Crown evolution kinematics of a camellia oil droplet impacting on a liquid layer

The phenomenon of droplet impact on an immiscible liquid is encountered in a variety of scenarios in nature and industrial production. Despite exhaustive research, it is not fully clear how the immiscibility of the liquid on which a droplet impacts affects the crown evolution. The present work experimentally investigates the evolution kinematics of a crown formed by the normal impact of a camellia oil droplet on an immiscible water layer. Based on discussion of dynamic impact behaviors for three critical Weber numbers(We), the radius of the crown and its average spreading velocity are compared with those of previous theoretical models to discuss their applicability to the immiscible liquid. The evolution kinematics(morphology and velocity) are analyzed by considering the effects of the We and layer thickness. Furthermore,the ability of crown expansion in radial and vertical directions is characterized by a velocity ratio. The results show that our experimental crown radius still follows a square-root function of evolution time, which agrees with the theoretical predictions. The dimensionless average spreading velocity decreases with We and follows a multivariate power law, while the dimensionless average rising velocity remains constant. The velocity ratio is shown to linearly increase with We,demonstrating that the rising movement in crown evolution gradually enhances with We. These results are helpful for further investigation on the droplet impact on an immiscible liquid layer.

Effect of the liquid temperature on the interaction behavior for single water droplet impacting on the immiscible liquid

The interaction of single water droplet impacting on immiscible liquid surface was focused with the temperature varying from 50℃ to 210℃.The impact behavior is recorded with a high-speed camera running at 2000 frames per second.It is found that droplet diameter,oil temperature,andWeber number have important influences on impact behaviors.Three typical phenomena,including penetration,crater-jet,and crater-jet–secondary jet,were observed.Penetration only occurs when the Weber number is below 105.With Weber number increasing to 302,the jet begins to appear.Moreover,to gain deeper physical insight into the crater formation and jet formation,the energy of droplet impingement onto the liquid pool surface was estimated.The oil temperature has a significant effect on the energy conversion efficiency.High temperature is beneficial to improve energy conversion efficiency.

Numerical simulation of droplet impacting liquid surface by SPH

Droplet impacting liquid surface is not only the extremely prevalent phenomenon in the nature and industrial production but also the extremely complicated problem of strong non-linear transient impact and free-surface flow. On the basis of the two-dimensional viscous incompressible N-S equations, this paper conducts a study of numerical simulation on the problem of droplet impacting liquid surface （water beads） of water container in certain initial velocity by the method of SPH （smoothed particle hydrodynamics）. The effect of surface tension is considered between surface particles by searching the free surface particles in the course of study; the effect of initial impact has been solved by use of artificial viscosity; at the same time, the side-wall virtual particles and image virtual particles are both introduced to deal with the boundary condition, which has solved the boundary defects quite well and eliminated the instability of real particles dropped to the comer of container. The calculated results form the distribution chart of particles, flow field chart, pressure chart and the displacement and velocity variation curve of different particles. The comparison between simulated results and experimental photos shows that the simulation is effective. This paper compares the variational curves for fluctuations of liquid surface qualitatively through adopting the methods of level-set, BEM and SPH, respectively at last. The simulated results show that it will produce strong non-linear phenomena, such as the splash of liquid, discrete liquid surface, and strong wave of free liquid surface, when the droplet impacts liquid surface; in the course of impacting, the movement of liquid particles exhibits the characteristic of oscillation; the method of SPH has certain advantages of dealing with the large deformation problem of free surface.

Evolution of cavity size and energy conversion due to droplet impact on a water surface

The deformation characteristics of the cavity due to droplet impact on a water surface are experimentally investigated.Dimensional analysis shows that the characteristic values of time,depth,and horizontal diameter can be taken as 10^(-3)times the ratio of surface tension to the product of viscosity coefficient and gravitational acceleration,the maximum depth,and the maximum horizontal diameter,respectively.The evolutions of the dimensionless cavity sizes for different values of Weber number(We)coincide for 220<We<686.A partial-sphere model of cavity is established based on experimental observations.Energy models are then derived,and the energy conversions are calculated to identify the relationship between these conversions and cavity deformation.It is found that the kinetic energy model established under the hypothesis proposed by Leng is no longer applicable when the dimensionless time t^(*)<3.5,owing to deviations from the geometric model.

