Abstract
Emulsions play a crucial role in many industrial processes, making their characterization essential. We introduce a novel optical approach for analyzing emulsions generated in a microfluidic chip using structured light. The approach is based on the investigation of the transverse Doppler shift detected in the probe beam. Laguerre-Gaussian vortex beams, carrying orbital angular momentum (OAM), probe water-in-oil droplets flowing through curved microchannels, with frequency shifts in the transmitted light revealing droplet motion. This label-free method enables real-time, integrated measurements in dense emulsions, overcoming conventional velocimetry limitations, giving a twist to the characterization technique of emulsions. Crucially, we show that this compact and robust technique is also sensitive to droplet size, density, and flow regime, offering a powerful tool for lab-on-a-chip emulsion analysis.