VOF SIMULATION OF SYMMETRY BREAKING LAMELLA DYNAMICS DURING SINGLE DROPLET IMPACT ON A CUBIC PILLAR

The dynamics of droplet impact on confined surfaces underpins applications from inkjet printing to spray cooling, yet the governing mechanisms of spreading and retraction remain actively investigated. Recent experiments examined spreading dynamics in normal (pillar edge) and diagonal (pillar corner) directions using high-speed imaging of water and isopropanol impacts. The droplet initially spreads symmetrically, then undergoes rotational transformation, forming a distinct rectangular lamella resembling a liquid umbrella. To gain deeper insights into these dynamics, this study employs high-resolution VOF simulations to analyze lamella formation on cubic pillars. Results match experiments well, capturing the asymmetric lamella evolution. The simulations reveal a lower spreading velocity in the diagonal direction compared to the normal one, with stronger deceleration in the diagonal case, particularly for water. Velocity profiles indicate negligible boundary layer influence on rotational transformation for low-viscosity fluids. The domain-integrated energy budget confirms that kinetic energy loss during spreading is mainly due to viscous dissipated, with surface energy forming the lamella and total energy nearly conserved. Additional simulations with a droplet of 20% higher viscosity reveal more developed boundary layers and clearer rim asymmetries. In this case, the rim remains consistently thicker in the normal direction than in the diagonal one, aligning with theoretical expectations and indicating that viscous effects enhance rim deceleration along the diagonal. Overall, this study provides new insights into the interplay of viscosity, boundary layer dynamics, energy redistribution, and rim evolution in droplet impacts on confined geometries.