SIZE DISTRIBUTION

A commercial laser light scattering (LLS) particle sizer (Malvern Mastersizer2000) is used to estimate the volume distribution of our samples of irregular particles. This instrument measures the phase function of the sample at 633 nm (and optionally at 466 nm) in a certain scattering angle range with special attention to the forward scattering peak. The volume distribution of projected surface-equivalent spheres that fits the measured phase function is obtained by using either the Lorenz-Mie theory or the Fraunhofer approximation for spheres. A detailed study of the range of validity of the LLS particle sizes can be found in Gómez Martín et al. JQSRT, 241, 2020. 

Briefly:

For spherical particles at visible wavelengths (λ=633 or 466 nm):

    Fraunhofer can be applied for r>0.5 μm for very absorbing particles, and generally for r>3μm (λ=633 nm). This limit would be reduced by a factor of ∽ 0.7 if the measurements are carried out at 466nm.

    The Mie model with the correct refractive index constrains the intrinsic applicability method to r> 0.1μm. Claims of sensitivity below this limit must be regarded with scepticism if additional supporting techniques (e.g. polarimetry) are not considered.

For irregular particles at visible wavelengths (λ=633 or 466 nm):

    The limit of Fraunhofer is lowered to r∽ 1μm (λ=466 nm) even for transparent particles. Fraunhofer cannot be generally trusted below that limit.

    Mie extends the validity almost to the lower limit for spheres (r∽0.3μm) if the real part of the refractive index (n) is high. Note that if n is uncertain, it is safer to assume values at the upper end of the uncertainty limit.

Click here for an extensive explanation of the size distributions.