  We analyze both numerically and experimentally the stability of the steady
jetting tip streaming produced by focusing a liquid stream with another liquid
current when they coflow through the orifice of an axisymmetric nozzle. We
calculate the global eigenmodes characterizing the response of this
configuration to small-amplitude perturbations. In this way, the critical
conditions leading to the instability of the steady jetting tip streaming are
determined. The unstable perturbations are classified according to their
oscillatory character and to the region where they are originated (convective
and absolute instability). We derive and explain in terms of the velocity field
a simple scaling law to predict the diameter of the emitted jet. The numerical
stability limits are compared with experimental results finding reasonable
agreement. The experiments confirm the existence of the two instability
mechanisms predicted by the global stability analysis.
