Transport and reaction in microscale segmented gas–liquid flow
We use micro particle image velocimetry (μPIV) and fluorescence microscopy techniques to characterize microscale segmented gas–liquid flow at low superficial velocities relevant for chemical reactions with residence times of up to several minutes. Different gas–liquid microfluidic channel networks of rectangular cross section are fabricated in poly(dimethylsiloxane) (PDMS) using soft lithography techniques. The recirculation motion in the liquid segments associated with gas–liquid flows as well as the symmetry characteristics of the recirculations are quantified for straight and meandering channel networks. Even minor surface roughness effects and the compressibility of the gas phase induce loss of symmetry and enhance mixing across the centerline in straight channels. Mixing is further accelerated in meandering channels by the periodic switching of recirculation patterns across the channel center. We demonstrate a new, piezoelectrically activated flow injection technique for determining residence time distributions (RTDs) of fluid elements in multiphase microfluidic systems. The results confirm a narrowed liquid phase RTD in segmented flows in comparison to their single-phase counterparts. The enhanced mixing and narrow RTD characteristics of segmented gas–liquid flows are applied to liquid mixing and in sol–gel synthesis of colloidal nanoparticles.