Hi all,
I am trying to conduct Dynamic Light scattering experiments on polymer sample using Malvern Zetasizer ZS. One question I have is to whether it is okay to use complex viscosity instead of normal viscosity while preparing the SOP for the experiment?
Thank you,
Richa Ghosh
Richa Ghosh
The use of the terms complex viscosity and normal viscosity leads to errors. Compare the viscosity units with the units in the instructions and use the correct viscosity.
Hi, there is the semi-empirical Cox-Merz rule which allows to numerically equal viscosity as a function of shear rate (in rad/s) with the complex viscosity as a function of frequency. As far as I know, no one was able to explain, why it should work in first place, but it is known to work well for almost all rheologically simple fluids, including polymer solutions. So, unless you have a very unusual polymer, you can use the complex viscosity instead.
However, I must wonder why you don't measure the standard viscosity. What kind of rheometer do you have? DLS requires a dilute solution to avoid interference between the dissolved chains (or actually, their solvation shells). We also have Zetasizer in our lab and we usually use a concentration of around a few miligrams per milliliter, depending on the sample and its Mw. At these conditions, the quality of the oscillatory measurements are quite poor and it's just better to use a steady flow (DHR2 rheometer by TA Instruments).
Petr Lepcio Thank you for your response! I am trying to extract Rheological measurements using DLS. The SOP for the DLS however requires input of viscosity. There is a document from Malvern which covers which viscosity should be referred to. https://macro.lsu.edu/HowTo/MALVERN/PDF/MALVERN_FAQ_OTHER/Which%20viscosity%20value%20should%20be%20used%20in%20Dynamic%20Light%20Scattering.pdf
However, the viscosity using a Rheometer didn't give a very stable value for some reason and the Complex Viscosity came out rather clean and reliable. However, the Complex viscosity consists of both storage and loss moduli. Hence, I was wondering whether I should only consider the loss moduli/frequency at shear rate of 0?
Any advice, will be appreciable!
In that case, you can use the complex viscosity from the first Newtonian plateau (at a low-frequency limit).
I have become a little bit out of touch since my retirement but as far as I am aware, knowledge of viscosity is not required for the determination of diffusion coefficient D from dynamic light scattering. The value of D follows directly from the decay rate, which is derived from the experimentally measured autocorrelation function (see e.g. Eq. 11 in J. Stetefeld et al., Biophys. Rev. 8 (2016) 409-427, HTML via Google Scholar; see also ‘dynamic light scattering’ in Wikipedia). Knowledge of viscosity η is necessary if one wants to measure hydrodynamic radius Rh of a particle or polymer by means of the Stokes-Einstein relationship. In that case D has to be extrapolated to infinite dilution and η is the viscosity of the medium (J. Stetefeld et al., Eq. 12). Assuming that this medium is a common solvent one can take the solvent Newtonian viscosity at the temperature of interest. For conversion of DLS results into rheological parameters I have added a paper by T.G. Mason et al., J. Mol. Struct. 383 (1996) 81-96.
Peter A. M. Steeneken , you are right, viscosity is not needed to determine the diffusion coefficient, but only for calculating the particle size. However, that is usually the information of interest.