In situ confocal Brillouin spectroscopy for depth-resolved measurements of fluids in microfluidic flow

Characterizing fluid mechanics in microfluidic systems is challenging using conventional rheology due to the small sample volume and the need for mechanical contact or tracer particles. Here, a confocal Brillouin spectroscopy platform is integrated with a microfluidic device for in situ, depth-resolved characterization of fluids under pressure-driven flow. The system measures Brillouin frequency shift and the spectral linewidth within a ∼60µm microchannel without perturbing the flow. Experiments with water and a 1 wt% collagen solution show well-defined channel interiors and stable spectral parameters across flow rates. This tracer- and label-free optical approach enables spatially resolved Brillouin measurements in microfluidic environments and establishes a baseline for probing flow-coupled effects in more complex fluids and geometries.