Small angle light scattering apparatus for analysis of single micrometric particles in microfluidic flows

The fast characterization of micrometric particle is becoming of increasing importance. The measurement of shape and the index of refraction of a particle allows very accurate characterization of the analysed material. A CCD-camera based small angle light scattering (SALS) apparatus has been developed to characterize single micrometric particles. The measured scattering vector spans the range 0.02 - 6.8 1/µm. The incident laser light is collimated to a spot of about 50 µm in diameter at the sample position with a divergence lower than 0.045 rad. Such a small collimated laser beam distinguishes this system from previous small angle light scattering instruments described in literature and opens the possibility to perform SALS in quiescent and in-flow conditions in small microfluidic channels. By properly designing the micro-channel and using a viscoelastic liquid as the suspending medium, it is able to realize a precise 3D focusing of the target particles. The forward scattering emitted from the particle is collected by a lens with high numerical aperture. Further, at the focal point of that lens a homemade beam stop blocks the incident light. Finally, a second lens maps the scattered light on the CCD sensor, allowing for the collection of far field images on short distances. Measurements with mono-disperse polystyrene particles, having nominal radii of 0.95, 1.64, 2.08, 2.90, 3.04, and 4.01 µm, both in quiescent and in-flow conditions have been realized. Experiments in-flow conditions allow the measurement of the single particle scattering profile. Results are validated by comparison with calculations based on the Lorenz-Mie theory. Measurements of real multiplexed particle solutions, with particles down to 1 µm in radius confirmed the possibility to use this SALS apparatus. Moreover, initial analyses of microgel particle structures in quiescent conditions over time have been carried out. This analysis can be extended for more complex systems, like multi-shell, or non spherical particles in terms of single particle characterization.