The impact of a compound drop on a solid surface: the influence of shell viscosity

A study of a compound drop impacting a surface is presented. The drop is of single-core, shell type, with the shell containing oil at three viscosities (1, 10, and 100 cSt) and the core containing water. The influence of varying viscosities and the Weber number (We) on impact is reported. The spread dynamics are found to be majorly influenced by the shell. With a rise in shell viscosity, the maximum spreading factor (β_max, maximum spread's ratio to the drop's pre-impact diameter) declines for a given impact velocity. At higher impact velocities, finger formation at the outer periphery and splash are comprehended for the low and medium-viscosity shells, respectively. The shell forms a film for the core to spread and retract, and thereby influences the core's spreading and retracting dynamics. At lower Reynolds numbers (Re) for the 1 cSt shell and all Re for the 10 cSt shell drops, total separation of the core at the end of retraction is reported. Partial separation is observed for lower viscosity shells at higher Re, while no separation is found in higher viscosity shells. The physics of the core jet length until separation is explored. To verify the shell's viscosity influence, the measured β_max is compared with a theoretical model developed indigenously. A good comparison is observed. The novelty lies in demonstrating a systematic experimental study of compound drops focused on shell viscosity, and in developing a simplified model to predict the maximum spread.