We develop a model to predict the fragmentation limit of drops colliding off-centre. The prediction is excellent over a wide range of liquid properties and it can be used without adjusting any parameter. The so-called stretching separation is attributed to the extension of the merged drop above a critical aspect ratio of 3.25. The evolution of this aspect ratio is influenced by the liquid viscosity and can be interpreted via an energy balance. This approach is then adapted to drop-jet collisions, which we model as consecutive drop-drop collisions. The fragmentation criterion is similar to the one observed for drop-drop collisions, while the evolution of the stretched jet aspect ratio is modified to account for the different flow fields and geometry.
Keywords: drop-drop collisions; drop-jet collisions; in-air microfluidics; stretching separation; viscosity.