2.2. Fabrication Process and Experimental Investigation

The micromixers with obstacle structures embedded on both sidewalls of a Y-shaped rectangular microchannel (200 µm in width and 50 µm in height) were fabricated through a soft-lithography process using a thick negative photoresist (SU-8 3050, MicroChem Corp., Newton, MA, USA) as a mold. The SU-8 master mold was replicated in PDMS (Silpot 184, Dow Corning Toray Co., Ltd., Tokyo, Japan) after curing at 80 °C for 40 min. After punching holes for the inlet and outlet ports (1.0 mm in diameter), the microchannels were sealed with a glass substrate (S9213, Matsunami Glass Ind., Ltd., Osaka, Japan) using a silicone-based adhesive double-sided tape (No. 5303W, Nitto Denko Corp., Osaka, Japan). The water contact angles of PDMS and the adhesive tape were 108° and 102°, respectively.

To investigate the mixing behavior in the fabricated micromixers, fluorescence imaging was performed using an inverted microscope (TE2000-U, Nikon, Tokyo, Japan). Pure water and fluorescein-dyed water (0.1 mol/m³) were introduced into the microchannel from individual inlet ports at the same flow rates ranging from 0.5 to 50 µL/min with two syringe pumps (YSP-201, YMC Co., Ltd., Kyoto, Japan) (equivalent to total flow rates ranging from 1 to 100 µL/min and Re range of 0.13 to 13 in the microchannel). The experimental investigation of the pressure drop used two types of pressure-driven micropumps with different maximum pressure capabilities (Flow EZ^TM 345 mbar and 7000 mbar used in the flow rate ranges of 1–10 µL/min and 20–100 µL/min, respectively; Fluigent SA, Le Kremlin-Bicêtre, France) equipped with a flow sensor (Flow Unit, Fluigent SA). The micropumps controlled the desired flow rate of water (colored with blue food color, 0.1% w/v) introduced into a microchannel. Additionally, they facilitated the measurement of the overall pressure drop required for liquid delivery from one of the two inlet ports (the other was not opened in the PDMS devices) to an outlet port.