Numerical Study on Hydrodynamic Forces for Micro Particle Detachment by Droplet Impact

This paper presents the results of a numerical investigation of micro-sized particle removal by droplet impact. Computational fluid dynamics simulation is used to calculate the flow distribution of droplet impact on a flat surface. The hydrodynamic forces exerted on the particle are then computed. Key factors controlling particle removal are discussed. Both hydrophilic and hydrophobic surfaces are considered. The flow distributions,especially the front edge expanding upon impact at microscale,strongly depend on surface wettability. The associated hydrodynamic forces on the particles vary accordingly. In addition, the impact on a dry surface can produce higher removal efficiency than that on a wet surface. Under the same impact conditions, the drag force exerted on a particle residing on a dry surface can be three orders of magnitudes larger than on a wet surface. Improving droplet impact velocity is more effective than improving droplet size.

Numerical simulation on the mechanism of the normal impact of two droplets onto a thin film

The normal impingement process of two water droplets upon a thin film on the solid surface was numerically investigated. The numerical treatment was based on the finite volume solution of the Navier-Stokes （N-S） equations combined with the volume of fluid method （VOF）. Physically reasonable results for the process of two droplets impacting on the thin film were obtained. The effects of the droplet velocity, the fihn thickness and the spacing between the two droplets on the splash and spread process of the impact were examined.

Simulation of Droplet Impact onto Horizontal and Inclined Solid Surfaces with Lattice-Boltzmann Method

In this paper, the lattice-Bohzmann method is used to investigate the droplet dynamics after impact on horizontal and inclined solid surface. The two-phase interparticle potential model is employed. The model is found to possess a linear relation between the macroscopic properties （ surface tension σ and contact angle α） and microscopic parameters （ G, G, ）. The flow state of the droplet on the surface is analyzed in detail, and the effects of surface characteristic, impact velocity, impact angle, the viscosity and surface tension of the liquid are investigated, respectively. It is shown that the lattice-Bohzmann method can not only track exactly and automatically the interface, but also the simulation results have a good qualitative agreement with ones of the previous experimental and numerical studies.

Simulation solution for micro droplet impingement on a flat dry surface

This paper presents a computational fluid dynamics approach for micro droplet impacting on a flat dry surface. A two-phase flow approach is employed using FLUENT VOF multiphase model to calculate the flow distributions upon impact. The contact line velocity is tracked to calculate the dynamic contact angle through user defined function program. The study showed that the treatment of contact line velocity is crucial for the accurate prediction of droplet impacting on poor wettability surfaces. On the other hand, it has much less influence on the simulation of droplet impacting on good wettability surfaces. Good fit between simulation results and experimental data is obtained using this model.

Droplet impact on wetted structured surfaces

In this study,we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities.The time evolution of crown geometry is strongly affected by the surface structure.When the thickness of the liquid film is larger than the structure height,the expanding speed of the crown base radius is independent of the structure width.However,if the liquid film thickness is equal to the structure height,the crown base expands slower as the structure width increases.Surface structures have strong effects on the crown height and radius,and can prevent ejected filament from breaking into satellite droplets for certain cases.For the liquid film with the thickness equal to the pillar height,both the crown height and the radius exhibit non-monotonic behaviors as the pillar width increases.There exists one pillar width which produces the smallest crown height and the largest crown radius.

