Experimental Study Of Jet Generation During The Impact Of Viscous Droplets On Liquid Surfaces

The phenomenon of droplet impact on liquid surfaces is widely found in nature and industrial applications.Although the impact of droplets is a classic problem and has been extensively studied,its dynamics is still not fully understood due to the interaction between the droplet and the liquid surface and the complicated flow dynamics.In this study,by using the high-speed imaging and image processing techniques,the impact dynamics of droplets on liquid surfaces is studied.This study mainly focuses on the physics of splash,jet and crown formation above the liquid surface,and the coalescence of the droplet with the liquid surface.It is generally accepted that a Worthington jet occurs when a droplet impacts onto a liquid pool.However,in this experimental study of the impact of viscous droplets onto a less-viscous liquid pool,another jet was identified besides the Worthington jet,forming a two-jet phenomenon.The two jets,a surface-climbing jet and the Worthington jet,may appear successively during one impact event.By carefully tuning the impact conditions,it is found that the two-jet phenomenon is jointly controlled by the droplet-pool viscosity ratio,the droplet Weber number.The mechanism of the surface-climbing jet is completely different from that of the Worthington jet: the liquid in the pool climbs along the surface of the droplet and forms a liquid layer which converges at the droplet apex and produces the surface-climbing jet.This surfaceclimbing jet has a very high speed,i.e.,an order of magnitude higher than the droplet impact speed.The effects of the impact speed,droplet viscosity,droplet size,and surface tension on the surface-climbing jet are also analyzed.In addition,the dynamic processes of droplets impacting on sessile droplets and the liquid films are studied.A similar surface-climbing jet phenomenon is also observed during these two types of impact processes,but it is different from that when the droplets impact the liquid pool.During this impact,the impacting droplets quickly interact with the solid substrate,and the impact pattern is affected by the solid substrate.When the droplet impacts the sessile droplet,the coalescing droplet will spread and retract.When a droplet impacts a liquid film,a cavity will form at the interface of the liquid film.A crown structure will form around the cavity.The size of the sessile droplet and the thickness of the liquid film affect the formation of the jet.