Flow-field dynamics during droplet formation by dripping in hydrodynamic-focusing microfluidics

Area of Science:

• Fluid dynamics
• Microfluidics
• Interfacial phenomena

Background:

• Hydrodynamic focusing is a key microfluidic technique for droplet generation.
• Understanding droplet formation mechanisms is crucial for applications in drug delivery and materials science.
• Previous studies often assumed shear-driven droplet formation in similar setups.

Purpose of the Study:

• To investigate the primary mechanism initiating oil droplet formation in a cross-channel microfluidic device.
• To analyze the flow field dynamics during droplet generation under specific experimental conditions.
• To differentiate between shear and extensional flow contributions to droplet pinch-off.

Main Methods:

• Microscopic particle image velocimetry (micro-PIV) was employed to visualize and quantify the continuous phase flow field.
• A pressure-driven cross-channel microfluidic device was utilized for droplet generation.
• Temporal analysis of instantaneous flow fields was performed in the dripping regime.

Main Results:

• Droplet formation was observed to be initiated by an extensional flow, not shear stress.
• The oil droplet stream temporarily plugged the outlet channel, inducing a pressure difference.
• Release of this pressure, driven by water from side channels, caused interfacial pinch-off and droplet formation.

Conclusions:

• Extensional flow, induced by temporary channel plugging, is the dominant mechanism for droplet formation in this microfluidic system.
• Shear forces are not the primary drivers of droplet initiation under low flow rate and moderate capillary number conditions.
• The findings provide new insights into droplet generation dynamics in microfluidic devices.