymographs. Our in situ LSI results show that droplets undergo delamination and cracking processes arising during droplet drying, which are confirmed by post mortem SEM imaging.

INFLUENCE OF BUBBLE GROWTH AND LIQUID FILM INSTABILITIES ON DROPLET IMPACT PHENOMENA UNDER SATURATED BOILING REGIMES

Evaporation and boiling are processes that occur in many industrial applications involving multiphase flows. For liquid films, however, studies are scarce regarding heat and mass transfer mechanisms and require further research. The main objective of this work is to evaluate bubble formation and detachment, followed by the impact phenomena. Therefore, an experimental setup was built and adapted for this purpose. A borosilicate glass impact surface is placed over a heat source, which consists of an aluminum block with four embedded cartridge heaters that heat the liquid film by conduction. Water and n-heptane are the fluids adopted for the experimental study, as the differences in thermophysical properties allow for a wider range of experiments. Study cases include dimensionless temperatures of θ > 0.6 for similar impact conditions. In terms of bubble formation, n-heptane displays smaller bubble diameters and higher release rates, whereas water exhibits larger bubbles and lower rates. Qualitatively, liquid film temperatures close to the saturation temperature do not reveal a direct influence on the crown development and posterior secondary atomization. For later stages of the impact, the central jet height and breakup are influenced by the film temperature, which is associated with the variation of thermophysical properties.

Super-cooled droplet impact and ice accretion on an aircraft engine nacelle lip

This manuscript concentrated on the icing phenomenon on the lips of an engine nacelle. Numerical simulation was employed to establish the airflow, supercooled droplet impact, and engine nacelle ice accretion models. The study investigated the airflow field near the lip, the characteristics of supercooled droplet impact, and the ice accretion near the lip under different flight conditions. The computational results indicated that due to the effects of nozzle contraction and flow extraction, a low-pressure, high-velocity, and low-temperature region appeared on the inner side of the lip, leading to ice accretion on the nearby wall. The maximum collection efficiency on the lip surface at 20, 000 ft was slightly higher than that at 10, 000 ft, with a maximum value of approximately 0.6. In the computed conditions of this study, as the flight altitude and Mach number increased, the ice thickness on the lip surface increased and the ice spread inward, posing a significant risk of ingestion after detachment.