Active Micromixer of Microfluids via Plasmonic Marangoni Convection

Chemical reactions can be efficiently carried out in microfluidic devices using a small amount of solution, meaning that fewer resources are required and less liquid waste is generated, making them energy-efficient and eco-friendly devices. However, the microchannel of the device has a narrow channel width, so the reaction solution exhibits laminar flow. Although this laminar flow is good for transporting the solution, efficiently conducting a chemical reaction is difficult in such a flow. Therefore, the aim of the present study is to produce a micromixer in which diffusion can be easily controlled in the microchannel. In this work, efficient stirring of the fluid was achieved only under laser irradiation by utilising the local light-to-heat conversion that occurs on plasmonic metal nanostructures placed in the microchannel. The gold nanostructure that covers a two-dimensional colloidal crystal, which was easily manufactured at low cost, has a high temperature of 200 °C or more only when irradiated with a laser, and the solution is vapourised and forms bubbles with a diameter of approximately 10 µm. The temperature difference on the bubble surface causes Marangoni convection, which can be used to efficiently stir the reaction solution in the assembled microfluidic device.