I see in many papers, the rotational viscometer is used to characterize the behaviour of STFs which yields apparent viscosity vs shear rate graph. And the shear rate that triggers thickening is determined at the abrupt jump in apparent viscosity. This is all clear.
However, my question is about what follows immediately after the abrupt viscosity jump. In all if not most of these graphs in papers, I see a drop of apparent viscosity right after the peak viscosity (i have attached herewith a graph with the region in question circled for clarification). My question, is why is the viscosity decreasing after the peak viscosity? Or is it a limitation of the viscometer? Could you please help me with this question?
I would say, at high shear rate the sample tends to behave like a solid and become slippery at the wall, i.e. flow discontinuity happens as a result of wall slip. Somehow, limitation of the viscometer. This may be confirmed by using a serrated wall/plate or sandblasting of the wall!
Or it could be due to the intense structural linkages build-up at the peak viscous value followed by structural breakdown leading to a subsequent fall of the viscosity of the medium.
At first glance, so abrupt cases as shown in Fig 5 should be reasonably interpreted as due to wall slip. I agree with A Nematollahzadeh. Tests with serrated walls should be used to confirm it. Probably, the use of serrated walls will change the appearance of the curve to a slow decreasing. On the other hand, the not so abrupt decay of the viscosity after the maximum could be initially adscribed to structure breakdown, as V Pandey suggests. However, the actual structure breakdown should be confirmed using alternative rheological tests or complementary experiments.
According to hydroclusters theory, shear thickening is caused by the formation of hydroclusters. These agglomerates of rigid particles can be destroyed when higher shear stress is applied.
I examined a lot of shear thickening samples using a rotational viscosimeter with the same measurement program and the same parameters (plate-plate geometry, the same gap between plates). A drop of viscosity after peak viscosity was present for all of the samples, regardless of the maximum viscosity value (very broad range of max viscosity: 50-10000 Pa s). It suggests that this effect is not caused by wall slip.
Dear all, other presumed reasons are the disentanglement and mechanical degradation of chains at high sheering. Best regards
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