Reversible Stream Drop Transition in a Microfluidic Coflow System via On Demand Exposure to Acoustic Standing Waves

Area of Science:

• Fluid Dynamics
• Microfluidics
• Acoustic Manipulation

Background:

• The transition between stream and droplet regimes in coflowing fluids is typically controlled by adjusting capillary numbers (Ca).
• Existing methods lack precise control over regime transitions without altering flow conditions.

Purpose of the Study:

• To experimentally demonstrate a novel method for reversibly controlling coflow regimes using acoustic standing waves.
• To investigate the underlying physics governing acoustic-induced transitions between stream and droplet flows.

Main Methods:

• Utilizing acoustic standing waves to influence interfacial dynamics in a coflowing system.
• Fixing the capillary number (Ca) while varying acoustic parameters.
• Conducting linear stability analysis to understand transition mechanisms.

Main Results:

• Acoustic radiation force exceeding interfacial tension (Ca_ac > 1) induces reversible stream-to-droplet transitions for Ca < 1.
• Stream relocation is observed for Ca ≥ 1 under acoustic influence.
• Experimental observations are explained by pinching, advection, and relocation timescales.

Conclusions:

• Acoustic standing waves provide a new, non-invasive method to control coflow regimes.
• The study reveals a transition from convective to absolute instability driven by acoustic forces.
• This work opens possibilities for advanced microfluidic device control and manipulation.