Interface evolution of microdroplets in flow-focusing channels with varying aspect ratios

Abstract This study combines experimental observations and numerical simulations to comprehensively analyse the interface evolution of confined droplets in microfluidic devices with flow-focusing junctions under different aspect ratios. Microchannels with aspect ratios of 1, 1/2 and 1/3 are designed, where droplets are generated at the first flow-focusing junction, and three distinct flow patterns – no breakup, single breakup and multiple breakups – are observed at the second flow-focusing junction. The relationship between droplet length and flow parameters is established, investigating the effects of capillary number and channel aspect ratio on droplet breakup behaviour. It is found that the scaling exponent of the minimum neck thickness increases with the continuous phase flow rate. Numerical simulations are carried out to illustrate the shape evolution of a droplet in three-dimensional space, allowing the calculation of the curvature distribution of the interface. The scaling exponent of the mean radius of curvature in a channel with an aspect ratio of 1 differs from that in a channel with an aspect ratio of less than 1. These findings provide theoretical support for understanding droplet breakup dynamics and lay a foundation for optimising microfluidic device design and structural innovation.