Drop impacts are present in our everyday lives, from showering and washing the dishes to inkjet printing and many industrial processes, such as spray coatings and spray cooling. In many of these applications it may be undesirable to have air entrained within the drop when it impacts a surface. As a drop approaches a surface, the gas beneath the drop is unable to fully escape resulting in a rising pressure which becomes sufficient to form a dimple in the bottom center of the drop. Therefore, when the drop makes contact with the surface, it is around the perimeter of this dimple, thus entrapping a disc of air which contracts into a minute bubble. In this dissertation, we study the very early time dynamics of the formation of the central air disc under a variety of circumstances using ultra-high-speed interferometry at rates up to 5 million frames per second. We show the effects of the liquid viscosity for viscosities spanning 7 orders of magnitude, for impacts of drops onto solid surfaces or a film of the same liquid. We find that the size of the air disc is weakly dependent on the drop viscosity to the -1/9 power. We also explore the extended gliding of the drop on a less than 160 nm thick film of air. For impacts onto a solid surface, this gliding layer is rupture in multiple random locations and each localized contact wets the surface at extreme rates compared with the expected viscous-capillary velocity. For impacts onto liquid films, the localized contacts are rarely observed and the gliding layer ruptures at a uniform location. The central bubble contracts much faster than expected in this case as well. Furthermore, we study the effects of reducing the ambient air pressure discovering a compressible and rarified-gas regime wherein the drop makes a double contact with the surface. Lastly, we study the effects of nano-scale surface roughness on the central bubble and the formation of thick bands of microbubbles around the periphery of the air disc.

Dans la galvanisation à chaud, les aciers sont protégés contre la corrosion par une mince couche de zinc obtenue par immersion dans un bain d’alliage de zinc. Avant ce processus, les tôles d'acier subissent un recuit de recristallisation afin d'éliminer l’écrouissage après laminage à froid. Les conditions de recuit utilisées réduisent le film d'oxyde de fer natif, ce qui favorise la mouillabilité de la surface de l'acier par le zinc liquide. Cependant, les nouveaux aciers à haute résistance contiennent des quantités importantes d'éléments d’addition, tels que le silicium et le manganèse. Ces élements diffusent à la surface de l'acier pendant le recuit de recristallisation et forment des particules ou des films d'oxyde par oxydation sélective externe. Si le fer pur est bien mouillé par le zinc liquide, ces oxydes ne le sont pas et leur présence à la surface peut entraîner des défauts dans le revêtement final.Pour étudier l'influence de la taille et de la distribution des oxydes sur le mouillage par le métal liquide, nous avons étudié un mouillage non réactif du plomb liquide sur une surface hétérogène texturée Fe / silice en utilisant la technique de chute de goutte.Ces surfaces ont été conçues par dépôt chimique en phase vapeur assisté par plasma, suivi d'un procédé photolithographique.Après l'impact, la goutte s'étend jusqu'à son diamètre d'étalement maximal. S’ensuit une phase de recule de la goutte. Pendant son recul, la goutte est plus ou moins retenue, en fonction du taux de couverture de silice, sur le fer pur: phénomène d’accrochage-glissement. Sur les surfaces à faible teneur en silice, ce phénomène entraîne une déformation de la forme de la goutte qui est plus allongée dans un sens et quelquefois à la division de la goutte.Il a été démontré que le mouillage est affecté principalement par la fraction de surface de la silice.Enfin, nous avons modélisé les différentes phases de l'étalement de la goutte sur ces surfaces hétérogènes. Des modèles de littérature ont été revus et adaptés et nous avons proposé des modèles macroscopiques de l'oscillation de la goutte pendant son étalement. / In hot-dip galvanizing, steel sheets are protected against corrosion by a thin layer of zinc obtained by immersion in a zinc alloy bath. Before this process, the steel sheets undergo recrystallization annealing to eliminate stresses after cold-rolling. The annealing conditions used reduce the native iron oxide film, which promotes the wettability of the steel surface with liquid zinc. However, new high-strength steels contain significant quantities of addition elements, such as silicon and manganese. These elements diffuse on the surface of the steel sheets during recrystallization annealing and form oxide particles or films by selective external oxidation. If pure iron is well wet with liquid zinc, these oxides are not and their presence on the surface can lead to defects in the final coating.To study the influence of oxide size and their distribution on liquid metal wetting, we studied a non-reactive wetting of liquid lead on a heterogeneous Fe / silica textured surface using the dispensed technique.These surfaces were designed by plasma-assisted chemical vapour deposition followed by a photolithographic process.After impact, the drop extends to its maximum spreading diameter. This is followed by a phase of drop receding. During this, the drop is more or less retained, depending on the silica coverage rate, on the pure iron: stick-slip motion. On surfaces with low silica content, this phenomenon causes a deformation of the drop shape which is more elongated in one direction and sometimes at the division of the drop.We showed that wetting is mainly affected by the surface fraction of silica.Finally, we modelled the different phases of drop spreading on these heterogeneous surfaces. Literature models were reviewed and adapted and macroscopic models of the oscillation of the drop during its spreading were proposed.

