Pinch-off mechanism for Taylor bubble formation in a microfluidic flow-focusing device

The present work aims at studying the nonlinear breakup mechanism for Taylor bubble formation in a microfluidic flow-focusing device by using a high-speed digital camera. Experiments were carried out in a square microchannel with cross section of 600 × 600 μm. During the nonlinear collapse process, the variation of the minimum radius of bubble neck (r 0) with the remaining time until pinch-off (τ) can be scaled by a power–law relationship: $$r_{0} \propto \tau^{\alpha } .$$ Due to the interface rearrangement around the neck, the nonlinear collapse process can be divided into two distinct stages: liquid squeezing collapse stage and free pinch-off stage. In the liquid squeezing collapse stage, the neck collapses under the constriction of the liquid flow and the exponent α approaches to 0.33 with the increase in the liquid flow rate Q l. In the free pinch-off stage, the value of α is close to the theoretical value of 0.50 derived from the Rayleigh–Plesset equation and is independent of Q l.