Investigation on asymmetric splitting behavior of droplets through T-junction with different-length branches

The utilization of T-junction microchannels with different lengths of branches is considered as the preferred method for asymmetric splitting to generate microdroplets with various sizes. In this study, a T-junction microfluidic chip with different-length branches is designed and an experimental platform is set up to visualize droplet asymmetric splitting in the chip, with a focus on elucidating microfluidic droplet asymmetric splitting behaviors. Through detailed analysis of the interface evolution and changing of the characteristic interface morphological parameters during the splitting process, the splitting flow patterns are clearly recognized and the underlying hydrodynamic mechanisms are revealed. Moreover, the influencing factors and their regulation rules on the asymmetric splitting volume ratios are also exposed. The results indicate that there are four types of flow patterns during droplet asymmetric splitting, i.e., splitting with obstruction and tunnel (SOT), splitting with tunnels and consequent droplets flowing in the opposite direction (ST-O), splitting with tunnels and consequent droplets flowing in the same direction (ST-S), and no splitting (NOS). A phase diagram of droplet splitting flow patterns is given. With the increasing initial droplet length for a given Capillary number, the splitting flow pattern experiences a transition from the NOS to the SOT through the ST-S and the ST-O. The splitting process comprises three distinct stages: the entering stage, the squeezing and splitting stage, and the post-splitting stage. During the squeezing and splitting stage, the competition of upstream continuous phase squeezing and interfacial tension causes asymmetric splitting by uneven shrinkage of the droplet neck. Additionally, it is found that the droplet final splitting volume ratio (V_l/V_s) increases with droplet length in the SOT flow pattern, while it decreases with droplet length in the ST-O and ST-S flow patterns. Importantly, the final V_l/V_s of the SOT pattern remains similar with increasing Ca for the same droplet length. However, the final V_l/V_s of the ST-O and ST-S pattern decreases with increasing Ca.