The impact of liquid drops on solid surfaces is a ubiquitous phenomenon in our everyday experience; nevertheless, a general understanding of the dynamics governing droplet impact remains elusive. The impact event is understood within a commonly accepted hydrodynamic picture: impact initiates with a rapid shock and a subsequent ejection of a sheet leading to beautiful splashing patterns. However, this picture ignores the essential role of the air that is trapped between the impacting drop and the surface. We describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. We directly visualize the rapid spreading dynamics succeeding the impact of a droplet of fluid on a solid, dry surface. We show that the approach of the spreading liquid toward the surface is unstable, and lift-off of the spreading front away from the surface occurs. Lift-off ensues well before the liquid contacts the surface, in contrast with prevailing paradigm where lift-off of the liquid is contingent on solid-liquid contact and the formation of a viscous boundary layer. We show that when a drop impacts an atomically smooth mica surface, a strikingly stable nanometer thin layer of air remains trapped between the liquid and the solid. This layer occludes the formation of contact, and ultimately causes the complete rebound of the drop. / Engineering and Applied Sciences

An aircraft may experience inlight ice accretion and corresponding reductions in performance and control when the vehicle encounters clouds of super-cooled water droplets. The EADS-IW Surface Engineering Group is investigating passive anti-icing possibilities, such as functional and ice phobic coatings. Ice-resistant coatings require investigating droplet impact on dry surfaces and wet films, including microscopic effects such as droplet splashing. To investigate droplet impacts, a volume of fluid (VOF) flow solver was used for droplets impacting dry and wetted hydrophobic and superhydrophobic surfaces, focusing on meso-scale simulations. The effects of structured, micro-scale surface roughness and the effects of a thin wet film on the surface, corresponding to a saturated surface under high humidity conditions, were investigated. Axisymmetric domains produced acceptable results for smooth, dry surfaces. It was determined that in order to properly predict behavior of droplets impacting surfaces with structured micro-scale roughness, three-dimensional simulations are recommended.

In order to study inlight ice adhesion at the droplet-scale, a strategy is presented to simulate the impact and solidification of a supercooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled Volume of Fluid and Level-Set method to capture the air-water interface and an Enthalpy-Porosity method to capture the liquid-solid interface. Calibration of a simulation parameter, Amush, is performed in order to match experimental data for different surface types and surface temperatures. The calibrated simulation strategy is applied to low-speed, inlight icing conditions. The effects of surface variation and droplet diameter variation are investigated, providing insight into the icephobicity of superhydrophobic surfaces. Numerical results suggest that large droplets (approximately 200 micron-diameter) will freeze and adhere to a superhydrophobic surface.

Icing on aircraft wings is one of the leading causes of aircraft crashes. It is mainly caused due to accumulation of ice or snow on the wing surface due to impact with supercooled droplets when passing through clouds at high altitudes, causing loss of lift obtained by the wings. It was found that droplet impact characteristics are dependent on droplet size, surface roughness, surface material hydrophobicity, and droplet impact velocity. As a continuation of the study of droplet impact contact characteristics by varying surface roughness and impact velocity, this study focuses on droplets impacting the vibrating surface at frequencies between 2-7 kHz. Atomization (water drop splitting into smaller droplets and ejecting from the surface) has been observed at different rates for all frequencies. The first set of data is collected by keeping roughness constant and increasing the amplitude of the vibration to observe the critical amplitude at which atomization is initiated. The surface roughness is varied for the second set of experiments. The data is quantified using image processing of the high-speed videos to obtain the rate of ejection for each case. / Master of Science / Icing on aircraft wings is among the leading causes of crashes, which involves small freezing water drops sticking to the wing surface thus reducing the lift. This study is an investigation to experimentally observe how small water droplets interact with surfaces vibrating at high frequencies when impacted. Surface roughness, materials, droplet velocities, and frequency of vibration have been varied and the droplet was captured using high-speed photography to study their effect on the aforementioned interaction. Glass, PET-G. and aluminum having specific roughness were fabricated using laser and chemical etching. Atomization (water drop splitting into smaller droplets and ejecting from the surface) has been observed at different rates for all frequencies. A relation between the amplitude of the vibration and the rate of atomization was found. The effect of varying frequencies and surface roughness has also been documented.

Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.   Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.   The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed. / <p>QC 20160329</p>

This study proposes a method to enhance the anti-icing capabilities of superhydrophobic surfaces by utilizing vibration to further reduce contact time of an impacting droplet in addition to keeping the droplet in the Cassie-Baxter regime, where surface adhesion is lower than the opposing Wenzel regime. We tested this with two methods: by investigating the effects of vibration normal to the plane of a superhydrophobic surface being impacted by water droplets in a room temperature environment, with the surface horizontal in a room temperature environment and tiled in a subzero degree environment. The amplitude and frequency of the vibration were varied in our experiments. Our results show that the mean contact time of a 10µL droplet consistently decreased linearly as the vibration frequency increased, though the standard deviations drastically increased. The ice accretion in the second phase of the testing also had significant variance, which obfuscated any reliable trend from the introduction of vibration.

Nous étudions les mécanismes d’endommagement dus à l’érosion pluie sur des matériaux massifs et sur des matériaux revêtus. Pour cela, un banc d’essais spécifique a permis de mener des recherches en laboratoire. Le principe de l’essai repose sur l’émission de jets à haute vitesse et à haute fréquence.L’étude des matériaux massifs met en évidence un mécanisme de propagation de fissures par fatigue qui aboutit à des pertes de matière. Ces mêmes endommagements ont été observés sur des échantillons usés en service. Il a été confirmé que la dureté augmente la résistance à l’érosion pluie des matériaux métalliques.L’étude des revêtements s’est focalisée sur 2 technologies, à savoir la pulvérisation cathodique magnétron, qui est un procédé de déposition phase vapeur, et la projection thermique sous flamme supersonique. Les revêtements obtenus par projection thermique (dont l’épaisseur était supérieure à 200 μm), se sont révélés moins résistants à cause d’un manque d’adhérence ou de la présence de défauts au sein du matériau. Les revêtements obtenus par PVD (dont l’épaisseur était inférieure à 30 μm) ont permis d’obtenir des gains de résistance significatifs. Dans tous les cas, quel que soit la technologie utilisée, l’adhérence du revêtement s’est révélé être un paramètre critique en ce qui concerne la résistance de la surface à l’érosion pluie. Enfin, une simulation numérique en dynamique a permis d’étudier les champs de contraintes dans des feuillets métalliques, et ce, en faisant varier leurs épaisseurs, les matériaux qui les composent, et la vitesse d’impact. Les résultats de la simulation tendent à prouver que la propagation des ondes de contraintes dans le matériau peut entrainer des phénomènes de sur-contraintes dans les feuillets les plus fins à cause de la réflexion des ondes sur la face antérieure de la plaque. / The study deals with the rain erosion mechanisms of both bulk and coated materials. For that purpose, a specific test bench has been built, enabling laboratory research. The principle of the test is based on the emission of high-speed water jets at high frequency.Studying bulk materials confirmed the positive influence of hardness on rain erosion resistance for metallic materials. The mechanism found responsible for material losses is the propagation of fatigue cracks. These fatigue damages were also observed on in-service worn out samples.The analysis of coated materials focused on two covering technologies, namely physical vapor magnetron sputtering deposition (Magnetron PVD) and high velocity oxy-fuel coating spraying (HVOF). The coatings obtained by HVOF (> 200μm) weren’t resistant enough due to lack of adhesion and/or due to specific defects within the material. The coatings obtained by PVD (< 30μm) have yielded to significant improvements on the surface resistance. However, the adhesion of the coating appeared as a critical parameter for the rain erosion resistance.Finally, a numerical simulation has been designed to study dynamic stress fields in metal sheets. Moreover the model allowed the sheets thicknesses, the materials, and the speed of impact to vary. The simulation showed that the propagation of stress waves in the material may cause over-stresses phenomena in the thinner sheets, due to the reflection waves on the back side of the plate.

