Microfocusing using the thermal actuation of microbubbles

This paper describes the microfocusing in a microchannel using the thermal actuation of a pair of microbubbles. A microbubble was produced from de-ionized (DI) water with an integrated microheater, and the volume was controlled by applying voltage. The microfocusing was demonstrated with a polydimethylsiloxane (PDMS) device consisting of two layers. The top layer included a microchannel that was 300 μm wide and 50 μm high. It was flanked by a pair of reservoirs. The bottom layer provided a microheater underneath the reservoir. Upon heating, DI water boiled immediately over the microheater and formed a microbubble that came out of the reservoir in a perpendicular direction toward the fluid. The fluid was focused from 300 to 22 μm, as the distance between the apexes of the arch-shaped microbubbles was shortened due to expansion, which was maintained at a flow velocity up to approximately 17.8 mm s −1 . The temperature of the water in the reservoir was estimated to reach the boiling point within 62 or 160 ms, depending on the substrate.