I have had so much difficulties with measuring PSD of fumed silica using DLS with 90 degree light scattering. The data do not make sense and I wonder if the particles aggregate very fast or there is any other reason for the weird behavior of fumed silica with our DLS. The count rate is very low (
I think the problem is with the suspension. It recommended to make a homogeneuous suspension and angle can be 172 degree back scattered or check reference parameters. Good luck!
There are a number of issues measuring fumed silica. First, it consists (in suspension) of long fused threads or chains of primary particles in the 10 - 20 nm region. Have a look under SEM or TEM. These primary particles cannot be separated into their fundamental particles under any circumstances. The particles are already aggregated. Here's a webinar (open in IE only; not enabled for mobile viewing). Look at slides 9 and 10 in particular:
Dispersion and nanotechnology http://tinyurl.com/hpywsge
Second, the material has a huge pore volume and this means that the effective relative refractive index (RRI) is close to unity. When suspended in water the system is virtually indexed-matched and nearly invisible. The combination of the 2 factors above leads to very weak scattering. Measurement of the powder with dry laser diffraction will show aggregated sizes in the 1 - 10 micron (yes microns!) region. The higher RRI in the dry helps the analysis.
The sensitivity at 90 degrees could be non-ideal, and dilution could potentially help (up to a certain point, still need to avoid number fluctuation effects http://www.materials-talks.com/wp-content/uploads/2018/03/FAQ-What-are-number-fluctuations.pdf ).There may be an additional complication with these samples due to rotational diffusion effects:
Article Dynamic light scattering of dispersed fumed silica aggregates
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