We study the formation of nanoliter-sized droplets in a microfluidic system composed of a T-junction in PEEK and tubing in Teflon. This system, practical for a 'plug and play' set-up, is designed for droplet-based experiments of crystallization with a statistical approach. Hence the aim is to generate hundreds of droplets identical in size and composition and spatially homogeneous. Therefore, parameters of control are droplet size and frequency. However, the geometry of the T-junction is not perfect and, moreover, its channels are circular, as opposed to the planar geometries with rectangular cross-sections that are usually used. However, based on 3D experiments and 2D simulations, we observe the same regimes of droplet generation in circular channels as in planar geometries, and with the same stability. Therefore, we refer to velocities instead of flow rates to characterize the system. Then we define operating range in terms of droplet size and frequency through empirical relations using total velocity, velocity ratio and capillary number, to ensure homogeneous droplets in channels of 500 mu m and 1 mm diameters. (C) 2015 Elsevier Ltd. All rights reserved.