Yield-stress fluids foams: flow patterns and controlled production in T-junction and flow-focusing devices

Area of Science:

• Rheology and Fluid Dynamics
• Microfluidics and Soft Matter Science

Background:

• Yield-stress fluids exhibit a critical stress threshold before flow initiation.
• Controlling bubble formation in microfluidic devices is crucial for producing stable foams.
• Existing methods for foam production often lack precise control over bubble size and frequency.

Purpose of the Study:

• To investigate the formation dynamics of yield-stress fluid foams in millifluidic flow-focusing and T-junction devices.
• To establish a phase diagram for bubble production regimes based on gas pressure and fluid flow rate.
• To develop and compare methods for achieving steady-state bubble production.

Main Methods:

• Experimental study of bubble formation in millifluidic devices under varying gas pressure and yield-stress fluid flow rates.
• Development of a theoretical model for the pressure at bubble formation onset, incorporating wall slip effects.
• Comparison of two control strategies: gas pressure regulation and gas flow-rate regulation.

Main Results:

• Identification of three distinct operating regimes: gas-fluid co-flow, transient bubble production, and fluid-only flow.
• A predictive model for bubble formation pressure was derived, considering wall slip.
• Both gas pressure and flow-rate regulation were demonstrated as effective methods for steady bubble production.

Conclusions:

• The study provides a comprehensive understanding of yield-stress fluid foam formation in microfluidic systems.
• The developed phase diagram and control methods facilitate predictable and stable production of dry yield-stress fluid foams.
• These findings offer practical pathways for controlled microfoam generation in various applications.