Droplet Splashing Research Articles

The surface morphology and dynamic impact properties with rebounding and splashing of water droplet on phase separation and breath figure assisted electrospinning films

ABSTRACT Electrospinning provides a versatile, efficient and low-cost method for the preparation of continuous nanofibres from various polymers. In this study, the polyhedral oligomeric silsesquioxanes (POSS) block copolymer was synthesized via atom transfer radical polymerization. The smooth fiber, porous fiber or hierarchically porous microspheres were prepared by electrospinning from POSS block copolymer, poly(vinylidene fluoride) (PVDF) and aluminium oxide (Al2O3). The influence of copolymer concentration, the ratio of the solvents, the diameter and concentration of the Al2O3 on the surface morphology were investigated. Porous fibers and porous microspheres were prepared by regulating the ratio of the solvents from the phase separation and breath figure methods. The dynamic behavior of the water droplet with the constant volume impacting on the electrospinning films were reported. The morphology evolution, restitution coefficient, the change of energy of the water droplets are examined. The droplet bounces several times on the superhydrophobic surface, while the droplet remains pinned and does not rebound when the contact angles was lower than 150°. On the other hand, the water droplets were splashed on the Al2O3 based electrospinning films. Finally, the mechanical properties of the electrospinning films were investigated.

Splashing of liquid droplet on a vibrating substrate

The unsteady axisymmetric problem of a liquid drop impacting onto a rigid vibrating substrate is studied. Initially, the drop is spherical and touches the flat substrate at a single point. Then, the substrate starts to move toward the drop and vibrate with a small amplitude and high frequency. The early stage of the impact is studied by using the potential flow theory and the Wagner approach in dimensionless variables. The effect of the substrate vibration on the drop impact is described by a single parameter. It is shown that the vibration of the substrate leads to oscillations of the pressure in the contact region. The low-pressure zone periodically appears in the wetted part of the substrate. The low-pressure zone can approach the contact line, which may lead to ventilation with separation of the liquid from the substrate. The magnitude of the low pressure grows in time. The acceleration of the contact line oscillates with time, leading to splashing of the droplet with the local increase of the thickness of the spray jet sheet at a distance from the contact line. The phase shift of the substrate vibration with respect to the impact instant is not studied. Splashing can be produced only by a forced vibration of the substrate. The impact onto an elastically supported rigid plate does not produce splashing. The obtained results and the theoretical model of the initial stage of drop impact are valid for certain ranges of parameters of the problem.

Anti-splashing properties of sticky superhydrophobic surfaces

We used a high-speed camera to investigate the effect of pore structures on the anti-splashing properties of superhydrophobic surfaces. Zinc oxide (ZnO) nanowires with open air pockets and anodic aluminum oxide (AAO) with sealed air pockets were synthesized and subsequently treated with a low-energy self-assembled monolayer to render their surfaces superhydrophobic. The water contact angle of the superhydrophobic ZnO surface (SH-ZnO) was almost identical to that of the superhydrophobic AAO surface (SH-AAO); however, SH-AAO exhibited greater adhesion than SH-ZnO, which we attributed to the difference in the structures of air pockets. In addition, when a water droplet impacted SH-AAO, the surface effectively suppressed droplet splashing because of high adhesion; by contrast, the conventional superhydrophobic surface (i.e., SH-ZnO) promoted droplet splashing because of the presence of an air layer between the impacting water and the solid surface. The anti-splashing behavior of SH-AAO was observed not only for the impact of a water droplet but also for the impact of a water jet on the surface.

A decoupled and stabilized lattice Boltzmann method for multiphase flow with large density ratio at high Reynolds and Weber numbers

A decoupled lattice Boltzmann method (LBM) with improved numerical stability is developed from the conventional multiple relaxation time (MRT) model by eliminating the second-order mutual effects of viscosity modes. The dependency between relaxation times and viscosity is decoupled to remove the O(u3) errors in the original model, and thus a stabilization strategy is proposed by adjusting relaxation times without changing the local viscosity under the mass and momentum conservations. This new model still stays in the conventional MRT framework and keeps the advantages of simplicity and efficiency of LBM implementation. The decoupled MRT is demonstrated to be valid for high-speed flow with Galilean invariance. Moreover, the pseudopotential model is employed in this paper to present the improvement of numerical stability in simulating the large-density-ratio multiphase flows at high Reynolds (Re) and Weber (We) numbers. Droplet collision is achieved at density ratio 1153, Re = 34474 and We = 28212. Droplet splashing is achieved at density ratio 6637, Re = 34474 (and 71928) and We = 30538 (and 259). Droplet impacting on dry wall is achieved at density ratio 5720, Re = 11462 (and 22982) and We = 13573 (and 166). All collision regimes are simulated by the decoupled MRT from low Re (We) to high Re (We).

