The Effect of Nanoparticle Morphology on the Measurement Accuracy of Mobility Particle Sizers

The influence of particle morphology on the accuracy of nanoparticle size distributions measured by the engine exhaust particle sizer spectrometer (EEPS, TSI Model 3090) was studied using the scanning mobility particle sizer (SMPS, TSI Model 3936) as a reference. The EEPS shows higher total number concentrations with the maximum relative difference up to 67 % and smaller number median mobility diameters for polydisperse silver nanoparticles generated in the laboratory. To provide a quantitative explanation of the difference, generated polydisperse nanoparticles were classified as monodisperse particles with the initial equivalent mobility diameter (d m1) and sintered in the second furnace at different temperatures (room temperature to 600 °C), to change their morphologies for the comparison tests. Without sintering (room temperature), results show that the measured mobility diameter (d m2) of the EEPS is smaller than that measured by the SMPS when d m1 is larger than 30 nm and the difference increases as d m1 is increased from 30 to 300 nm. But the difference decreases as the morphology of particles is changed from branched chain agglomerates to spheres for d m1 less than 80 nm and the sintering temperature higher than 200 °C. Theoretical analysis shows that the mean charge per agglomerates is more than that of spheres resulting in overestimation of the electrical mobility and underestimation of the d m2 by the EEPS.