Multiphase flow in microfluidics is an increasingly growing field, especially in biotechnology. For instance, a steady-state slug flow would benefit lab-on-a-chip drug delivery methods. This flow would not only use minute amounts of reagents, but it would also decrease the sample processing time. Thus, researching a steady-state plug flow in a microchannel is beneficial to the drug delivery field.
Five PMMA, directly-milled microchannels [2: Aspect Ratio 1 (with and without pressure ports, 2): Aspect ratio 2 (with and without pressure ports), and 1: Aspect Ratio 3 (without pressure ports)] were manufactured. These channels were then cleaned, and a PMMA cover slip was thermally bound. In addition, a test setup was constructed to create steady gas and liquid flows to input into the channel. The gas flow is controlled by highly-accurate and a fast gas flow controllers, and a pressurized liquid reservoir maintains a steady liquid flow rate. A microscope equipped with a CCD camera captures images of flow within the microchips.
Two techniques were used to capture flow pictures: a back illumination method with a low recording frequency, and a laser illumination method with a high recorded frequency were used to capture frames. Each channel was tested by using a set liquid flow rate of either 0.05 ml/min (low) or 0.10 ml/min (high). While the liquid flow rate was held constant, the gas flow rate was incrementally adjusted by 0.05 ml/min. Frames were taken at each increment. Bubbly, slug, and annular flow regimes were observed. OPTIMAS, image processing software, was used to extract size distributions of gas bubbles and liquid plugs and to extract bubble velocities. Bubble and plug size distributions were use to assess whether or not the flow is steady state. For the low liquid flow of 0.05 ml/min, all channels reached a steady state. However, the volumetric flow ratio ranges of steady state are different for each channel. For the high liquid flow rate of 0.10 ml/min, steady flow was not obtained in the AR1 channels. There were some minor differences in stable flow between the non-pressure port microchannels and the corresponding pressure-ported microchannel, probably due to slight differences in machining. The maximum pressure drops in AR1 and AR2 channels were 6 and 3 PSIG.
The extracted velocity data included the velocity at the centroid, minimum and maximum point on the bubble. The bubbles did not deform by much (3%) because these three velocities are relatively equal. In addition, the velocities indicate that the bubble slips in the microchannel.