No abstract

International audienceThe collision of water drops against small targets was studied experimentally by means of a high-speed photography technique. The drop impact velocity was about 3.5 m/s. Drop diameters were in the range of 2.8-4.0 mm. The target was a stainless steel disk of 3.9 mm diameter. The drop spread beyond the target like a central cap surrounded by a thin, slightly conical lamella bounded by a thicker rim. By mounting a small obstacle near the target, surface-tension driven Mach waves in the flowing lamella were generated, which are formally equivalent to the familiar compressibility driven Mach waves in gas dynamics. From the measurement of the Mach angle, the values of some flow parameters could be obtained as functions of time, which provided insight into the flow structure. The liquid flowed from the central cap to the liquid rim through the thin lamella at constant momentum flux. At a certain stage of the process, most of the liquid accumulated in the rim and the internal part of the lamella became metastable. In this situation, a rupture wave propagating through the metastable internal part of the lamella caused the rim to retract while forming outwardly directed secondary jets. The jets disintegrated into secondary droplets due to the Savart-Plateau-Rayleigh instability. Prior to the end of the retraction, an internal circular wave of rupture was formed. It originated at the target and then it propagated to meet the retracting rim. Their meeting resulted in a crown of tiny droplets. A theoretical analysis of the ejection process is proposed. (C) 2002 American Institute of Physics

International audienceThe dynamics of a circular liquid lamella resulting from the collision of a water drop with a small disc-like target was studied experimentally and theoretically. Such a type of collision also acts as a model of drop impacts on plane surfaces in the absence of liquid friction, and therefore for more widespread collisions of drops of inviscid liquid with solid surfaces. We propose a simple model to describe the dynamics of the lamella resulting from the drop impact and also predict the structure of the liquid flow in the lamella. It is based on the observations that during the drop collision with the target, the liquid is ejected at an approximately constant flow rate with a velocity that significantly decreases in time. The resulting distributions of velocities, local flow rates and film thickness in the lamella are calculated. Besides, we have measured the distribution of the local Weber numbers by generating Mach-like rupture waves (we have called them Mach-Taylor waves) in the lamella, which follows the Taylor theory of disintegration of fluid sheets. Unknown parameters of the model are obtained from the comparison between the theoretical expression for local Weber number and the experimental data. The time evolution of the lamella diameter was obtained by numerical integration of the model. It was found that during the lamella life, zones of metastability could be formed in the lamella. In these zones a propagating rupture hole cannot be transported away by the flow and it Yields to destabilization. One metastability zone expands from the target towards the external rim, and it is the opposite for the other one