The impact of outlet microchannel dimension on viscoelastic droplet formation was investigated using a high-speed digital camera.Dilute polyethylene oxide(PEO)-glycerol aqueous solution was used as the dispersed phase,while mineral oil with 4%(wt.)surfactant sorbitanlauric acid ester(Span 20)was used as the continuous phase.It was found that the decrease of the outlet orifice size would result in the decrease of the droplet size and the increase of the frequency of the droplet formation.There exists a critical capillary number,beyond which the monodisperse droplets would be formed.Moreover,the critical capillary number decreases with the decrease of the outlet orifice size.The breakup behaviors of the viscoelastic thread for droplet formation could be divided into two stages: the "flow-driven" stage controlled by the inertia of dispersed phase and the dynamical radial pressure of continuous phase,and the "elastocapillary" stage mainly controlled by the elastic and capillary forces.In addition,the influence of the outlet orifice size on the breakup dynamics and interface shape of the viscoelastic thread was still investigated in both stages,respectively.This study is conducive to the fully understanding of the intrinsic differences of viscoelastic droplet formation with various microchannel dimensions.Meanwhile,it would provide the theoretical and experimental foundation for further selection and design of microfluidic devices.