Numerical simulation on partial coalescence of a droplet with different impact velocities

Partial coalescence is a complicated flow phenomenon.In the present study,the coalescence process is simulated with the volume of fluid(VOF)method.The numerical results reveal that a downward high-velocity region plays a significant role in partial coalescence.The high-velocity region pulls the droplet downward continuously which is an important factor for the droplet turning into a prolate shape and the final pinch-off.The shift from partial coalescence to full coalescence is explained based on the droplet shape before the pinch-off.With the droplet impact velocity increasing,the droplet shape will get close to a sphere before the pinch-off.When the shape gets close enough to a sphere,the partial coalescence shifts to full coalescence.The effect of film thickness on the coalescence process is also investigated.With large film thickness,partial coalescence happens,while with small film thickness,full coalescence happens.In addition,the results indicate that the critical droplet impact velocity increases with the increase of surface tension coefficient but decreases with the increase of viscosity and initial droplet diameter.And there is a maximum critical Weber number with the increase of surface tension coefficient and initial droplet diameter.

SPH Modeling of Droplet Impact on Solid Boundary

A droplet undergoes spreading,rebounding or splashing when it impacts solid boundary,which is a typical phenomenon of free surface flow that exists widely in modern industry.Smoothed particle hydrodynamics(SPH)method is applied to numerically study the dynamical behaviors of the droplet impacting solid boundary,and both the spreading and rebounding phenomena of the droplet are reproduced in the simulation.The droplet deformation,flow fields and pressure fields inside the droplet at different moments are analyzed.Two important factors,the initial velocity and diameter,are discussed in determining the maximum spreading factor,revealing that the maximum spreading factor increases with the increase of the impact velocity and droplet diameter respectively.

Continuous droplet rebound on heated surfaces and its effects on heat transfer property: A lattice Boltzmann study

A thermal multiphase lattice Boltzmann(LB) model is used to study the behavior of droplet impact on hot surface and the relevant heat transfer properties.After validating the correctness of the codes through the D^(2) law,the simulations of intrinsic contact angle and the temperature-dependent surface tension are performed.The LB model is then used to simulate the droplet impact on smooth and micro-hole heated surface.On the smooth surface,the impinging droplet is reluctant to rebound,unless the intrinsic wettability of the solid surface is fairly good.On the micro-hole surface,however,the micro-holes provide favorable sites for generating a high-pressure vapor cushion underneath the impinging droplet,which thereby facilitates the continuous droplet rebound.For the continuously rebounding droplet.The time evolution of volume and temperature display obvious oscillations.The achievable height of the rebounding droplet increases as the intrinsic wettability of the solid surface becomes better,and the maximum transient heat flux is found to be directly proportional to the droplet rebounding height.Within a certain time interval,the continuous rebounding behavior of the droplet is favorable for enhancing the total heat quantity/heat transfer efficiency,and the influence of intrinsic wettability on the total heat during droplet impingement is greater than that of the superheat.The LB simulations not only present different states of droplets on hot surfaces,but also guide the design of the micro-hole surface with desirable heat transfer properties.

Study on Nonlinear Coupling Wave Model for Liquid Droplet-Solid Impact

In order to investigate the material corrosion by liquid droplet solid impact, a nonlinear coupling wave model adopted to analyze the impact between the spherical liquid droplet and an elastic solid plane has been developed. Many usable results such as the dimensionless pressure in the contact plane of liquid solid and inside the liquid droplet, the equivalent stress distribution inside the solid, the effect of solid elasticity on the impact, and the locations of the maximum equivalent stress in different...