This thesis studies a droplet of ferrofluid impacting a liquid water pool. The ferrofluid is oil-based and therefore immersible in water. The shape of the ferrofluid drop at impact is changed by using an electromagnet underneath the liquid pool. The magnet is turned off by an external trigger just before the drop collides with the liquid pool surface, to stop the magnetic interaction. The prolate or oblate shape of the drop has an influence on the cavity formation and evolution after the impact. The experiments look specifically at the maximum depth and diameter of the cavity, as a function of the drop impact shape for the same impact velocity. This is done over a range of impact velocities. The prolate drops generate deeper cavities than spherical or oblate drops. Furthermore, a study is conducted on the jet formation that occurs during the cavity collapse to investigate the influence of droplet shape on the jet velocity.

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The interaction of a liquid drop with a solid surface is being actively studied to understand practically encountered scenarios such as the impact of fuel spray droplets onto the walls of engine combustion chamber, the formation of thermal barrier coating on the surfaces of turbine blades, the process of ink-jet printing, etc. The surface topography of solid surface is one of the major parameters influencing the dynamics of drop-surface interaction process. Understanding the precise role of surface topography features such as micro asperities and grooves on the spreading and receding processes of impacting liquid drops is crucial for the improvement in abovementioned applications. Recent developments in the fabrication of micro- and nano-structures on solid surfaces provide fabulous opportunities to investigate the role of single/multiple micro asperities and grooves on the dynamics of impacting drops. The thesis deals with an experimental work on the impact of water drops on stainless steel surfaces comprising unidirectional parallel grooves. A group of six target grooved surfaces covering a wide range of surface wettability were considered. The target surfaces were prepared using the techniques of photolithography, electro discharge machining, and laser machining. Scanning electron microscope and optical surface profilometer were used to characterize the groove texture geometrical parameters of the target grooved surfaces. The experiments of drop impact were carried out in an experimental apparatus consisting of a liquid drop generator, a substrate table, and a digital video imaging system. Free-falling distilled water drops released from a certain height were allowed to impact normally on the target surfaces. The image sequences of drop impact dynamics were constructed from the images captured using the digital video imaging system. Majority of the drop impact experiments were captured using a high speed video camera operating with frame speed ranging from 3000 to 10000 fps. For the target grooved surfaces, the impact dynamics was analyzed for the impacting drop liquid oriented both in the direction perpendicular to the grooves ( ) and in the direction parallel to the grooves (||) via independent test runs. The captured digital frames were used to deduce the temporal variation of impacting drop parameters such as drop contact diameter, drop contact angle, and drop height at the center of impacting drop with the aid of image processing software. The impacting drops were characterized in terms of Weber number, We expressed in terms of drop impact velocity and drop diameter measured just before the start of impact process. The study covered We ranging from 1.8 to 170. In general, the groove texture on the solid surface influences the drop impact process for all We examined in the study. The effect is more pronounced for the receding of impacting drops. For high We drops, the groove texture enhances the perturbations seen at the periphery of spreading lamella. The study showed quantitatively that the drop impact process on a target grooved surface comprising unidirectional parallel grooves develops a non-axisymmetric drop flow on the grooved surface exhibiting different spreading and receding processes of impacting drop liquid in the directions perpendicular ( ) and parallel (||) to the grooves. The maximum spreading diameter reached immediately after the completion of early inertia-dominated spreading in is less than that obtained in || due to the loss of drop kinetic energy caused by the pinned motion of drop liquid in . The non- axisymmetric drop flow on the target grooved surface develops a difference between the frequencies of contact angle oscillation of impacting drop liquid in  and ||. The frequency difference in contact angle oscillation causes the beating phenomenon in the temporal variation of the contact angle anisotropy, Δθ and drop height at the center of impacting drop, Z. For a given target grooved surface, the experimental measurements suggested that the beat frequency is almost independent of We. The temporal variation of Δθ and Z do not show the traces of beating phenomenon for the impact of high We drops. Owing to the non-axisymmetric drop flow, the final equilibrium drop shape is eccentric for the impact of low We drops and approaches a circular shape for the impact of high We drops. The role of groove texture geometrical parameters is seen in the drop impact process via the surface wettability especially for the impact of low We drops. Larger surface roughness factor makes the target grooved surface to exhibit hydrophobic characteristics.

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.

