Droplet-induced optical effects in an opto-microfluidic cross-configuration system

A comprehensive description of all the optical phenomena occurring when light interacts with a moving dispersed phase in a constrained environment such as a real microfluidic channel is needed to perform a quantitative analysis as well as predictive one. This requires identifying fingerprints in the detected optical signal that are doubtlessly correlated with the shape and content type of the dispersed phase from those connected to uncertainties of the optical detection systems and/or instabilities in the microfluidics apparatus leading to dispersed phase size distribution. This article aims to model all the droplet-induced optical effects in an opto-microfluidic cross-configuration system and quantify how diffraction, transmission, absorbance, and reflection contribute to the overall response in the detected intensity after light-matter interaction. The model has been tested in the case of water droplets dispersed in hexadecane continuous phase as generated in an opto-microfluidic platform where optical waveguides are fully integrated with the microfluidic channels, so that light illuminates the flowing droplets from the channel wall and collected on the opposite side. A critical discussion of the impact of geometry and constrains is proposed as well as the impact of each contribute in terms of fingerprints in the detected signal. The good agreement obtained demonstrates the potentialities of both the derived model and the cross-configuration, getting information on droplet characteristics from the intensity arising from its light interaction.