Growth of Convective and Absolute Instabilities in Co-flowing Jets

We have shown recently that the dripping-to-jetting transition in co-flowing liquids is controlled by two non-dimensional numbers: the capillary number (\textbf{\textit{Ca}}$_{out})$ of the outer liquid and the Weber number of the inner liquid (\textbf{\textit{We}}$_{in})$. When jetting is forced by \textbf{\textit{Ca}}$_{out}$, the diameter of the jet narrows in the downstream direction and the drop size scaling is well predicted assuming that the Rayleigh-Plateau instability is convected along the jet to cause its break up. However, when jetting is forced by \textbf{\textit{We}}$_{in}$, the diameter of the jet widens in the downstream direction and the resultant drop size can not be predicted assuming that the Rayleigh-Plateau instability causes the jet to break up. Instead, we believe these jets break due to absolute instabilities.