I am calculating viscosity of my oil in water emulsion using a rotational viscometer with a disk shaped spindle. The fluid is less viscous upon observation. However when i measured the viscositu, the viscosity increases with increase in spindle speed and torque. If thats the case then which reading should be considered correct or please explain how to take reading correctly.
Hi Payas,
It looks like your emulsion is shear-thickening. I don't know about the details of your emulsion (oil volume fraction, droplet size, stabilised or no, etc.), but as far as I know it's fairly common for emulsions to have shear-dependent viscosity.
Have you compared your measurements against a known reference, e.g. pure water or pure oil? These should show Newtonian behaviour, so if you see spindle speed or torque dependence there too, it's probably a measurement artifact. If not, your emulsion is indeed shear-thickening, and there's not just one reading that's correct - you should create a viscosity curve (viscosity as a function of shear stress) or a flow curve (viscosity as a function of shear rate).
Hope this helps,
Sita
Without details on formulation (oil content) and measurement (disk & container geometry, rotation speed) this is hard to assess. But you might be seeing inertial effects at higher speeds, similar to "Taylor vortices" in Couette geometries. This effect would lead to higher "apparent viscosities", but in fact the higher torques would not caused by viscosity, nor allow to measure it.
The fluid you are experimenting is probably non-Newtonian. Such an observation could be due to changes in the state of aggregation or coagulation of the suspensions. Such behavior has already been observed in granular and clay-like colloidal suspensions in which the applied shear forces build up the interlinking of the structural elements giving rise to the apparent increase in viscosity. The buildup of the structures is often disrupted upon the removal of shear which makes the process reversible, but not always.
Certainly your the emulsion is non-Newtonian. Note down the spindle speed and torque that will help you in calculating shear rate . Take different values of spindle speed keeping the speed constant. With the increase in shear rate (spindle speed) the turbulence increases causing the droplet to break into smaller droplets which in turn increases the viscosity of the emulsion. Based on size of the droplet or the shear rate if you could find, you can establish the vortex system active their. So we will know how viscosity, droplet size and shear rates are related.
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