A Twist on Droplet Microfluidics: OAM‐Based Investigation of Emulsion Properties

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.