The research topic of liquid drop interaction with solid surfaces is being actively pursued to gain in-depth understanding of several practical cases such as the impingement of fuel spray droplets on surfaces like combustion chamber walls and piston top of an I.C. engine, heat transfer via spray impingement, ink-jet printing, etc. In most of the cases, the physical and flow properties of the liquid drop/spray may be fixed whereas it may be possible to tune the physical and chemical properties of the solid surface thereby enabling to control the interaction process. The present work belongs to the study of liquid drop-solid surface interaction process with special focus on the physical characteristics of solid surface. The thesis reports an experimental study of the dynamics of millimetric water drops impacted onto the junction of dual-textured substrates made of stainless steel. The dual-textured substrates consisted of hydrophobic (textured) and hydrophilic (smooth) portions. The entire textured portion comprised of parallel groove-like structures separated by solid posts/pillars. Two dual-textured substrates, which differ only in the geometry of their textured portions, were employed. Surface topography features of the dual-textured substrates were characterized using scanning electron microscopy (SEM) and optical surface profilometer. The wetting behavior of the textured and smooth portions of the substrates, quantified in terms of the equilibrium, advancing, and receding contact angles adopted by a water drop on the surface portions, was characterized experimentally through the methods of sessile drop formation, captive needle volume addition, and drop evaporation under ambient conditions. Free-falling water drops were impacted from a height onto the junction between the hydrophobic (textured) and hydrophilic (smooth) portions of the dual-textured substrates. A set of twelve different impact experiments were conducted on each of the target substrates with drop impact velocity (Uo) ranging from 0.37 to 1.50 m/sec. The dynamics of drop impact were captured using a high speed camera with frame rate ranging from 3000 to 10000 frames per second. From the captured frames, the temporal variations of the impacting drop parameters were measured using a MATLAB-assisted program. A systematic analysis of experimental data revealed the existence of four distinct regimes of drop dynamics on the dual-textured substrate: (a) early inertia driven drop spreading, (b) primary drop receding, (c) secondary spreading on the hydrophilic portion, and (d) final equilibrium regimes. It is shown that the drop impact dynamics during the early inertia driven impact regime remains unaffected by the dual-texture feature of the substrate. A larger retraction speed of impacting drop liquid observed on the hydrophobic portion of the substrate makes the drop liquid on the higher wettability/hydrophilic portion to advance further (secondary drop spreading). The net horizontal drop velocity towards the hydrophilic portion of the dual-textured substrate decreases with increasing drop impact velocity. The available experimental results suggest that the movement of bulk drop liquid away from the impact point during drop impact on the dual-textured substrate is larger for the impact of low inertia drops. A semi-empirical model, based on the balance of the wettability gradient, contact angle hysteresis, and viscous forces acting on impacted drop liquid on the substrate, is formulated to predict the movement of bulk drop liquid away from the impact point (ξ). A satisfactory comparison between the model predictions and the experimental measurements is reported for the variation of ξ with Uo.

Dans cette thèse nous nous intéressons au problème de l’impact d’une goutte sur une surface solide. Nous proposons pour cela de nous placer dans un cadre plus général en utilisant les analogies existantes avec d’autres problèmes d’impact. Dans la première partie de ce manuscrit nous proposons de revisiter le problème de l’impact de goutte pour les temps courts à la lumière de son problème dual à savoir l’impact d’un objet solide dans un bain liquide lorsque l’inertie est l’effet dominant. De cette analogie est déduit un modèle reposant sur la théorie des écoulements potentiels. L’analyse asymptotique nous permet de dégager à l’ordre dominant les mécanismes essentiels de ce problème puis nous mettons en évidence la structure autosimilaire des champs de pression et de vitesse induits par l’impact. La structure de la couche limite est également étudiée. Les prédictions théoriques issues de ce modèle sont comparées à des solutions numériques obtenues à l’aide d’un solveur des équations de Navier-Stokes. Nous étudions ensuite les temps intermédiaires de l’impact, correspondants au moment où la solution autosimilaire cesse d’être valide et nous déterminons les causes de cette transition. Dans la troisième partie nous étudions un cas particulier d’évolution aux temps longs en revisitant le problème de l’impact d’une goutte sur un disque de même taille. Nous obtenons les solutions analytiques pour les champs de pression et de vitesse à l’instant initial et nous proposons ensuite différentes directions de recherche pour l’étude de l’évolution de la nappe liquide induite par l’impact. Nous finissons ce manuscrit par une brève introduction aux impacts de goutte de fluides à seuil. / In this thesis we consider the problem of drop impact onto a solid surface. In order to study this phenomenon we consider a more general framework by using analogies with some other impact problems which are a priori very different. In the first part of the thesis we propose to revisit the inertia-dominated drop impact problem for short times at the light of the dual problem defined by the impact of a solid object onto a liquid bath. We deduce from this analogy a model based on potential flow theory. Then asymptotic analysis is used to determine the essential mechanisms of the problem at leading order. This approach reveal a self-similar structure both for the velocity field and the pressure field induced by the impact. The structure of the boundary layer is also studied. Theoretical predictions deduced from this model are compared with numerical solutions obtained with the Navier-Stokes multiphase flow solver Gerris. Then we study the impact for intermediates times which correspond to the period of the breakdown of the self-similar solution. The origin of the transition is determined by using new numerical experiments. In a third part we propose to study a particular case of long time evolution by revisiting the problem of drop impact onto a solid target matching its own size. We obtain analytical solutions for pressure and velocity fields at initial time by using pressure impulse theory and we propose few research directions for the study of the evolution of the liquid sheet induced by the impact. This thesis ends with a brief introduction to drop impact of Bingham fluids.