Plane-symmetric co-flow capillary for the generation of monodisperse droplets

This study proposes a novel co-flow capillary microfluidic device that can generate highly monodisperse droplets and polymeric microspheres. The device mainly consists of two self-aligning special-shaped polymeric capillaries. The outer capillary features a gradually contracting and expanding geometry, and the inner has an elliptical cross section at the end. The elliptical nozzle of the inner capillary fits into the contraction region of the outer capillary, and so assembled device, namely a plane-symmetric co-flow capillary, benefits from the self-alignment of the capillaries. The design and manufacturing process of the device are outlined, including a discussion on how the processing conditions affect the capillary geometry. Subsequently, the proposed device is used for droplet generation tests, and the diameter distribution of generated droplets and their influencing factors are investigated. The droplet generation mechanism with the elliptical nozzle is discussed with the help of modeling and simulation. Furthermore, monodisperse porous polymeric microspheres are fabricated using the proposed device, and their porous features are characterized. The results show that the proposed device can produce monodisperse droplets with a mean diameter of a few hundred micrometers and a coefficient of variance (CV) of less than 1%, reflecting the stability of the device. Additionally, porous polymeric microspheres could be successfully produced, and the CV of the size distribution is only around 1%.