Wetting behavior in the inertial phase of droplet impacts onto sub-millimeter microstructured surfaces

Hypothesis: 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 F S < 5 ∘ and θ F S = 120 ∘ ± 5 ∘ . Several experiments are reproduced using our in-house code FS3D to show the internal flow fields. Findings: 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. • The shape of the wetted area in the inertial phase of a droplet impact on a structured surface depends on whether spreading is top- or groove-driven. • For groove-driven flow, the fluid can follow a single groove as a jet or split up at groove intersections. • If flow-splitting is dominant, a diamond shape develops, as the spreading distance is dominated by the taxicab distance in the grooves. • The axial spreading factor increases more than the diagonal one for higher impact velocities, as diamond-groove-driven spreading is dominant. • If the droplet impact is dominated by jetting, a star-like pattern can develop.