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In air, microfluidics based on the collisions between drops and a continuous liquid jet appears very promising to encapsulate drops in spherical liquid shells or in cylindrical continuous jets forming precursors to spherical capsules and fibres, respectively. Yet to date, only a few investigations of drop-jet collisions have been made [Visser et al. Sc. Adv. 2018, Planchette et al. PRF 2018]. Motivated by potential biomedical applications for which the restrictions on the materials go beyond liquid jettability and drop producibility, this work investigates the effects of liquid wettability and miscibility on the outcome of drop-jet collisions.
Restricting our study to head-on collisions, we consider three different pairs of liquids. The liquids silicon oil, n-hexadecane and an aqueous solution of ethanol and glycerol are selected to have comparable surface tension (24 mN/m +/- 4 mN/m) and viscosity (5 mPa.s). They are combined with the same aqueous glycerol solution (68mN/m; 5 mPa.s) reducing the material properties variations to those of their interfacial tension and providing pairs of immiscible liquids, totally and partially wetting, and a pair of miscible liquids.
By varying the relative drop-jet velocity and the drop spacing, we observe five different regimes of collision outcomes for all combinations of liquids: drop-in-jet, fragmented drop-in-jet, encapsulation with and without satellites, and mixed fragmentation. Given its importance for new applications, special emphasis is given to the drop-in-jet regime and its limits to the first occurrence of fragmentation. For each liquid pair, we vary the drop spacing between two extreme values and report for the whole studied range the threshold velocity between drop-in-jet and fragmentation. We show that the variations of both wettability and miscibility barely affect this threshold. We closely observe the collisions using dyed drops and two cameras providing orthogonal views and distinguish during the first phase of the collision, no significant difference between the three liquid pairs. Indeed for given relative velocity and drop spacing the drop deformation from a sphere to a bent lamella surrounded by a rim is similar for all jet liquids. The maximal drop surface extension is well predicted using the drop Weber number and the kinetics of this extension scales with the drop capillary time scale. In agreement with the experimental observations, we attribute the possible threshold shifts to the difference of kinetics during the drop recoil that constitutes the second phase of the collision.