Simple, Effective, and Ecofriendly Strategy to Inhibit Droplet Bouncing on Hydrophobic Weed Leaves

Despite small-molecule surfactants and polymers being widely used as pesticide adjuvants to inhibit droplet bouncing and splashing, they still have intrinsic drawbacks either in the easy wind drift and evaporation, the unfavorable wettability, or the usage of nonrenewable resources. In this paper, we found that upon droplet impacting, 1D nanofibers assembled from natural glycyrrhizic acid (GL) could pin on the rough hydrophobic surface and delay the retraction rate of droplets effectively. Using GL as a tank-mixed adjuvant, the efficiency of glyphosate to control the weed growth was improved significantly in the field experiment, which addressed the dilemmas of current adjuvants elegantly. Our work not only provides a constructive way to overcome droplet bouncing but also prompted us to verify in future if all 1D nanofibers assembled from different small molecules can display similar control efficiencies.

Dynamic Response of PVDF Cantilever Due to Droplet Impact Using an Electromechanical Model.

The dynamic response of a polyvinylidene fluoride (PVDF) cantilever beam under excitation of water droplet impact is investigated by developing an electromechanical model. In the model, the governing equations of beam motion and output voltage are derived in the theoretical way, such that the voltage across the PVDF layer and the cantilever deflection can be predicted. The motion of the beam is described by the multi-mode vibration model through which more accurate results can be obtained. The predicted results of the model are validated by the experiment. Combined with the experiment and the model, the effect of surface wettability on droplet-substrate interaction mechanisms is investigated, which provides an insight into the improvement of mechanical-to-electrical energy conversion efficiency in raindrop energy harvesting (REH) applications. Results show: (1) the droplet splash on a super-hydrophobic beam surface has a positive effect on voltage generation. The splash limit that affects the reaction force of the impacting droplet is experimentally determined and greatly dominant by the Weber number. (2) Small-scaled droplets in splash regime allow generating higher voltage output from a super-hydrophobic beam surface than from an untreated hydrophilic beam surface. (3) Tests of successive droplet impacts also show that a super-hydrophobic surface performs better over a hydrophilic surface by producing constant peak voltage and higher electrical energy harvested. In this case, the voltage measured from the hydrophilic surface decreases gradually as the water layer is accumulated. Overall, the electromechanical behaviors of a super-hydrophobic PVDF cantilever sensor can be well predicted by the model which shows a great potential in energy harvesting by maximizing the inelastic collision upon droplet-substrate interactions.

Experimental study on secondary droplets produced during liquid jet impingement onto a horizon solid surface

Experiments were preformed to measure the mass and size of secondary droplets splashed during liquid jet impingement onto a horizontal solid surface. To explore the effects of liquid hydraulic properties such as viscosity and surface tension, pure water, two aqueous solutions of glycerin, and two aqueous solutions of ethanol were used as the test solutions. Using the experimental data accumulated in this work, dimensionless correlation was developed for the splash ratio (ratio of the droplet splash rate to the jet flow rate). Here, it was assumed that the mass of secondary droplets splashed per impact can be expressed as a function of the impact Weber number and the Ohnesorge number. The correlations for the Sauter mean diameter and the size distribution of secondary droplets were also proposed. It was shown that the size distribution of the secondary droplets can be fitted with the log-normal distribution. Two parameters determining the log-normal distribution profile were correlated by the impact Weber number.

