μ-PIV study of the formation of segmented flow in microfluidic T-junctions

We present transient measurements of the flow field during the formation of bubbles in a microfluidic T-junction. Using microscopic particle image velocimetry (mu-PIV), velocities in a 800 pm squared T-junction were measured with a spatial resolution of 20 x 20 x 100 mu m(3). Three-dimensional velocity distributions in the continuous phase were constructed from planar velocity fields measured at four different depths. The image acquisition frequency was phase-locked to the bubble frequency by triggering the system on the passing of a bubble. This allowed for the measurement of the velocities at different instances in the periodic formation cycle with a high temporal resolution (similar to 5 ms). The shape and movement of the interface between the two immiscible phases were measured by means of a high speed camera. It was observed that at no instance, the growing bubble fully blocks the channel. At least 25% of the insurgent liquid creeps through the menisci in the corners of the channel. The quantification of this leakage allows the prediction of bubble volume and the amount of liquid separating the bubbles without fitted parameters. Prior to break-up, the rapid constriction of the bubble neck starts when the neck radius equals one-fourth of the channel width. This coincides with flow reversal around the bubble, which can be explained from the Laplace pressure terms. (c) 2007 Elsevier Ltd. All rights reserved.