Liquid film instability and secondary droplet generation in viscous drop impact with a solid surface

This study investigates the splash phenomenon of high-viscosity liquid droplets impacting a solid surface, focusing on the paradoxical suppression of splashing at high impact velocities. While droplet impact is crucial for applications like inkjet printing, the conditions governing splash formation, especially its suppression, are not fully understood. This research aims to elucidate the mechanism and threshold for splash suppression at increased impact velocities and to determine the influence of ambient gas pressure on this phenomenon. The experimental methodology involved two primary investigations. First, silicone oil droplets of varying viscosities were impacted onto a solid plate at velocities ranging from 6.43 m/s to 14.9 m/s under atmospheric pressure. Second, the post-impact behavior of 10 cSt silicone oil droplets was observed under reduced-pressure conditions. The impact process was visualized using a high-speed camera. Results confirmed the existence of a velocity regime where increasing the impact speed prevents splashing; for example, 10 cSt silicone oil splashed at 7.69 m/s but not at 8.77 m/s. This suppression is attributed to the inhibition of a thin liquid film (lamella), which is a prerequisite for splash at lower speeds. Moreover, observations in the reduced-pressure environment confirmed that a decrease in the ambient gas pressure also suppresses splash occurrence even in the high-viscosity region. For 10 cSt silicone oil, the splash threshold was found to be near an ambient pressure of 80 kPa, as impacts at lower pressures resulted in no splash.