  Most commercial microfluidic droplet generators rely on the planar
flow-focusing configuration implemented in polymer or glass chips. The planar
geometry, however, suffers from many limitations and drawbacks, such as the
need of specific coatings or the use of dedicated surfactants, depending on the
fluids in play. On the contrary, and thanks to their axisymmetric geometry,
glass capillary-based droplet generators are a priori not fluid-dependent.
Nevertheless, they have never reached the market because their assembly
requires art-dependent and not scalable fabrication techniques. Here we present
a new device, called Raydrop, based on the alignment of two capillaries
immersed in a pressurized chamber containing the continuous phase. The
dispersed phase exits one of the capillaries through a 3D-printed nozzle,
placed in front of the extraction capillary for collecting the droplets. This
non-embedded implementation of an axisymmetric flow-focusing is referred to
{\it co-flow-focusing}. Experimental results demonstrate the universality of
the device in terms of the variety of fluids that can be emulsified, as well as
the range of droplet radii that can be obtained, without neither the need of
surfactant nor coating. Additionally, numerical computations of the
Navier-Stokes equations based on the quasi-steadiness assumption are shown to
correctly predict the droplet radius in the dripping regime and the
dripping-jetting transition when varying the geometrical and fluid parameters.
The monodispersity ensured by the dripping regime, the robustness of the
fabrication technique, the optimization capabilities from the numerical
modeling and the universality of the configuration confer to the Raydrop
technology a very high potential in the race towards high-throughput droplet
generation processes.
