When a viscous drop impacts onto a pool of lower viscosity liquid, a new jetting phenomenon is observed; an Apex Jet.

This jet is produced by the ejecta sheet which emerges from the free surface of the pool, moves up along and wraps around the surface of the drop. When this sheet of liquid converges and collides at the top apex of the drop it produces a thin vertical jet at velocities of more than 10 times the drop impact velocity.

This jetting occurs for a limited range of impact conditions, where the ejecta speed is sufficient for the sheet to travel around the entire drop periphery, while not so fast that it separates from the drop surface.

The lower bound for the jetting region is thereby set by a minimal Reynolds number, while the upper bounds are subject to a maximum Weber-number criteria. The strongest observed jets appear for viscous drops impacting onto liquid pools with the lowest viscosity as well as lowest surface tension, such as acetone and methanol.

Jetting has also been observed for drops which are immiscible with the pool liquid, under a different range of impact conditions. However, jetting is never observed for pools of water, as the surface tension is then significantly larger than that of the drop.