Generation of micron-sized drops and bubbles through viscous coflows

In this paper, the generation of micron-sized drops (smaller than 1.5 μ m) is achieved through a simple device, suggested by the numerical experiments by Suryo and Basaran [R. Suryo, O.A. Basaran, Tip streaming from a liquid drop forming from a tube in a co-flowing outer fluid, Phys. Fluids 18 (2006) 082102], in which two liquids coflow coaxially under creeping flow conditions. Here we determine the appropriate experimental range of the dimensionless parameters in which tiny jets, over two order of magnitude smaller than the drop from which they originate, are formed. Moreover, the simplicity of the coflowing geometry has permitted to design a multiplexed device for the generation of emulsions with industrial applications. In the case when the inner fluid is a gas we find that, up to our optical resolution, conical tips are visualized if no gas is injected. For small values of the gas flow rate, and no matter how large the outer capillary number is, no stable long gas jets are formed. Instead, we observe the periodic production of bubbles from an unsteady conical tip with diameters of the order of one tenth the injection nozzle diameter.