Perturbations of liquid jets with an entering sphere in flow focusing

ABSTRACT The significant relationship between performance parameters of compound droplets and their geometries makes accurate control of the breakup of compound jets particularly critical. The physical mechanism behind the earlier and easier loss of stability in compound jets caused by inner dispersive interfaces is still not fully known, which constrains accurate prediction of the conditions that result in the breakup of compound jets. In the present study, three perturbation modes—namely, block-off, squeeze-through, and slip-through—are found to form between an inner sphere and a liquid jet. The conditions for the occurrence of block-off are accurately predicted by solving for the radius of the liquid jets. According to analyses of the velocity profiles of liquid jets, the key reason for the loss of stability in a quasi-steady jet interface caused by the entry of a sphere smaller in size than the jet cross-section is the degradation of tracking characteristics of the sphere due to viscous shear on the sphere. These findings provide a new explanation for the long-standing question that how the inner dispersive interface triggers jet instability and can help improve the accurate control of the breakup of compound jets containing inner dispersive interface.