This study presents an experimental and numerical investigation on the impact morphology and the onset of splashing for the interaction of a hexadecane drop with a thin wall-film of the same liquid. Experiments were carried out for Weber numbers between 300 and 1400 and a dimensionless film thickness between 0.1 and 0.5. The experimental and numerical setups are briefly outlined. The uncertainty of the experimental results is analyzed carefully. In agreement with previous studies, the impact morphology is mapped and divided into the three classes of splashing, deposition, and transition. However, two additional phenomena, the formation of a jet and a bubble, are observed. Their occurrence is not sporadic, rather it reproduces systematically both in one-component and binary systems. Since bubble and jet formation are observed in this study, the only unique impact feature for two-component interactions is the formation of holes inside the crown wall. For the splashing/deposition limit an empirical correlation, based on the K-factor, is proposed and compared to available splashing limits in literature. Taking into account the experimental error, all three splashing limits are in good agreement with the experimental results. Furthermore, the applicability of the in-house code FS3D for the computation of the impact morphology and splashing/deposition limit for one-component droplet wall-film interactions is demonstrated.

The shape of the wetted region after a droplet impact can vary significantly even in the early phase of the process. How much of the early spreading process occurs within the structures versus above the structures, flow regimes and local wetting at groove intersections can have effects on the sizes and shapes of the final wetted regions. Experiments and simulations: We experimentally study droplet impacts onto cubic pillars with , and side length, height and separation. Weber numbers ranged between 80 and 1 100, while Reynolds numbers varied between 1 150 and 10 600 using water and isopropanol droplets. The contact angle on a flat segment of the samples was modified between θ_FS<5^∘ and θ_FS=120^∘±5^∘. Several experiments are reproduced using our in-house code FS3D to show the internal flow fields. Diamond-shaped spreading patterns with edges aligned at 45^∘ to the structure pattern are observed. A transition between top-dominated (circular) spreading and diamond spreading occurs depending on the structure size and impact velocity. Groove intersections can act as flow dividers, causing spreading along a path with 90^∘ bends. For large structure sizes and impact velocities fluid jets can pass through the structures uninhibited.

An unsteady analytical solution is proposed to predict the spreading rate of the crown generated by an impacting droplet onto wetted walls. The modelling strategy is based on the direct integration of the boundary layer correction into the potential flow solution that leads to the well-established square-root time dependence. The original potential flow has the structure of an unsteady, stagnation point flow with decaying strength. For initial strengths of the potential flow $a_0 \geqslant 100\ {\rm s}^{-1}$ , we find that a self-similar solution can also be obtained for the boundary layer in the variables $\left (r \sqrt {a(t)/(\nu t)}, z \sqrt {a(t)/(\nu t)} \right )$ . The self-similarity of the solution enables a straightforward estimation of momentum losses during the spreading of the liquid layer along the wall. The proposed modelling approach yields an excellent agreement with experiments during the entire spreading phase. Moreover, it enables a smooth transition from the inertia-driven to the shear-controlled regime of crown propagation. In general, the analysis shows that momentum losses arising from viscous effects cannot be neglected during a significant portion of crown propagation, particularly for thin wall films.

This work presents a systematic experimental study of droplet impact onto a wet substrate. Four different silicone oils are used, covering a range of Reynolds number between $$116< \text{Re} <1106$$116 < Re < 1106 at two different initial wall film heights. The objective is to characterize the temporal and radial evolution of the velocity field within the crown crater by means of micro-PIV. Our findings show that the velocity field has the structure of an axisymmetric stagnation point flow with decaying strength a(t). The latter exhibits an exponential decay and can be explained in terms of the exponential decay of the pressure force exerted by the impacting droplet onto the wall film. In this context, the commonly accepted functional dependence $$a(t) \propto t^{-1}$$a ( t ) ∝ t - 1 represents only the first-order Taylor approximation of the exponential decay and has therefore only a limited temporal validity. The analysis also corroborates the existence of an inertial regime concerning the velocity field for $${\text{Re}} > 270$$Re > 270 . This is not observed at lower Re numbers due to the increased pressure losses caused by the extensional (normal) strain during the radial spreading of the lamella. To validate these findings a holistic approach is chosen, which combines numerical results, analytical solutions and experimental data from literature. In particular, by using the continuity equation, it is shown that the experimental decay of the wall film height can be reconstructed from the velocity measurements. Consilience of results from different approaches provides a robust validation of the micro-PIV data obtained in this work. Graphical