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 x 600 mu m. During the nonlinear collapse process, the variation of the minimum radius of bubble neck (r(0)) with the remaining time until pinch-off (tau) can be scaled by a power-law relationship: r(0) proportional to 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 alpha approaches to 0.33 with the increase in the liquid flow rate Q(1). In the free pinch-off stage, the value of alpha is close to the theoretical value of 0.50 derived from the Rayleigh-Plesset equation and is independent of Q(1).