  It is generally accepted that the 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, we identify another jet
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 condition, we find that the
two-jet phenomenon is jointly controlled by the droplet-pool viscosity ratio,
the droplet Weber number, and the droplet-pool miscibility. 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 surface-climbing 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 analysed. This study does not only provide
physical insights into the mechanism of droplet and jet dynamics, but also be
helpful in the optimisation of the droplet impact process in many relevant
applications.
