Could you please provide some more information on the experiment? In particular, what were the stresses and the values of viscosity obtained via both methods, how long was the experiment run in both cases, do you expect any chemical transformations, were the samples isolated from the atmosphere or not, were the same measuring geometries used or not. This could possible help to find out the particular reason.

Dear sir,

During my experiments I have used cone and plate geometry for all exp. And covered with solvent trap to minimize the chance of evaporation. Chemical transformation were observed with respect to temp, but with respect to time not observed. Particulars for both experiments as follows: (At particular concentration)

1. Viscosity vs Shear rate-(i.e, controlled rate ranging from 0.01-100 1/s, shear stress-0.5-60Pa, time-500sec): In this η0 value is 124 Pa calculated by applying cross model.

2. Creep:

a) Stress 10 Pa - 11000 sec- DJ/Dt -0.95. Therefore η0 value is 1.05

b) Stress 1 Pa- 11000 sec- DJ/Dt -0.95. Therefore η0 value is 1.19

At the first glance, I can see two possibilities.

First, in the flow curve experiment, how many points did you record? The setting was 500 sec per data point or per flow curve? In the latter case, the values could be far from equilibrium - depending on the details, the system could possibly approach the stationary state for tens and hundreds of seconds. Anyway, you could estimate the equilibration time from the creep curves - as the onset of purely linear signal with time. I would recommend (if not so in your previous measurement) to update the settings of the flow curve experiment so that the measurement time per point is at least no less than the onset of the stationary flow part in the creep curve.

Second, from the creep curve, some increase of the viscosity with decreasing stress was observed, probably, this is the onset of yield stress. Even though 0.5 Pa in the flow curve experiment is cca. the same as 1 Pa in the creep curve, the viscosity could drastically increase in that region if 0.5-1 Pa is about the yield stress value.

Sir thank you so much, I understand your observations. I have recorded 51 points in viscosity, and 500 sec per flow curve data measured. However, viscosity data 0.5 to 10 Pa is in the range of 140-110 Pa. Now I will again conduct creep experiments at lower stress, and then compare with the viscosity value.

Seeking for some other things in RG I found your question dear Kiran, and the interesting answers of Evgeny; they are to the point. I have to add, however, that the gels must be "fresh" on the rheometer and not pre-imposed to stresses. Several times, a reason to fail in reproduction of rheological experimental data is due to some simple mistakes. The so called "pre-conditioning" (especially in AR-G2 rheometer software) is very important; even the loading of the sample can cause "pre-conditioning". For instance the Agar gels are cracking, in case of inattentive sample loading! \\ Hope helping u Kiran.

I do agree with Thomas, I have had a lot of data dispersion just by changing a bit the "pre-shearing time", from sample to sample, it would be right consider leaving for some time (e.g. 10 min), the sample on the fixed plate before starting any test, to disipate any previous strain caused by putting the sample onto the plate

Thanks for all suggestions, they are really helps full for facing the practical problems. Experiments re-conducted for certain concentrations at lower shear ranges, and obtained values are comparable with the viscosity values. Thanks a lot.

Good for you, congratulations Kiran!!