Considering both the first nonaxisymmetric (n=1) and the axisymmetric (n=0) disturbances, a viscous spatial instability analysis of coflowing liquid-gas jets in capillary flow focusing is carried out. A detailed parametric study is performed to explore characteristics of the spatially amplified branch in a convectively unstable regime. The numerical results show that the Weber number and the velocity at the interface have significant influences on the transition between axisymmetric and nonaxisymmetric instabilities, whereas the other parameters such as the Reynolds number, the slope of the liquid velocity profile at the interface, the density ratio, and the viscosity ratio hardly change the transition. Nonaxisymmetric disturbances grow faster than axisymmetric ones for relatively high Weber numbers. Particularly, the comparison of the theoretical prediction with the experimental results reported by Si et al. [J. Fluid Mech.629, 1 (2009)] indicates that the spatial instability analysis is in better agreement with experiments than the temporal instability analysis for moderate and high Weber numbers.
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