The influence of viscosity on the outcome of collisions between liquid droplets and another immiscible liquid jet

We experimentally investigate collisions between a regular stream of droplets and a continuous liq-uid jet. In this work, the effects of viscosity on the outcome of immiscible drop-jet collisions are studied. To do so, a broad variation of liquid viscosity of both the drop and the jet liquid is consid-ered. Two basic liquid types are used, including three aqueous glycerol solutions for the drop and three types of silicone oil for the jet liquid. In total, five different totally wetting liquid pairs are investigated, where only the viscosity ratio λ = μd/μj is varied in the range 0.25 < λ < 3.50. The results indicate that, varying the relative impact velocity and the drop spacing, four main collision outcomes occur: Drops-in-jet, fragmented drops-in-jet, encapsulation with/without satellites, and mixed fragmentation. Generally, increasing viscosity stabilizes both the drops and the jet. Special focus is put on the first instants after the impact, during which the drop, totally engulfed by the jet, deforms from a sphere to a bent lamella with a surrounding rim. To observe the drop deformation, the drop liquid is dyed and the jet remains transparent. Two cameras are used providing orthogonal views. We show that the maximum extension of the drop inside the jet depends on the impact veloc-ity and the drop viscosity only. The drop maximum spreading can be well modeled by a scaling law combining the drop Reynolds and Weber numbers. In contrast to the drop extension, the one of the jet depends on both liquid properties. The drop fragmentation, which takes place during the recoil of the liquid structure, after both liquids have reached their maximum extensions, can be attributed to the end-pinching mechanism proposed by Stone et al. (1986).