Stacked confocal fluorescent images of microchannels at medium and high flow rate. Cases 5–8 were observed at Q = 100 μl/min. Cases 9–12 were observed at Q = 200 μl/min. The driving frequency and PZT transducer placement corresponding to each image and scale of images is analogous to that in Fig. 5 (i.e., Cases 5, 6, 9, 10: “channel PZT” transducer; Cases 7, 8, 11, 12: “air pocket PZT” transducer; Cases 5, 7, 9 11: 2.93 MHz; and Cases 6, 8, 10, 12: 2.62 MHz).