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Drop Formation in Co-flowing Fluid Streams
Yuanyuan Cui and Nivedita R. Gupta

We numerically simulate the formation of a drop at the tip of a needle with a coflowing fluid stream. The drop and the bulk phase are treated as incompressible Newtonian fluids. A hybrid volume-of-fluid method with a front-tracking scheme is implemented to study the problem. As the flow rate of the outer fluid or the Bond number is increased, the drop breaks faster with smaller volumes. As the viscosity of the drop phase or capillary number is increased, the drop takes longer to break with larger volumes. Jetting mode of breakup is observed at higher flow rate ratios and drops with a negative curvature at the rear are observed for higher Bond numbers. We also consider the effect of adding soluble surfactants on the drop formation process in the co-flowing system. Surfactants accumulate at the fluid interface in the vicinity of the neck generating Marangoni stresses that slow down the breakup process.

Keywords: drop formation, break up, co-flowing, dripping, jetting, surfactant, necking

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