Adding 2-3 drops of glycerol might change the viscosity of the sample significantly which might impact light scattering. Please check the resulting viscosity and in DLS software, you can add the viscosity parameter accordingly to get the correct results. Also, check whether the cuvette is compatible with glycerol.

Ghazal Shariat Panahi

The measurement of zeta potential is influenced by the refractive index. Viscosity affects the stability of the slurry; a drop of glycerin in water will not change either.

Ghazal Shariat Panahi

In our hands this is no problem. As mentioned by Nabodita Sinha , this will affect the dynamic viscosity, so you need to adjust it. We've been using this well designed web page as resource:

http://www.met.reading.ac.uk/~sws04cdw/viscosity_calc.html

You should also adjust the refractive index (here a list for 20 °C):

http://edge.rit.edu/edge/P13051/public/Research%20Notes/refractive%20index%20glycerin%20water.pdf

If you also want to perform DLS on your sample you might see a glycerol peak below 1 nm - in our hands it comes out around 0.5 nm in the intensity weighted distribution.