Impact of water drops on small targets

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

Impact of drops of surfactant solutions on small targets

“…These values do not significantly differ from the ones obtained for water by Rozhkov et al (2002). Under the same impact conditions, it was found v + = 2.87 ± 0.61, v − = −1.91 ± 0.41, v r = 2.39 ± 0.37 and v = 0.48 ± 0.37 m s −1 at t ≈ 5.5.…”

“…For all the solutions, the drops form a thin liquid lamella with a relatively thick toroidal rim like the water ones (Rozhkov et al 2002).…”

“…Rozhkov et al (2002Rozhkov et al ( , 2004 observed the collapse of the lamella because of the rim retraction (which is recalled to be equivalent to the propagation of an external rupture wave) and sometimes because of the propagation of an internal rupture wave starting from the target. With Silwett L77 drops, a new type of lamella collapse occurs characterized by the random nucleation of holes, giving rise to a web-like structure that consists of long-living liquid filaments (figure 3).…”

“…The use of a small disc-like target as a substrate instead of a plate eliminates the influence of the viscous drag on the solid surface, and makes possible the detailed observation of hydrodynamic effects that are usually screened by viscosity. Rozhkov et al (2002Rozhkov et al ( , 2004 showed that the radial expansion of the lamella is equivalent to the radial ejection of a liquid sheet from a point source with ejection velocity v s and flowrate q s . Let us introduce the dimensionless quantities, ), t is the time, r, the radial coordinate, v, the local velocity and q, the local flow rate defined as the amount of liquid that flows through a circular contour of radius r per unit time, i.e.…”

“…The rim motion results from the balance between the momentum flux provided by the liquid flow into the rim, the rim inertia and the capillary forces exerted on the rim by the two film surfaces (Rozhkov et al 2002(Rozhkov et al , 2004. From the knowledge of V (t,Y ) and Q(t,Y ), we could numerically calculate the rim trajectories Y l = Y l (t) (Y l = r l /d i , r l is the rim radial coordinate, figure 1) for different values of We i that appeared as the single input parameter.…”