We report an ultrafast synchrotron x-ray phase-contrast imaging study of the primary breakup mechanism of a coaxial air-assisted water jet. There exist great similarities between the primary (jet) and the secondary (drop) breakup, and in the primary breakup on different length scales. A transition from a ligament- to a membrane-mediated breakup is identified around an effective Weber number ${\mathrm{We}}^{\prime }\sim 13$. This observation reveals the critical role an effective Weber number plays in determining the atomization process and strongly supports the cascade breakup model.

• Received 29 August 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.154502