A high‐order phase‐field based lattice Boltzmann model for simulating complex multiphase flows with large density ratios

SummaryBased on phase‐field theory, we develop a simple and robust single relaxation time (SRT) lattice Boltzmann (LB) model for simulating complex multiphase flows with large density ratios (up to 2000). The approach utilizes two LB equations (LBE), one is used to describe the interface behavior and the other is used to calculate the hydrodynamic properties. To improve the accuracy and stability in capturing interface, the high‐order LB model derived through the fourth‐order Chapman‐Enskog expansion analysis is applied to Cahn‐Hilliard equation. For solution of the flow field, a modified particle distribution function in the pressure‐velocity formulation is constructed to correctly consider the effect of local density variation and continuous pressure field. With such improvement, the proposed multiphase LB model is able to maintain numerical stability for the problem with very large density ratio, lower relaxation parameter and complex interface. For validation, a series of benchmark cases are carried out. Specially, the full potential of the proposed model is validated by bubble bursting at a free surface, bubble rising and droplet splashing with a density ratio of 2000. In all test cases, the obtained numerical results agree well with the reference data or the analytical solutions.

Surface roughness effect on a droplet impacting a thin film using pseudo-potential lattice Boltzmann method

A tunable surface tension pseudo-potential lattice Boltzmann method (LBM) is applied to study a droplet splashing on a thin film over a rough surface. Our study focuses on the crown evolution processes influenced by various roughness parameters, including the protrusion height and the distance between two protrusions. The total kinetic energy of the crown is introduced to study the evolution process. The results indicate that more kinetic energy is consumed in the collision process and that the crown has a shorter dimensionless height in the case of a rough surface than with a flat surface. A threshold dimensionless protrusion height exists at which the energy consumption is minimized and the crown height is maximized. The dimensionless distance between two protrusions may affect the symmetry of the liquid crown but does not influence the kinetic energy consumed in the impact process. Neither the protrusion height nor the distance between two protrusions has a significant effect on the crown radius evolution process. This study shows that the proposed LBM pseudo-potential model is an effective tool for predicting the process of a droplet impacting a thin film in the presence of complex boundaries.

Asymmetric droplet splashing

Asymmetric droplet splashing is observed during oblique impacts and under variable ambient pressure. These two effects are viewed as symmetry breaking factors which lead to new forms of splashing, including upward-only splashing and wing splashing.

Considerations in the use of slit lamp shields to reduce the risk of respiratory virus transmission in coronavirus disease 2019.

The use of slit lamp shields has been recommended by the American Academy of Ophthalmology as an infection control measure during the coronavirus disease 2019 pandemic. However, there is limited evidence regarding its efficacy to reduce viral transmission risks. We aim to provide an evidence-based approach to optimize the use of slit lamp shields during clinical examination. Respiratory droplets from coughing and sneezing can travel up to 50 m/s and over a distance of 2 m, with a potential area of spread of 616 cm. Slit lamp shields confer added protection against large droplets but are limited against smaller particles. A larger shield curved toward the ophthalmologist and positioned closer to the patient increases protection against large droplets. A potential improvement to the design of such shields is the use of hydrophilic materials with antiviral properties which may help to minimize splashing of infectious droplets, reducing transmission risks. These include gold or silver nanoparticles and graphene oxide. Slit lamp shields serve as a barrier for large droplets, but its protection against smaller droplets is undetermined. It should be large, positioned close to the patient, and used in tandem with routine basic disinfection practices.

Numerical investigation of water droplet impact on horizontal beams

Droplet impact on elastic beams is considered as a novel model of energy transfer which is a promising alternative in applications of energy harvesting. The transient impact process is dominated by the fluid–solid interaction and the capillary effect. The numerical model based on SPH method allows predicting the droplet dynamic behaviors due to super-hydrophobic (SH) surfaces. The predicted results are also compared with relevant experiments to verify the robustness and flexibility of the model. For fixed-fixed beams, typical regimes, namely spherical-shaped rebound, pancake-shaped rebound and splashing of droplet, are identified. The elasticity of beam causing the earlier lifting-off phenomenon of droplet is investigated in detail. By comparison, cantilever beams repel the droplet in a smoother way and large deformation of the beam is considered. The slipping-off phenomenon is expected to occur under specific conditions on soft cantilevers. The effect of elasticity plays a key role in the maximum deflection and oscillating frequency for both types of beams. This work examines the effectiveness of the framework based on the numerical model which provides further understandings for droplet impacts. It may lay the foundation for practical applications, such as engineering piezoelectric raindrop energy harvesters and plant leaves repelling raindrops.

