The motion of long bubbles in microchannels using a meter-long, rectangular capillary on a chip.

Area of Science:

• Fluid dynamics
• Microfluidics
• Interfacial science

Background:

• Understanding gas-liquid interface dynamics is crucial in microfluidics.
• Previous studies on long bubbles in capillaries lacked extensive validation, especially in noncircular geometries.
• The behavior of bubbles in complex fluids like surfactant and nanoparticle solutions is not fully understood.

Purpose of the Study:

• To investigate the flow behavior of long gas bubbles in noncircular capillaries.
• To validate theoretical models of bubble dynamics in microchannels.
• To determine if fluid composition (water, surfactant, nanoparticles) affects bubble drag.

Main Methods:

• Fabrication of a novel microfluidic device with a meter-long, noncircular channel (35 μm x 100 μm x 1 m).
• Study of long bubble flow in capillaries wetted by deionized water, surfactant solutions, nanoparticle solutions, and mixtures.
• Measurement of pressure drop across the bubbles as a function of flow rate and fluid properties.

Main Results:

• The pressure drop and drag of long bubbles scale with the Capillary number (Ca) as Ca^2/3 over the range 10^-7 < Ca < 10^-4.
• This Ca^2/3 relationship was experimentally confirmed for bubbles in noncircular capillaries, consistent with prior theory for surfactant solutions.
• Contrary to the initial hypothesis, the Ca^2/3 drag-velocity relationship was observed for all tested fluids: deionized water, surfactant solutions, nanoparticle solutions, and their mixtures.

Conclusions:

• The drag-velocity relationship for long gas bubbles in noncircular capillaries is independent of the wetting fluid's composition (water, surfactant, nanoparticles).
• The established Ca^2/3 scaling law for bubble drag is robust and applicable across a range of simple and complex fluids.
• The developed microfluidic device provides a reliable platform for studying gas-liquid flow dynamics in microchannels.