Ultrasound transmission spectroscopy: In-line sizing of nanoparticles

Nanoparticles are increasingly used in a number of applications, e.g. coatings or paints. To optimize nanoparticle production in-line quantitative measurements of their size distribution and concentration are needed. Ultrasound-based methods are especially suited for in-line particle sizing. These methods can be used for opaque dispersions and at high concentrations. However, using ultrasound to measure nanoparticles is challenging: despite the use of high frequencies the scattering is close to the Rayleigh regime (ka ≪ 1) and the information contained in the measurements is limited. In this work the performance of an ultrasonic particle sizing method is evaluated using SiO₂ nanoparticles. The measurement method is based on ultrasound transmission spectroscopy. The presence of nanoparticles affects the propagation of ultrasound in the medium, which is measured over a frequency band of 50 - 250 MHz. The wave propagation effects are then interpreted using the inversion of a physics model. The investigated dispersions consisted of SiO₂ nanoparticles (1.4 and 2.0 vol%) dispersed in water. Four batches, provided by Nano-H S.A.S., had monomodal size distributions with mean sizes 150, 300, 420 and 440 nm. Two bimodal size distributions were investigated: 1) a mix of 50% 302 nm and 50% 422 nm particles, and 2) a mix of 50% 150 nm and 50% 422 nm particles. As a reference the size distributions were measured using an optics based Malvern Zetasizer. The mean particle sizes and concentrations were similar to the reference, with differences between 4.5 and 19% and between 3 and 15%, respectively. The shape of the particle size distributions obtained by the ultrasonic instrument were similar to that of the reference. Also, the ultrasound instrument was able to produce correct results for both mono- and bimodal size distributions. The temperature of the mixture did not have a significant influence on the results.