Insights on the impact of a plane drop on a thin liquid film (original) (raw)
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Droplet impact on a thin liquid film: anatomy of the splash
Journal of Fluid Mechanics
We investigate the dynamics of drop impact on a thin liquid film at short times in order to identify the mechanisms of splash formation. Using numerical simulations and scaling analysis, we show that it depends both on the inertial dynamics of the liquid and the cushioning of the gas. Two asymptotic regimes are identified, characterized by a new dimensionless number$J$: when the gas cushioning is weak, the jet is formed after a sequence of bubbles are entrapped and the jet speed is mostly selected by the Reynolds number of the impact. On the other hand, when the air cushioning is important, the lubrication of the gas beneath the drop and the liquid film controls the dynamics, leading to a single bubble entrapment and a weaker jet velocity.
Experimental and numerical studies of droplet impact on flowing liquid films
2018
Droplet impact on flowing liquid films constitutes an important research area due to its manifold applications both in industry and day-today living. Previous studies have, however, ignored the contributions of stochastic waves to the drop impact dynamics. In this project, an experimental study of droplet impact on controlled flowing liquid films is carried out. The aim of the study is to provide an understanding of the contributions of the spatial structures of waves to drop impact dynamics on flowing films. The experimental facility consists of a falling film rig which comprises film flow, film control, and droplet-generation units, as well as a high-speed imaging system. In a preliminary study, the effect of film control on the dominant wave propagation modes is investigated. Two classes of waves are identified, namely the gamma I and II wave families, which are characterised both qualitatively and quantitatively and confirmed to be in good agreement with previous studies in the literature. Studies on the interaction patterns between doubly-excited planar pulse waves on an uncontrolled flowing film surface are then carried out to provide insight into the interaction patterns of waveforms on flowing liquid films. Distinct interaction modes are found to be of central importance to understanding the complex wave interactions which could lead to interfacial 'turbulence'. The effect of film control on the impact dynamics of both low and highinertia drops is then studied. In both studies, the impact surface is divided into the "wave hump", "flat film", and "capillary waves" regions. For low-inertia drop impacts, film control is observed to have a qualitative and quantitative effect on the length of liquid columns formed in a partial coalescence outcome, the pinch-off time, as well as the size of the ejected daughter droplets. Qualitative differences included a complete change of the outcome, with other outcomes such as 'bouncing', 'sliding', and 'total coalescence' observed in the low-inertia drop impact scenario. For high-inertia drop impacts, the effect of film control is studied in the morphology of the crown produced in a splash outcome as well as the distinctive attributes of the ejected droplets.
Drop impact onto a liquid layer of finite thickness: Dynamics of the cavity evolution
Physical Review E, 2009
In the present work experimental, numerical, and theoretical investigations of a normal drop impact onto a liquid film of finite thickness are presented. The dynamics of drop impact on liquid surfaces, the shape of the cavity, the formation and propagation of a capillary wave in the crater, and the residual film thickness on the rigid wall are determined and analyzed. The shape of the crater within the film and the uprising liquid sheet formed upon the impact are observed using a high-speed video system. The effects of various influencing parameters such as drop impact velocity, liquid film thickness and physical properties of the liquids, including viscosity and surface tension, on the time evolution of the crater formation are investigated. Complementary to experiments the direct numerical simulations of the phenomena are performed using an advanced free-surface capturing model based on a two-fluid formulation of the classical volume-of-fluid ͑VOF͒ model in the framework of the finite volume numerical method. In this model an additional convective term is introduced into the transport equation for phase fraction, contributing decisively to a sharper interface resolution. Furthermore, an analytical model for the penetration depth of the crater is developed accounting for the liquid inertia, viscosity, gravity, and surface tension. The model agrees well with the experiments at the early times of penetration far from the wall if the impact velocity is high. Finally, a scaling analysis of the residual film thickness on the wall is conducted demonstrating a good agreement with the numerical predictions.
The role of time in single drop splash on thin film
Experiments in Fluids, 2004
This paper reports on an experimental study of the impact of water drops on thin liquid films. The morphology of the impact was studied by still photography, and quantitative results were obtained by a proper image analysis technique. The time evolution of various parameters like the crown diameter, the crown height, and the secondary drop diameters are reported, and these experimental parameters are correlated and compared to available theoretical models. A particular set-up of the acquisition system allowed us to photograph the splash from below the solid wall, allowing the first estimation of the crown thickness and the total number of jets protruding from the crown rim as a function of time. The results indicate that, for the range of the parameters investigated, there is not a strong dependence on the film thickness. The evolution of the crown height depends on the impact Weber number, whereas its growing velocity and the crown thickness evolution are almost independent of Weber number.
Abstraet-A numerical study is presented of the drainage and rupture of the liquid film between two drops whose centres approach each other at constant velocity. The considerations are restricted to the partially-mobile case (in which the drop viscosity is rate-determining) and to small approach velocities. The latter restriction permits a transformation of the governing equations to a single universal form, which is solved with the help of boundary integral theory. As in the constant force case, the numerical results show the formation of a dimple but the final drainage behaviour differs considerably. Finally, the influence of van der Waals forces is investigated and the results are shown to correspond well with a simple model proposed earlier for the effective critical film-rupture thickness.
