Excitation-frequency determination based on electromechanical impedance spectroscopy for a laser-microfabricated cavitation microstreaming micromixer

Abstract

We for the first time propose and validate an electromechanical impedance spectroscopy (EMIS) technique to determine the resonance frequency (fr) of micromixers for effective cavitation microstreaming. Theoretical fr predictions of the oscillating bubble have been inaccurate because it was assumed that the bubble had a spherical shape even though bubbles are commonly trapped in low-profile air pockets, and thus their shapes are not spherical. Empirical excitation-frequency search has been employed but it was time-consuming and could overlook the exact value of fr. Strong electromechanical coupling between a piezoelectric transducer and a microfluidic chip (i.e., a mechanical structure) allows straightforward, rapid fr determination (∼4.5 min) using EMIS. To validate the EMIS method, we compare microstreaming patterns generated by bubbles excited at four different resonance frequencies using a high-speed imaging setup: 1) fr,B, the theoretical fr assuming a spherical bubble, 2) fr,Beq, the theoretical fr based on the concept of an “equivalent spherical bubble” having a surface area equal to the vibrating air-water interface area, 3) fr,Pf, the EMIS-based fr of a piezoelectric transducer, and 4) fr,Cf, the EMIS-based fr of the microfluidic chip bonded with the transducer. After confirming that fr,Cf yields the strongest cavitation microstreaming, the air-pocket shape is designed with consideration of the bubble stability and mixing effectiveness. A micromixer chip with the designed air pockets is excited at fr,Cf and exhibits rapid homogenization (37.2 s) of diluted ink and deionized water in the mixing chamber of a substantial volume (61.9 μL). Additionally, an experimentally validated, adhesive-tape-supported laser microfabrication technique allows rapid prototyping (∼10 min) of polymethyl methacrylate microfluidic chips with an excellent dimensional accuracy, machining resolution, and surface smoothness. We expect the proposed EMIS-based technique for resonance-frequency determination and the simple, rapid laser microfabrication method to be widely employed for bio-/chemical microfluidic applications in the near future.