Relationship Between Attenuation and Viscosity for Newtonian and Non-Newtonian Fluids 1

[KimWonGun]

After consulting several resources, I am still unclear about the relationship between viscosity and attenuation for Newtonian and non-Newtonian fluids. Stokes Law states that attenuation is proportional to (dynamic) viscosity. That makes intuitive sense for both Newtonian and non-Newtonian fluids. Yet several resources suggest that attenuation increases when viscosity DECREASES for a thixotropic material under shear stress. What basic principle am I overlooking?

 

[Engdoitbetter]

Dear KimWonGun,

I'm not a material expert so do not rely on my word (I've just read the wikipedia article on Thixotropy!). My guess is the following:

- newtonian fluid always behave the same (i.e. the fluid dissipates energy always in the same way), so in case of laminar flow (the same as Stokes' Law) an increase in viscosity leads to greater attenuation, because energy is dissipated through the velocity gradient between the various layers of the flow (tau = mu * du/dy).

- thixotropic fluids do NOT behave always the same: when at high viscosity, they're almost like solids so there isn't much slip between the various layers; therefore the dissipated energy is low (i.e. viscosity is high but the velocity gradient is very low, then work is low). At low viscosities, slip increases so that a greater amount of energy (i.e. work done by viscosity) is dissipated.

Hence my guess is that slip rate increases much more than the correspondent decrease in viscosity, changing abruptly the way energy is dissipated.

Hope it helps.

Stefano