Numerical investigation of Weber number and gravity effects on fluid flow and heat transfer of successive droplets impacting liquid film

Droplet-based high heat flux dissipation technique under multi-gravitational environments has gained increasing research attention due to the increased requirements of heat dissipation in advanced air-/space-borne electronics.In this paper,a threedimensional model was developed to investigate the impact of continuous droplets on liquid film under various Weber numbers and gravity loads.In other words,the effects of Weber number and gravity load on the flow and heat transfer characteristics were investigated.The results demonstrated that the dissipated heat flux was positively correlated with both Weber number and gravity load.A large Weber number indicated larger kinetic energy of a droplet,leading to a greater disturbance on the impacted film area.When the Weber number was doubled,the average wall heat flux could be enhanced by 36.3%.In addition,the heat flux could be boosted by 5.4%when the gravity load ranged from 0 to 1g.Moreover,a weightless condition suppressed the vapor escape rates on the heating wall where the volume fraction of the vapor on the wall could increase by 20%under 0g,leading to deteriorated heat transfer performance.The novelty in this paper lies in the accurate three-dimensional modeling of an aerospaceoriented droplet impacting two-phase heat transfer and fluid dynamics,associating macro-scale thermal performance to microscale thermophysics mechanisms.The findings of this study could guide the development of aerospace-borne spray cooling facilities for advanced aerospace thermal management.

Rapid surface texturing to achieve robust superhydrophobicity,controllable droplet impact,and anti-frosting performances

Robust superhydrophobic surfaces with excellent capacities of repelling water and anti-frosting are of importance for many mechanical components.In this work,wear-resistant superhydrophobic surfaces were fabricated by curing a mixture of polyurethane acrylate(PUA)coating and 1H,1H,2H,2H-Perfluorodecyltrichlorosilane(HFTCS)on titanium alloy(TC4)surfaces decorated with micropillars pattern,thus,composite functional surfaces with PUA coating in the valleys around the micropillars pattern of TC4 were achieved.Apparent contact angle on fabricated surfaces could reach 167°.Influences of the geometric parameters of micropillars pattern on the apparent contact angle were investigated,and the corresponding wear-resistant property was compared.Droplet impact and anti-frosting performances on the prepared surfaces were highlighted.An optimized design of surface texture with robust superhydrophobicity,controllable droplet impact,and anti-frosting performances was proposed.This design principle is of promising prospects for fabricating superhydrophobic surfaces in traditional mechanical systems.

Dynamic spreading characteristics of droplet impinging soybean leaves

The study on the deposition efficiency of pesticide droplets on soybean leaves can provide the basis for reducing pesticide quantity and increasing pesticide efficiency during the application of soybean plant protection machinery.The movement behavior of droplet impinges on the plant leaf surface is affected by many factors,among which the most important and the easiest to adjust are spray droplet size and impingement velocity.By changing the droplet size and impact velocity and using Fluent simulation software,the pesticide droplet hitting the soybean leaf surface was simulated and a test platform was established to verify the simulation results.The conclusions are as follows:The longitudinal roughness of soybean leaves is higher than the transverse roughness,the longitudinal pressure of soybean leaves is higher than the transverse pressure during the impact process,and the velocity of droplet spreading along the longitudinal is lower than that of spreading along the transverse;although soybean leaf surface has high adhesion,droplet losses still exist when droplet impact velocity is relatively high.The maximum spreading diameter of the droplet increases first and then decreases with the increase of impact velocity.At the same time,the maximum spreading diameter of droplet increases with the increase of particle size.The droplet deposition was best at 1.34 m/s impact velocity and 985μm particle size.This conclusion can provide optimal operation parameters for soybean plant protection operation which can be used to guide soybean plant protection operation,improve control effect,reduce quantity and increase efficiency.

Effect of Feather Elasticity of Kingfisher Wing on Droplet Impact Dynamics

We experimentally studied droplet impact dynamics onto wing feathers of kingfishers. Distilled water droplets with a fixed diameter of 2.06 mm were used as drop liquid and the initial impact velocities of droplets varied from 0.28 m· s^-1 to 1.60 m·s^-1. Two high-speed cameras were utilized to capture the impact process of water droplets onto the wing feather surface from both horizontal and vertical directions. Two states of the feathers （elastic and inelastic） were considered to study the influence of elasticity. At the entire impact ve- locity range we studied, regular rebound, bubble trapping and jetting, partial pinning and partial rebound of droplets on inelastic wing feather surface were observed as the initial impact velocity increased. However, only one dynamic behavior （regular rebound） was found on the elastic wing feather surface. The elasticity plays a more important role in the direction difference of droplet spreading than wing feather microstructure. The contact time of water droplets on the elastic wing feather surface was less than that on the inelastic surface within the range of Web numbers from 1.06 to 36 under test conditions.

