Modified capillary number to standardize droplet generation in suction-driven microfluidics

In droplet microfluidic devices with suction-based flow control, the microchannel geometry and suction pressure at the outlet govern the dynamic properties of the two phases that influence the droplet generation. Therefore, it is critical to understand the role of geometry along with suction pressure in the dynamics of droplet generation to develop a predictive model. We conducted a comprehensive characterization of droplet generation in a flow focusing device with varying control parameters. We used these results to formulate a scaling argument and propose a governing parameter, called as modified capillary number (Ca_L), that combines normalized droplet volume with geometrical parameters (length of dispersed and continuous phase channels) and flow parameters (interfacial tension, phase viscosity and velocity) in a power law relationship. Ca_L effectively captures the transition from squeezing to dripping regimes of droplet generation, providing essential insights into the design requirements for suction-driven droplet generation. These findings are key to standardize microfluidic flow-focusing devices that can achieve the desired droplet generation behavior with optimal pressure consumption.