Experiments in Fluids
The present paper investigates experimentally the splashing dynamics of two-component droplet wall film interactions. Over a wide range of Weber numbers and dimensionless film thickness, the different combinations of two low surface tension fluids, e.g. hexadecane and hyspin, including their corresponding one-component interactions, have been considered. As a first step, the splashing morphology is examined and the respective similarities with open literature data are reported. In addition, the splashing dynamics is investigated evaluating quantitatively the time evolution of crown height and diameter, the total number of liquid jets (fingers) generated at the upper crown rim as well as the total number of ejected secondary droplets including their corresponding diameter, cumulative volume and velocity magnitude. The results are analysed by various post-processing procedures aiming to provide a large dataset, which can be efficiently used for the validation of numerical models. Furthermore, the importance of the impact morphology for understanding the impact dynamics is pointed out.
Film Drainage between Colliding Drops at Constant Approach Velocity: Experiments and Modeling
Journal of Colloid and Interface Science, 2000
Experiments and modeling of the drainage of the thin liquid film between two deformable spherical drops approaching each other at constant velocity in another liquid are being presented. Two numerical models based on the lubrication theory have been developed considering the cases of immobile or mobile drop interfaces. The absolute film thickness and the thinning rate have been measured using laser interferometry for a wide range of capillary numbers. In all studied cases, the model with immobile interfaces was found to give the best predictions of the experimental time evolution of the film thickness and radial expansion. These results made it possible to derive a typical time scale of the drainage process.
Conference Paper: ILASS 2016, Brighton, UK, 2016
The phenomenon of a single drop impact onto a thin liquid film (e.g. wall-film) is encountered over a wide range of natural and engineering applications. Among different drop impact morphologies, splashing has drawn great attention over the last decades due to its relatively complex morphology that is characterised by liquid fingers at the upper crown rim, and subsequent secondary droplet ejection. In this paper, we show experimentally new splashing phenomena that are mainly observed for two-component droplet wall-film interactions. In particular, it is shown for very thin liquid films that even if the impact energy is insufficient to generate crown-type splashing, the drop impact may result in an ejection of a great amount of secondary droplets that are generated from a crown breakdown starting from the wall-film surface. Experiments are carried out with different combinations of hyspin and n-hexadecane as test liquids and over a range of drop Weber numbers (WeD=400-1600) and normalised wall-film thicknesses (h/D = 0.035 − 0.1). In addition, Particle-Tracking-Velocimetry (PTV) algorithms are used to determine velocity vectors of the secondary droplets for typical splashing as well as deposition cases. The results are analysed by various post-processing procedures aiming to quantify and clarify the impact morphology and dynamics of such a crown bottom breakdown formation. Introduction The impact of droplets onto thin liquid films finds great applicability in a variety of natural and industrial processes such as raindrops, ink-jet printing, spray cooling, fuel injection in engines, etc. A considerable amount of research has been therefore focused on the impact morphology of droplet wall-film interactions, e.g. a review can be found in Yarin [1]. Among different impact morphologies, splashing [2, 3] has drawn great scientific interest due to its relatively complex and fascinating features such as jets (liquid fingers) originating from the cusps of the upper crown rim and subsequent secondary droplet formation. For one-component droplet wall-film interactions, and among a wealth amount of published works, a threshold between splashing and non-splashing regimes has been established as a function of wall-film thickness, Weber and Ohnesorge numbers [4, 5, 6]. In addition, Cossali et. al [7] determined a number of characteristic splashing features such as number of jet fingering, secondary droplets and crown diameter, also as a function of time [8]. Contrary to one-component droplet wall-film interactions, and despite the significant industrial interest and applicability , the amount of investigations focused on two-component impact dynamics is very limited in the literature. For example, impacting fuel droplets on the cylinder surface of Diesel engines may dilute the oil lubrication performance, whilst, if the impact energy is sufficient to generate splashing, secondary droplets that consists of a combination of fuel and lubrication oil maybe ejected in the combustion chamber increasing dramatically the emissions of the engine. Thoroddsen et al. [9] investigated the impact of water/glycerine droplets on thin ethanol films. The authors observed thousands of small holes in the crown as a direct consequence of the surface tension difference between the drop and the film liquids that presumably result from Marangoni-driven flows and instabilities. Banks et al. [10] used water and several aqueous solutions in order to investigate the effect of droplet and wall-film viscosities on two-component droplet wall-film interactions. They found that the mechanism of crown formation strongly depends on the wall-film properties, while a small dependence of splashing on the droplet properties was also observed. In the studies of Geppert et al. [11, 12], various combinations between hyspin and hexadecane as test liquids were investigated. The authors reported similarities about the impact outcome of one and two-component droplet wall-film interactions, although a number of holes in the crown was observed for the latter case similar to Thoroddsen et al. [9]. A first correlation for the onset of splashing for two-component droplet wall-film interactions considering the Weber number (We), the average Ohnesorge number (Oh) and the normalised wall-film thickness (δ), was also reported [12]. More recently, Geppert et al. [13] found that, similar to one-component droplet wall-film interactions [7], the number of liquid fingers at the upper crown rim increases with reducing the wall-film thickness and the average Ohnesorge number of the drop and the film. This paper is also related to two-component drop wall-film impact dynamics and morphology. Here we report that for very thin wall-film thicknesses (0.035 < δ < 0.1), even if the impact energy is insufficient to generate crown-type splashing, a breakdown that originates from the wall-film surface ejects upwards a large amount of fast and small secondary droplets. Therefore, a new splashing phenomenon is observed, even at a deposition range of the
Multiple drops impact onto a liquid film: Direct numerical simulation and experimental validation
Computers & Fluids, 2021
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