This study experimentally investigates the effect of viscosity on the outcomes of collisions between a regular stream of droplets and a continuous liquid jet. A broad variation of liquid viscosity of both the drop and the jet liquid is considered, keeping other material properties unchanged. To do so, only two liquid types were used: aqueous glycerol solutions for the drop and different types of silicone oil for the jet liquid. Combining these liquids, the viscosity ratio λ = µ drop /µjet was varied between 0.25 and 3.50. The collision outcomes were classified in the form of regime maps leading to four main regimes: drops-in-jet, fragmented drops-in-jet, encapsulated drops, and mixed fragmentation. We demonstrate that, depending on the drop and jet viscosity, not all four regimes can be observed in the domain probed by our experiments. The experiments reveal that the jet viscosity mainly affects the transition between drops-in-jet and encapsulated drops, which is shifted towards higher drop spacing for more viscous jets. The drop viscosity leaves the previous transition unchanged, but modifies the threshold of the drop fragmentation within the continuous jet. We develop a model that quantifies how the drop viscosity affects its extension, which is at first order fixing its shape during recoil and is therefore determining its stability against pinch-off.
I. INTRODUCTIONThe large number of recent scientific publications dedicated to encapsulation shows the increasing need for reliable, precise and scalable technologies. This demand is mainly motivated by the biomedical and pharmaceutical industries, which strive to deliver actives as efficiently and safely as possible [1][2][3][4]. The development of cell culture and tissue engineering requires, beyond the necessity of cell feeding and harvesting, a mean, to manipulate and assemble the cells, which can be achieved by their regular and controlled encapsulation into a matrix [5][6][7][8]. The need of encapsulation is also rising in less demanding applications such as in the production of cosmetic, food-products, agricultural inputs, and in depollution tasks [9][10][11][12][13][14].To tackle these challenges, several methods have been proposed. For the production of well controlled spherical capsules, the technology of choice is microfluidics. Indeed, since researchers have been using this toolbox, many microdroplet based applications emerged, including chemical micro-reactors, multiple emulsions and cell capsules [15][16][17][18]. Yet, while present in the scientific community since decades, microfluidics has barely made it to industries. Beside the need for precise chips requiring appropriate design and manufacture, the risk of clogging remains, the main issue which considerably limits scale-up possibilities [19][20][21]. Regarding the production of fibers, which are especially desirable for medical and biomedical applications [22][23][24], the state of the art relies on coaxial or emulsion electrospinning. The former, however, enables only the production of core-sh...
