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.

Equally sized droplets made of the same liquid are known to either bounce, coalesce, or separate under collision. Comparable outcomes are observed for immiscible liquids with bouncing, encapsulation instead of coalescence, and separation with two or more daughter droplets. While the transitions between these regimes have been described, the liquid distribution arising from separation remains poorly studied, especially in the case of head-on collisions, for which it cannot be predicted. This distribution can be of three types: either two encapsulated droplets form (single reflex separation), or a single encapsulated droplet plus a droplet made solely of the encapsulating liquid emerge, the latter being found either on the impact side (reflexive separation) or opposite to it (crossing separation). In this paper, a large number of experimental and simulation data covering collisions with partial and total wetting conditions and Weber and Reynolds numbers in the ranges of 2–720 and 66–1100, respectively, is analyzed. The conditions leading to the three liquid distributions are identified and described based on the decomposition of the collision in two phases: (i) radial extension of the compound droplet into a lamella and (ii) its relaxation into an elongated cylindrical droplet. In accordance with these two phases, two dimensionless parameters, Λ=ρi/ρoWei−1/2 and N=νo/νi σo/σio, are derived, which are built on the collision parameters and liquid properties of the encapsulated inner droplet (i) and the outer droplet (o) only. The combination of these two parameters predicts the type of liquid distribution in very good agreement with both experimental and numerical results.

Single droplet impacts onto thin wall-films are a common phenomenon in many applications. For sufficiently high impact velocities, the droplet impact process consists of three phases, i.e., initial contact stage, droplet deformation with radial momentum transfer inducing an upward rising lamella, and crown propagation. Here, we present the results of a combined numerical and experimental study focusing on the early dynamics of the impact process. Specifically, the effects of the initial droplet shape, wall-film thickness, and contact line motion are analyzed. Prior to impact, an oblate spheroidal droplet shape was observed. Using direct numerical simulation, we show that the droplet shape affects the impact dynamics only during the first two phases, as it is one of the key parameter influencing the correct prediction of the impact zone. The contact line propagation is described by a square-root-time dependence R¯CL=ατ for both, dry and wetted surfaces. On dry surfaces, the advancement of the contact line is determined by the rolling motion of the truncated droplet. On wetted surfaces, the value of the α-parameter is controlled by two concurrent effects, namely, rolling motion and wall-film inertia. For impact onto thin films, the rolling motion prevails. With increasing wall-film height, the droplet penetrates into the soft substrates and wall-film inertia becomes the controlling factor. These insights into the early impact dynamics on wetted surface are important for the formulation of a unified modeling approach.