Measuring the polarized complex forward-scattering amplitudes of single particles in unbounded fluid flow: CAS-v2 protocol

A detailed protocol for measuring the complex forward-scattering amplitude S(0°) of single particles, the Complex Amplitude Sensing version 1 (CAS-v1), has recently been developed and used for characterizing environmental particles. However, interpretations of the S(0°) data need a priori assumptions on the particle's shape, and applications of the method have mostly been limited to the particles suspended in liquids. Here, we thoroughly upgrade the CAS technique to perform quality-controlled S(0°) measurements at two independent polarizations for particles suspended in gases and liquids. The polarization-resolved S(0°) sensing constrains the particle's shape and improves the physical interpretability of data. An optical coherence backscattering detection technique enables non-invasive yet precise constraints of the particle's location in the sensing region, realizing precise S(0°) measurements even for aerosol particles introduced via a gas jet without using a flow microchannel. The CAS-v2 protocol proposed here will be useful as a fast yet detailed particle measurement technique for laboratory and field studies.