Impact of drops on a dry smooth surface was studied at elevated chamber pressures and low Reynold's numbers to characterize the effect of chamber pressure on drop splashing and spreading. Two drop sizes of methanol, ethanol, propanol, hexadecane and diesel were tested for impact speeds between 1.5 - 3.3 m/s and pressure of upto 12 bars. Splash ratio, unlike the results of Xu et al, increased sharply with decreasing impact speed suggesting that drop speed is a more critical parameter for splash. Drop splashing was also found to be affected by drop shape, with drop distortion having a significant impact on splash promotion or suppression. In accordance with existing theory, drop spreading and maximum spread factor were found to be independent of pressure in the regime tested. These observations provide new insights and comparison data for evaluating and modeling the behavior of alternate fuels like ethanol.

Cette thèse porte sur la formation du splash lors de l'impact de gouttes sur substrat solide. Alors que l'influence du gaz environnant a souvent été négligé par le passé, des expériences récentes ont montré que la pression du gaz pouvait contrôler la formation du splash lors de l'impact. Dans cette thèse la formation du splash est étudiée lorsque deux paramètres du gaz varient: la densité et la viscosité dynamique, paramètres par leur rapport aux propriétés du liquide. Deux mécanismes de splash sont identifiés: le splash-jet lorsqu'un jet est formé avant le contact de la goutte avec le solide, le splash-détachement lorsque le splash se forme après le contact liquide-solide. Un diagramme de phase entre ces différents mécanismes est obtenu en fonction des paramètres du gaz. L'influence d'autres paramètres, en particulier l'angle de contact est également étudiée. Finalement, le cas de l'impact d'une goutte sur un liquide très visqueux est étudié à la fois théoriquement, numériquement et expérimentalement. Une méthode numérique originale a été développée afin de prendre en compte la frontière entre les deux liquides et le solide et la comparaison avec des expériences réalisées au laboratoire est très prometteuse. Suivant la valeur de la viscosité du liquide impacté, l'impact se comporte comme dans le cas d'un impact sur surface solide. / A splash is observed under certain conditions as drop impacts on solid. Gas has been generally neglected in the splashing mechanism because of the large liquid/gas density and viscosity ratio. However, experiments demonstrated recently a genuine role of the surrounding gas. Under incompressible assumption, this thesis aims to understand the gas effect in the splashing mechanism using both analytical and numerical methods. By changing the gas density or viscosity, two mechanisms of splashing are identified: ''jet-splash'' and ''detachment-splash''. Curved transition frontiers between outcomes in function of the density and viscosity ratio are found. Both gas inertial and viscous effects are crucial in the splashing formation. The creation and lift-up of the ejecta (the small jet for a jet-splash and the thin liquid sheet for a detachment-splash) is the origin of splash and an aerodynamic force makes the lift-up occur. The contact angle can influence the impact outcome, since a hydrophilic contact angle can eliminate a splash while a hydrophobic contact angle promotes the splash. Finally, drop impact on highly-viscous liquid is investigated. A theoretical model is proposed to deal with the triple-phase dynamics in the numerics. By increasing the viscosity of the liquid basin, dynamics varies from a ''wave-like regime'' to a ''solidification regime''. Experiments of an ethanol drop impacting on a highly-viscous liquid (honey) basin are executed. The basin performed as a solid and the complete suppression of splashing by decreasing the gas pressure is observed. Drop shapes predicted by simulations agree with the experiments.

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.