Numerical simulation of a water droplet splash: Comparison between PLIC and HRIC schemes for the VoF transport equation

A wide range of natural and industrial processes, such as a droplet falling in the ocean as well as the injection of fuel in an internal combustion engine, are physically explained by the study of multiphase flows. The understanding of these processes is a fundamental step in order to optimize their use in industrial applications. Computational tools have been increasingly used for that purpose. This paper concerns the simulation of a water droplet of 2.9 mm diameter impacting onto a water pool with two different free falling velocities, 1.55 m/s and 2.5 m/s. The simulations were run using the Volume of Fluid method, which captures the interface between two fluids by solving a transport equation for the volume fraction. As demonstrated in many previous investigations, the solution of this phase transport equation is far from trivial. Therefore, in order to better understand the influence of the scheme for the advection term of the VoF transport equation, two different schemes are used, namely HRIC and PLIC. The CFD software Convergent Science Inc.’s CONVERGE TM CFD, which adopts adaptive mesh refinement techniques (AMR), is used to carry out the simulations. The results are then compared to experimental data. It is concluded that the VoF method with both schemes is able to track the surface between the fluids with good accuracy. Furthermore, the PLIC scheme maintains a sharper interface than the HRIC scheme as well as is more accurate and computationally efficient than the HRIC. These observations can be extended to other two-phase flow simulations.

Conservation of personal protective equipment for head and neck cancer surgery during COVID-19 pandemic.

BackgroundCOVID‐19 pandemic has led to a global shortage of personal protective equipment (PPE). This study aims to stratify face shield needs when performing head and neck cancer surgery.MethodsFifteen patients underwent surgery between March 1, 2020 and April 9, 2020. Operative diagnosis and procedure; droplet count and distribution on face shields were documented.ResultsForty‐five surgical procedures were performed for neck nodal metastatic carcinoma of unknown origin (n = 3); carcinoma of tonsil (n = 2), tongue (n = 2), nasopharynx (n = 3), maxilla (n = 1), and laryngopharynx (n = 4). Droplet contamination was 57.8%, 59.5%, 8.0%, and 0% for operating, first and second assistant surgeons, and scrub nurse respectively. Droplet count was highest and most widespread during osteotomies. No droplet splash was noted for transoral robotic surgery.ConclusionFace shield is not a mandatory adjunctive PPE for all head and neck surgical procedures and health care providers. Judicious use helps to conserve resources during such difficult times.

Identifying the transition region between splashing and spreading of fuel drops

Whether an impacting droplet splashes or spreads is determined by a number of parameters, and a transition region exists between the two behaviors.

Three-Dimensional droplet splashing dynamics measurement with a stereoscopic shadowgraph system

In the current study, a novel three-dimensional (3D) measurement technique is established using a high-speed stereoscopic shadowgraph system, which is applied to investigate the 3D splashing dynamics of silicone oil dropping on liquid films quantitatively. Both crown-type splashing and crown wall bottom breakdown splashing morphologies are involved for measurement and comparison. Based on the shadowgraph images, the secondary droplets diameter, number and mass fraction are determined. The three-dimensional (3D) coordinates of the secondary droplets during the splashing processes are reconstructed and tracked, based on which the 3D trajectories, velocities, ejecting angles and kinetic energy are calculated and analysed. It is found that the secondary droplets in crown bottom breakdown cases have a larger mass fraction and kinetic energy than that in crown-type splashing cases. The measurement indicates that the radial velocity increases with the decreasing of film thickness, while the vertical velocity does not vary too much. Significant disparities between two-dimensional (2D) and three-dimensional (3D) velocity magnitudes as well as kinetic energy have been identified, which indicate that accurate time-resolved 3D measurements are of great importance for quantitative investigation of splashing phenomena and the high-speed stereoscopic shadowgraph system has been proved to be able to play a role.

Experimental study of flame expansion induced by water droplet impact on the burning cooking oil

A series of experiments on water droplet impact on the burning peanut oil were performed to explore the effects of droplet size and falling height on flame expansion scale. Flame expansion phenomenon accompanied by smoke and flame hairs was captured and its mechanism was clarified in detail. Both droplet size and falling height have significant effects on the maximum flame expansion scale. Droplet size determines the flame expansion scale by influencing the mixed amount of water vapor and combustible fuel gas, while the falling height affects the flame expansion scale by suppressing oil droplet splash. Furthermore, the effect of falling height on the flame expansion scale is only significant at smaller droplets. With the increase of water droplet diameter (2.0–4.5 mm) and falling height (45–95 cm), the maximum height of flame expansion is increased to 6–15 times, and correspondingly the maximum volume has an increase of 66742–484073 times.