Dans le cadre de la modélisation et de l’expérimentation multi-échelles (projet LabEx MMCD pour les matériaux pour la construction durable) de l’Université Paris-Est Marne-la-Vallée, cette thèse de doctorat vise à modéliser et caractériser les micro-matériaux conçus par impact de gouttelettes de céramique fondue. Les applications de ces matériaux revétus de couches minces sont des traitements de surface pour la construction durable tels que la protection anti-corrosion, les barrières thermiques, le traitement du verre ou les renforts mécaniques. En particulier, nous nous concentrons sur la physique associée à la dynamique des gouttelettes liquides (l'aire de contact et le temps de contact entre la gouttelette et la surface) en effectuant une série de simulations numériques pour les écoulements diphasiques à petite échelle avec le code maison Thetis. Nous avons considéré des variations des conditions d'impact initiales ainsi que l’influence des forces d'inertie, capillaire et visqueuses sur la dynamique des gouttelettes. Nous nous sommes intéressés en particulier au diamètre d'étalement maximal, au temps d’étalement maximal et au temps de contact, sur des surfaces solides de mouillabilité variable. Le code est basé sur l’utilisation d’une méthode Volume-Of-Fluid. Il introduit une fonction auxiliaire régularisée pour estimer la courbure locale et la normale à l'interface. Les principaux liquides de référence adoptés sont l'eau et la céramique fondue, l'eau est choisie pour valider notre code en comparant les simulations aux résultats expérimentaux. La céramique fondue est adoptée car elle est largement utilisée en projection thermique pour créer des barrières thermiques et chimiques (couches anti-oxydantes) ainsi que des renforts mécaniques sur des échantillons spécifiques. Nous nous concentrons sur les cas où les surfaces sont hydrophobes, même si les cas hydrophiles sont également considérés dans les configurations de validation pour des raisons de généralité. Egalement, en introduisant une partie de calcul de l'énergie dans la thèse, une analyse énergétique détaillée de la gouttelette après l'impact est effectuée dans les phases d'étalement et de rétraction pour bien comprendre la dynamique à l'intérieur de la gouttelette. Nous trouvons que le temps de projection est inversement proportionnel à la vitesse d’impact, indépendamment de l’angle de contact lors de l’étalement au temps courts. Une nouvelle mise à l'échelle entre l'étalement maximal et le temps d'étalement est proposée. Celle-ci s'accorde très bien avec les résultats expérimentaux. Par ailleurs, nous introduisons cette mise à l’échelle dans une classe de modèle basée sur la conservation de l’énergie pour prédire l’étalement maximal adimensionné, ce qui permet de mieux prévoir l’étalement maximal adimensionné. Pour finir, une mise à l'échelle du temps de contact est proposée en termes de nombre d'Ohnesorge et de Reynolds / Under the framework of the LabEx Multi-Scale Modelling and Experimentation of Materials for Sustainable Construction, of Université Paris-Est Marne-La-Vallée, the present PhD thesis aims at modelling and characterizing micro-material designed by impact of molten ceramic droplets. The applications of thin coating materials are surface treatments for sustainable construction such as anti-corrosion, heat barrier, glass treatment or mechanical reinforcement of specific structures.In particular, we focus on the physics behind the liquid droplets' dynamics (the contact area and the contact time between the droplet and surface) by conducting a series of small scale multiphase flow numerical simulations with home-made code Thetis. All simulations are axisymmetric. We have considered variations of initial impact conditions, and studied the influence of inertial, capillary and viscous forces on the droplets' dynamics, especially the maximum spreading diameter, spreading time and the contact time, on solid surfaces. The code is based on Volume-Of-Fluid techniques and introduces an auxiliary smooth function to estimate the local curvature and the normal to the interface. The major reference liquid adopted are the water and the molten ceramic, the water is chosen to validate our code against available experiments at the beginning. The molten ceramic is adopted as it is widely used in thermal spray to built thermal and chemical barriers (anti-oxidant layers) as well as mechanical reinforcements on specific samples. We focus on the cases in which the surfaces are hydrophobic, even if hydrophilic cases are also considered in validation configurations for the sake of generality. Meanwhile, by introducing an energy calculation part in the code, a detailed energetic analysis of the droplet after impact is performed in both the spreading and retraction stage to have a deep understanding of the dynamics inside the droplet.We find the jetting time is inversely proportional to the impact velocity, independent of the contact angle in the early spreading. A new scaling between maximum spreading and spreading time is observed, and agrees well with experimental results. Further, we introduce this scaling into the model based on energy conservation to predict the maximum spreading factor, which provides better prediction on maximum spreading factor than existing literature references. Also a scaling of contact time is proposed in terms of Ohnesorge number and Reynolds number

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.