Studies of droplets formation regime and actual flow rate of liquid-liquid flows in flow-focusing microfluidic devices

Studies of droplet formation regime and actual flow rate of liquid-liquid flows were conducted in flow-focusing microfluidic devices using water or gellan solution as aqueous phase and an organic phase composed by soybean oil and polyglycerol polyricinoleate. The influence of flow rate and flow rate ratio (q) on droplets formation regime was evaluated. Squeezing regime was observed at q between 0.5 and 1, while dripping regime occurred at q lower than 0.5. The actual flow rate of the phases was calculated and similar values to the nominal one were found at low flow rates, but the actual values were lower than nominal one at high flow rates. Such differences were associated to the forces balance involved in the droplets formation by flow-focusing mechanism and the reduced dimensions of the channels, implying in an accumulation of the injected fluid. Dimensionless numbers of Reynolds (Rec), Weber (Wed), Capillary (Ca) and Eotvos (Eo) were calculated from the actual velocities. A higher viscosity difference between the phases and lower interfacial tension between water/oil phases led to higher values of Ca and Wed for water than for gellan solution as aqueous phase. Our results showed that the droplets size and formation was influenced by the physical properties of the phases and process conditions (input flow rate), which also defines the droplets formation regime. Therefore, understanding the dynamics of droplets generation in microchannels is essential to ensure that droplets can be created based on specific parameters, such as droplets formation regime and dimensionless numbers, overcoming the limitations of the scale-up (operational conditions and physical properties) of microfluidic systems.