(132a) A Mobile Device for Nanoparticle Characterization By Wide-Angle Light Scattering (WALS) and Laser-Induced Incandescence (LII)

The combination of the optical measurement techniques Laser-induced incandescence (LII) and Elastic Light Scattering (ELS) allows for an online, non-invasive and comprehensive characterization of nanoparticle systems. With LII the monomer size (primary particle diameter) and particle concentration (volume fraction) are accessible [1], aggregate size (radius of gyration) and aggregate structure (fractal dimension) can be derived from elastically scattered light [2].

We developed and improved a mobile measurement setup based on wide-angle light scattering (WALS) combined with two-color Time-Resoved LII (TiRe-LII) [3]. Conventional approaches for elastic light scattering techniques may suffer from limited angular resolution using only a few detectors at discrete positions or slow measurement rates with a single detector rotating in angular steps around the measurement object. With WALS the scattered light from a small punctual measurement volume is imaged onto a camera-chip using an ellipsoidal mirror. Consequently, scattering diagrams can be obtained from single-shot data over a wide angular range (10°-165°) at a high angular resolution (<0,5°). Here, a continous-wave fiber-laser (2 W maximum output power @ 532 nm) and an industrial CCD-camera were used to acquire scattering data. Evaluation of the scattering data in the Guinier- and Power-Law-regime yields effective values of radius of gyration and fractal dimension. For LII-measurements a pulsed diode-pumped Nd:YAG-laser (15 mJ maximum energy per pulse @ 1064 nm and 100 Hz maximum pulse frequency) is employed. The incandescence signal is detected at two different spectral regions by gated photomultiplier tubes and read-out by an oscilloscope (500 MHz, 4 GaS/s). Primary particle sizes were subsequently derived from an adequate theoretical cooling model considering decay time, particle temperature, ambient temperature and pressure. The volume fraction was determined relating the maximum intensity of the incandescence signal to a known volume fraction of a calibration source.

In practice, scattering data from real aerosols consists of the single scattering signals of aggregates of different morphology and only effective values for aggregate sizes can be determined. Inferring size distributions from the angular distribution of elastically scattered light is mathematically ill-posed as different distributions may lead to similar scatterings signals. Bayesian inference is used as a powerful statistical tool to derive probability densities for recovered size distributions of soot particles from a flat-flame burner (McKenna type). In another experiment, silica particles are synthesized from a liquid precursor and sintered afterwards. With increasing sintering temperature the fractal-like aggregates become more spherical and the scattering behavior shifts from the Rayleigh-Debye-Gans approximation for Fractal Aggregates (RDG-FA) towards Mie-theory. Here, a Principal Component Analysis (PCA) is conducted to correlate the scattering signals to the respective theory.

References

[1] H.A. Michelsen, C. Schulz, G.J. Smallwood, S.Will, Prog. Energ. Combust., 51: 2-48 (2015)

[2] C.M. Sorensen, Aerosol Sci. Technol. 35: 648-687 (2001)

[3] F.J.T Huber, M. Altenhoff, S. Will, Rev. Sci. Instr. 87: 053102 (2016)