High-precision optical measurements of microdroplet diameters using pixel intensity averaging and thresholding*

Droplet diameter measurements are often made in microfluidic applications using optical images, with an accuracy that is typically limited by the optical resolution. However, experiments in laser ablation, evaporation, and whispering gallery mode photonics may require droplet diameter measurements with nanoscale accuracy. Achieving such accuracy can also enable improvements in the precision and stability of drop generation and manipulation. We imaged optically single microdroplets in free flight using a high-resolution optical system with uniform illumination and high contrast. To achieve submicron precision, we calculated the average pixel intensity as a function of the distance from the center of the droplet, which provided a low-noise intensity profile from which we extracted the drop radius via thresholding. At the submicron scale, two systematic effects became the dominant source of diameter error: injection instabilities and illumination asymmetries. These effects can be measured and corrected for through non-linear intensity transformations, compensating for focus drifts, and by finding more accurate centers of the droplets. After corrections, we achieved a standard deviation of the diameter of less than 10 nm for droplets with diameters around 40 µm.