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One of the most complicated issues the aerospace and aviation industries are dealing with is aircraft icing. The impact and freezing process of a water droplet on a cold surface has been investigated over time in order to develop preventative methods for avoiding icing. In the present study, we examined the behavior of a water droplet impacting on an aluminum plate with a surface roughness of 0.01µm and surface temperature variation from room temperature to 0oC, −5oC, −10oC and −15oC. The effect of droplet impact Reynolds number along with surface temperature variation on non-dimensional parameters like spread factor, retraction rate, and spread velocity is analyzed. The increase in impact Reynolds number and droplet spread factor is observed with a rise in the initial height of the droplet. At a higher Reynolds number, inertial forces are dominant over viscous and capillary forces, while at a lower Reynolds number, surface temperature shows a significant effect. The graphical representation of droplet retraction rate indicates a decrease with lower surface temperature and a rise with higher Reynolds numbers. Moreover, the spread velocity of the droplet is higher with an increased Reynolds number, and surface temperature does not have a notable effect on it. A rapid transition of momentum from vertical to horizontal direction occurs, and droplet dissipates energy in overcoming the viscous effects. The effect of surface roughness variation coupled with surface temperature is investigated in detail for three different surface roughness of aluminum and glass. The increase in surface roughness and temperature enhance hydrophobic behavior by repelling the droplet, while reduced surface temperatures show hydrophilic behavior by causing adhesion of the droplet on surface. / Master of Science / The supercool water droplets exist in the atmosphere and whenever these droplets come in contact with a cold surface, ice is formed. This ice accretion phenomena is observed not only on aircraft's control surfaces, but also on jet engines, power transmission lines and wind turbine blades. Research is on going to understand the impact and freezing process of water droplets on different cold surfaces and subsequently devise methods for avoiding this phenomena. In the current research work, the droplet impact is analyzed on an aluminum plate with surface roughness of 0.01µm. The spread factor of the droplet indicates the liquid surface contact area, and an increase is observed at larger heights in spread factor, impact velocity, and Reynolds number due to high inertia. Then, the surface temperature is varied from 0oC to −5oC, −10oC and −15oC, and it is observed that as the viscous effects are higher at lower surface temperatures, the droplet dissipates more energy in overcoming the high viscous effects and the spread factor decreases . Moreover, the spread velocity of the droplet is the measure of rate at which the liquid-solid contact area increases. Initially the droplet has vertical momentum, and on impact it shifts from vertical to horizontal direction, as the velocity rises drastically after impact. Surface roughness is another important factor that affects the ability of a surface to repel (hydrophobic) and attract (hydrophilic) the droplet by affecting its spread rate. The more the surface roughness, the droplet spread factor reduces and droplet rebounds indicating the hydrophobic nature. While adhesion is observed at the lower surface temperature, even with high roughness, showing the hydrophilic nature.

L'objet de cette thèse est de modéliser et simuler des écoulements diphasique/triphasique à phase non miscibles. L'impact de plusieurs gouttes d' émulsion (eau/huile) sur une plaque mouillante l'huile est simulée. Ainsi, une méthode pour lisser l'interface (SVOF), afin d'obtenir une courbure précise, basée sur une méthode eulérienne de type "Volume Of Fluid" (VOF) spécifique au caractère multiphasique de l' écoulement est développée. Un modèle de ligne triple et un modèle de mouillabilité sont développés pour calculer les forces capillaires. Ces modèles et ces méthodes sont validées partir de données expérimentales puis utilisées pour simuler le Plate-Outet les écoulements macroscopiques au voisinage du cylindre de laminage / The scope of this dissertation is to model and simulate non-miscible multiphase °ows. Theimpact of several emulsion droplet on the wetting steel strip is simulated. So, the method ofsmooth VOF based on Eulerian "Volume Of Fluid" approach which is particulary adapted tointerfacial °ows is developed. The new method SVOF allows to calculate the curvature with abetter precison than other method. A wettability model and a triple line model are developedto calculate the capillary forces. This models and this methods validated on the experimentaldata and used to simulate the Plate-Out and the macroscopic °ows in neighbourhood of coldrolling system.

The inhibiting effect of a sharp edge on liquid spreading is well observed during drop interaction with textured surfaces. On groove-textured solid surfaces comprising unidirectional parallel grooves, the edge effect of posts results in the squeezing of drop liquid in the direction perpendicular to the grooves and the stretching of drop liquid along the grooves leading to anisotropy in drop flow, popularly known as wetting anisotropy which has been employed in several engineering applications. A recent study observed that the energy loss incurring at the edges of posts via contact angle hysteresis is primarily responsible for the anisotropic spreading of impacting drops on groove-textured surfaces. The present study aims to elucidate the role of edges on the spreading and receding dynamics of water drops. The experiments of drop impact are carried out on semi-infinite rectangular post comprising a pair of parallel 90-deg edges separated by a distance (post width) comparable to the diameter of impacting drop. The equilibrium shape of drops on the semi-infinite rectangular post is analyzed using open source computational tool Surface Evolver to optimize the ratio of initial droplet diameter to post width. Quantitative measurements of drop impact dynamics on semi-infinite rectangular posts are deduced by analysing high speed videos of impact process captured under three different camera views during experiments. Based on the role of post edges on impacting drops, different regimes of the impacting drops are characterized in terms of drop Weber number and the ratio of diameter of impacting drop to post width. Characteristic features of impact dynamics in each of the regimes are identified and discussed. It is seen that edges play a pivotal role on all stages of impact dynamics regardless of Weber number. Impacts in the regime of completely pinned drops on narrow posts are further analyzed to reveal characteristics of post-spreading oscillations.

Dynamic behavior of impacting water drops on superhydrophobic solid surfaces provides important details on the stability/durability of such solid surfaces. Multi-scale surface roughness combined with a layer of low energy chemical is an essential surface modification process followed to create superhydrophobic capabilities on solid surfaces. The present work aims at studying the effect of low energy surface coating on droplet impact dynamics by carrying out experiments of water drop impacts on rough solid surfaces with and without chemical modification. A group of six aluminium alloy (Al6061) surfaces (three pairs) are prepared. Roughness, characterized in terms mean surface roughness, Ra, is introduced to these metallic surfaces using sand-paper polishing, electric discharge machining (EDM), and chemical based surface etching process. Low energy surface layer is laid on the rough surfaces by coating NeverWet hydrophobic solution, octadecyl-trichloro-silane (OTS), and perfluorodecyltricholorosilane (FAS-17). The impact dynamics of water drops is analyzed by capturing high speed videos for a range of drop Weber number from 1 to 570 and the salient features of drop impact process on the coated rough surfaces are compared with the corresponding uncoated rough surfaces. A one-to-one comparison on the spreading, fingering, receding, and final equilibrium of impacting drops on the coated and uncoated target surfaces is presented. Upon coating NeverWet, the original surface features of the base aluminium surface are completely covered by the hydrophobic coating material resulting in a fresh top surface layer. The outcomes as well as the bounce-off characteristics of impacting water drops on the coated surface are comparable to those observed on lotus leaf. The surface morphology features of rough aluminium surfaces coated with OTS and FAS-17 are comparable to those of the corresponding uncoated surfaces. The quantitative measurements on primary spreading and maximum spread factor of impacting drops are largely unaffected by the presence of low energy chemical coating. The dominant effect of surface coating is seen on the receding of impacting drops and hence the final drop configuration. This behavior is more prominently seen on EDM fabricated rough surface (larger Ra) combined with OTS coating than that on etching based rough surface (smaller Ra) combined with FAS-17 coating highlighting the dependence of coating effect with roughness features.