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

Hypothesis: The dynamics of gas-liquid interfaces differs between aqueous surfactant and nanoparticle mixtures in rectangular cross-section capillaries.

Experiments: We designed and fabricated a new microfluidic device with a meter-long channel and a noncircular cross section (35 μm by 100 μm by 1 m) to study the flow behavior of long bubbles in capillaries wetted by water as well as surfactant and nanoparticle solutions. Flow in the novel symmetric loop device maintains essentially straight and inertialess conditions over a wide range of flow rates.

Findings: The pressure-drop versus velocity relationship of long bubbles in capillaries with noncircular cross sections has been studied theoretically but not extensively validated. The measured pressure drop (normalized with respect to interfacial properties) experienced by bubbles varies as Ca^2/3 over the range 10^-7<Ca<10^-4 where Ca=μUσ. Thus, the drag also scales as Ca^2/3. The measurements from our new device thereby confirm previous theoretical relationships of the flow of long gas bubbles wetted by surfactant solutions in noncircular capillaries. Contrary to the hypothesis, the Ca^2/3 drag-velocity relationship applies to gas bubbles in deionized water, nanoparticles in brine, surfactant in brine, and nanoparticle-surfactant mixtures in brine for the conditions studied.