SIMULATION OF THE TWO PHASE FLOW OF DROPLET IMPINGEMENT ON LIQUID FILM BY THE LATTICE BOLTZMANN METHOD

A Lattice Boltzmann Method （LBM） with two-distribution functions is employed for simulating the two-phase flow induced by a liquid droplet impinging onto the film of the same liquid on solid surface.The model is suitable for solution of twophase flow problem at high density and viscosity ratios of liquid to vapor and phase transition between liquid and its vapor.The roles of the vapor flow,the density ratio of liquid to vapor and the surface tension of the droplet in the splashing formation are discussed.It is concluded that the vapour flow induced by the droplet fall and splash in the whole impinging process may affect remarkably the splash behaviour.For the case of large density ratio of liquid to vapor a crown may engender after the droplet collides with the film.However,for the case of small density ratio of liquid to vapor a ＂bell＂ like splash may be observed.

Effect of droplet size on the jet breakup characteristics of n-butanol during impact on a heated surface

Jet breakup during droplet impacted on heated surface has received wide concerns due to its regularity.The Weber number(We)is a common used dimensionless parameter to classify and analyze the phenomenon,but it is unable to summarize all detailed phenomenon variations and related theory.Thus,the effect of droplet size on jet breakup was investigated by considering its significant difference in various scenarios.The behavior of n-butanol droplets with diameter range from 1.71 mm to 2.84 mm impacts a heated surface with jet breakup was recorded by backlit technology.Three parameters,the jet breakup time,the jet breakup length and the jet ligament diameter,were analyzed to illustrate the phenomenon.The results showed that these parameters are affected by the droplet size largely at 10<We<35,but affected lightly at higher Weber numbers.The sub-droplet diameter reduces with Weber number for all initial size droplets while the larger initial droplet corresponds to larger sub-droplet.The breakup time,breakup length and jet liga­ment diameter increase with the initial diameter.The time of jet breakup versus agrees well with the theory of Rayleigh instability regardless of droplet diameter.In addition,the jet breakup phenomenon was observed to evolve into pancake bouncing with an ultrafine high-speed jet when the We was above 50.

Understanding Spray Coating Process: Visual Observation of Impingement of Multiple Droplets on a Substrate

Spray coating is a facile deposition process with numerous existing and emerging applications. However,spray coating is a stochastic process comprising impingement of many droplets which upon impact on a heated substrate may dry or solidify individually or coalesce first to form a thin liquid film and then dry to yield a thin solid film. There is very limited knowledge on how this process occurs; therefore in this work, high speed imaging is used to visualize the spray coating process. Two model solutions including food-dye with properties like those of water, and poly(3, 4-ethylenedioxythiophene) : poly(styrenesulfonate)(PEDOT : PSS), a polymeric solution, are sprayed onto permeable glossy paper and regular impermeable glass substrates. Substrates are kept at room temperature and 80?C elevated temperature. In some cases, a vertical ultrasonic vibration is imposed on the substrate to study its effect on the coating process. It is observed that the spray coating process is highly random and stochastic. A higher substrate temperature results in a better coating process in that a more uniform and defect-free coating forms. Imposed vibration in the case of glossy paper substrates results in better droplet spreading and more uniform coating. The results also show that under the conditions of these experiments,impinged droplets dry individually or as islands of multiple coalesced droplets to form a coating. In other words,at used spray flow rate and spray droplet size, a continuous thin liquid film does not form prior to drying even at room temperature. Further systematic studies and high magnification lenses are required to visualize and understand the details of the process.