Oblique drop impact on thin film: Splashing dynamics at moderate impingement angles

The oblique drop impact on the thin film is numerically investigated in this paper with special attention paid to its splashing dynamics at moderate impingement angles (45° ≤ α ≤ 75°). A three-dimensional multiphase lattice Boltzmann flux solver associated with the diffuse interface method is adopted after being validated against reference data. Efforts are made to recover the complex flow features in the oblique drop impact on the thin film at various impingement angles, film thicknesses (δ), Ohnesorge numbers (Oh), and Weber numbers (We). We found that the later stage of radial propagation of the jet base and the free rim is dominated by inertia and can be well correlated with dimensionless time τ through the square-root law. The elevation of the free rim exhibits a linear relationship with time and varies with Oh and We, indicating its connection to the splashing of minor droplets, which is also an outcome of inertia, viscosity, and surface tension. Moreover, the onset of droplet splashing is generally insensitive to δ. Based on that, a correlation between the splashing limit and the impingement angle is established and shows good agreement with numerical results at moderate impingement angles.

Study of droplet splashing on a liquid film with a tunable surface tension pseudopotential lattice Boltzmann method

A tunable surface tension pseudopotential lattice Boltzmann method (LBM) with an axisymmetric boundary is applied to study a droplet splashing on a thin film. Our work focused on the crown behavior influenced by the different parameters, including the Reynolds number, Weber number, liquid film thickness, and gravity acceleration. In addition, the total kinetic energy of the crown is proposed to explain the evolution of the crown shape from the perspective of energy. Based on the achieved results, it is found that the crown radius changes with time are unaffected by these parameters and consistent with the power-law in previous studies. However, the height of the crown and the satellite droplet formation process are affected by the referred parameters. Besides, it is found that the energy consumption during the collapse process and splash angle are two key factors affecting the height of the crown. There is a threshold liquid film thickness with the maximum height of the crown, which is between 0.2 and 0.25 times the droplet radius. This study shows that the proposed LBM pseudopotential model is a robust and effective tool for the study of the droplet splashing on the thin film and has the potential to predict the droplet splashing phenomena in the presence of complex boundaries.

A Σ-ϒ two-fluid model with dynamic local topology detection: Application to high-speed droplet impact

A numerical methodology resolving flow complexities arising from the coexistence of both multiscale processes and flow regimes is presented. The methodology employs the compressible Navier-Stokes equations of two interpenetrating fluid media using the two-fluid formulation; this allows for compressibility and slip velocity effects to be considered. On-the-fly criteria switching between a sharp and a diffuse interface within the Eulerian-Eulerian framework along with dynamic interface sharpening is developed, based on an advanced local flow topology detection algorithm. The sharp interface regimes with dimensions larger than the grid size are resolved using the VOF method. For the dispersed flow regime, the methodology incorporates an additional transport equation for the surface-mass fraction (Σ-ϒ) for estimating the interface surface area between the two phases. To depict the advantages of the proposed multiscale two-fluid approach, a high-speed water droplet impact case has been examined and evaluated against new experimental data; these refer to a millimetre size droplet impacting a solid dry smooth surface at a velocity as high as 150 m/s, which corresponds to a Weber number of 7.6×105. Droplet splashing is followed by the formation of highly dispersed secondary cloud of droplets, with sizes ranging from 10 μm close to the wall to less than 1 μm forming at the later stages of droplet fragmentation. Additionally, under the investigated impact conditions, compressibility effects dominate the early stages of droplet splashing. A strong shock wave forms and propagates inside the droplet, where transonic Mach numbers occur; local Mach numbers up to 2.5 are observed for the expelled surrounding gas outside the droplet. Relative velocities between the two fluids are also significant; local values on the tip of the injected water film up to 5 times higher than the initial impact velocity are observed. The proposed numerical approach is found to capture relatively accurately the flow phenomena and provide additional information regarding the produced flow structure dimensions, which is not available from the experiment.