(314j) Probing the Dynamics of Microscale Bubble/Droplet Flows with On-Chip Pressure Sensors

Segmented microflows have found a wide range of applications and allowed to translate chemical and biological processes into continuous-flow formats while minimizing axial dispersion. Most previous studies have relied on optical micrographs of the fluid phase distribution in describing the flow characteristics.

Instead, our contribution focuses on the pressure that is expected to display a characteristic, Capillary-number dependent, spatiotemporal behavior. The characteristic behavior of the pressure signal is important during bubble/droplet break-up and during steady flow along the channel. Estimates of how the pressure varies in these two cases were so far based on analytical or numerical approaches and involve several assumptions: smooth walls, constant interfacial tension and complete wetting. Accurately measuring the dynamic behavior displayed by the pressure in bubble/droplet microchannel flow, therefore, provides for a very sensitive tool to discriminate between flow regimes, determine bubble/droplet velocities and ? in multichannel arrangements ? achieve flow synchronization.

We integrated piezoresistive pressure transducers in soft lithographically patterned microfluidic devices. On-chip integration significantly reduced the available dead volume and removed any unwanted bubbles in the sensing channel. The integrated pressure sensors were calibrated and electrically amplified. Dynamic pressure measurements at millisecond time resolution and a sensitivity exceeding the capillary pressure by at least one order of magnitude were obtained. The presented results were obtained for segmented gas-liquid and liquid flows at different Capillary numbers (0.001-0.05) and wetting conditions.