  A focused jet is generated from a concave free surface through an impact of a
tube. Experimental and numerical studies were conducted to explore the viscous
influences. The results show that a strong vorticity sheet is generated along
the free surface just after impact, and the liquid bulk is initialized
($t\le$$10^{-4}$s). After the impact, the liquid surface evolves freely and a
jet is ejected ($t\gg$$10^{-4}$s). Pronounced tangential flow is found, and the
material in the jet mainly originates from the viscous penetrating layer below
the free surface. According to the above observations, a vorticity mechanism of
the free surface evolution is proposed. Further, by combining the viscous and
interfacial tension effects, a scaling relation estimating the jetting velocity
is derived, which agrees with the numerical and experimental results well. The
work will be valuable for viscous droplets preparation.
