When the shearing occurs for a short time period at each shearing rate, the measurement is occurring in the transitional zone and not at a steady-state for each point. The short shearing time gives insufficient time for the fluid to respond. That is the reason why the data at 10s in pink is different from the data at 120s (which is 12 times longer) in grey/green.

Aroon Shenoy Thank you.

Dear Eric Dickson,

I don't really understand your question. Please tell me more in detail...

Hello, Nazaruddin Sinaga,

I measured the viscosity vs. shear rate for the same shear thickening fluid twice but I got different curves (the grey/green curve and the pink curve in the attached image). The only difference of measurement condition is the shearing time per point. If I set the shearing time per point as 120s then I get the grey/green curve; whereas if I set the time as 10s I get the pink curve. I'm asking why the change of shearing time results in 2 curves.

Please let me know your thoughts. Thank you.

Best wishes,

Eric

As Dr. Shenoy has suggested the equilibrium time for the both the measurements are different therefore, this results in different viscosity vs shear rate data.

viscosity also influence by temperature and state of flow

Time influences flow parameters depending on type of flow

It is hard to know exactly what is happening without knowing the fluid chemistry and microstructure.

It could be a chemical reaction that was going on very slowly, because your fluid wasn't well mixed, and when you sheared it, the reactants had a higher chance to meet each other. It could also be a sort of particle aggregation or molecule gelation that depends on particles or molecules coming into contact. Think about clouds making rain; it requires wind to make micro drops of condensed water to meet each other to coalesce into bigger drops, which are large enough to precipitate. It can happen by diffusion too, but this is a much slower process than shear rate. It could also be that your fluid was not enough mixed, so you have a multiphase flow, with the less viscous liquid surrounding pockets (big blobs) of the high viscous fluid. They mix with time...

If your fluid was getting less viscous, I would have guessed it was either due to temperature rising, chemical degradation into smaller molecules, or breakup of aggregates.

I would first try to see the microstructure in a microscope. If you see big blobs before shearing, and either no blobs or much smaller blobs, then bingo! It's a mixing effect. If it's not that, you'd need more sophisticated chemical analysis to find out what exactly is happening. If you don't care about learning what is happening inside the fluid, just use the steady-state curve.

Another easy test is to shear the fluid, stop, wait a few seconds, shear it again, stop, wait longer (minutes?), shear it again, wait longer (hours?), shear it again, ...

Does your fluid still behaves in the same way?

Hello Eric. It is as Dr. Shenoy has indicated. For non-Newtonian fluids, adequate time per point must be allowed at a given shear rate to allow that shear rate to fully move throughout the sample gap. In controlled shear rate, the sample responds immediately but there is the time required for the applied shear rate to reach the whole gap. If you were to take a single shear rate and shear your sample at that rate and plot shear stress over time, there is a finite amount of time required for the shear stress to become constant. That finite time is the time to steady state. The general rule of thumb is that for most samples, 1/shear rate is the amount of time needed to achieve steady state. In the olden days, we would manually determine the time to steady state by measuring at individual shear rates to see what time is needed for each but now most rheometers will do this for you. In RheoCompass, if you choose the preconfigured app called "Viscosity & Flow Curve V2.0" it is set up to automatically check for steady state, collect the point, then move to the next shear rate. You can also set your test up like this by going to the Details for the interval, then in the Data points setting Duration select By steady state. This all assumes that your temperature is constant and that your pre-test protocol is always the same. Non-Newtonian materials are also thixotropic meaning they break down with shear and some time is needed for them to recover that structure that is broken down such as in stirring before loading or even the bob/cone/plate going down onto the sample. We call this the shear history and the best way to control this is to establish a pre-shear interval followed by a hold interval so that all your samples see the exact same shear history. About that shear thickening, does your sample have particles in it? Normally one only sees shear thickening in dispersions. Another phenomena that causes what looks like shear thinning is leaving the laminar flow regime and entering turbulent flow. This transition is governed by Reynold's number which encompasses viscosity, density, gap, and speed. For water, usually this transition happens around 125 1/s. If you are using a larger gap, it will happen at lower shear rates.