Active Microfluidic Mixing Using Photoacoustic Laser Streaming from Gold-Implanted Optical Fibers and Glass Beads

Efficient microfluidic mixing remains a significant challenge due to the difficulty of generating advection and turbulence, especially as many applications demand reduced sample and reagent consumption for improved efficiency and lower costs. In this work, we introduce a new active mixing method─photoacoustic laser streaming (PALS)─which uses laser pulses to produce fluid jets. PALS integrates photoacoustic effects with acoustic streaming, transforming metal-coated surfaces into fluidic pumps without moving mechanical parts. To demonstrate the concept, we fabricated a fiber-based PALS pump by implanting gold (Au) onto the facet of an optical fiber. The fiber tip was immersed in liquids to perform mixing in a microplate (50 μL volume), achieving complete mixing in 3 min compared to hours required for diffusion alone. Additionally, we utilized Au-coated glass microbeads as PALS stirrers. Upon light excitation, the beads not only generated jets from their surfaces but were also recoiled by the jets, creating enhanced turbulence and more efficient mixing. While this method was demonstrated in a microplate, we anticipate even greater effectiveness in microfluidic channels, where traditional mixing techniques face significant limitations.