Investigation of the dynamic characteristics of compound droplet impacting on microcolumn arrays

An experimental study on the impact behavior of compound droplet on microcolumn arrays was performed. The deforming, spreading and retraction were investigated by varying Weber number (We = 25–250), inner liquid viscosity (μ = 5.92–21.1 mPa·s) and ratios of inner to outer radii (α = 0.35–0.72). The results show that droplets with larger α exhibits jet spreading in the early stage and slower retraction in the later stage. The increasing We yields larger inertia force, contributing to that the spreading pattern gradually changes from a rhombus shape to a circular shape. The spreading factor increases with the increasing inner liquid viscosity. In the retracting stage, the spreading factor increases with the decreasing α due to the additional surface tension force causing by the presence of inner interface in the rim. When the viscosity of inner liquid is considerably large, the microstructure inhibits the spreading process. The retraction maximum height on the microstructure increases with the increasing α. Furthermore, the energy dissipation equation was analyzed. It can be concluded that the increasing friction coefficient, contact area and viscosity lead to greater energy dissipation, significantly affecting spreading dynamics. This study provides vital information for fundamental understanding of the dynamic characteristics when the compound droplet impacts on the microcolumn arrays.