Is there any analytical way of estimating the viscosity of a ferrofluid (iron oxide) without the use of a rheometer [no magnetic field also]? I tried surveying the literature, but the papers on 'negative [ferrofluid] viscosity' [magnetic field present] are of no use to me.
There are several theoretical approaches regarding the rheology of suspensions of spherical particles, which are the commonly used ferrofluids. Einstein's theory is only sufficient for high diluted systems, Rosensweig gives a good analytic model which fits also for higher volume concentrations. For the general behaviour of colloidal suspension in dependency of the volume fraction and particle shape Mueller et al. (Proc. R. Soc. A 2010) gives a very good overview.
For Dilute concentrations you can use a simple Glass Capillary Viscometer. But the experiment with ff can be quite nasty! There are a few early papers (in 70's I guess) on such measurements.
There are lots of viscometry experiments/apparatuses to try, some of them are really simple/cheap too (Google them all for more details):
1) Bubble viscometers (compares the length of time it takes fora bubble to move from one end of a tube to the other when the tube is filled with a liquid).
2) Cone viscometers (compares the length of time it takes a certain volume of liquid to pour out through certain sized orifice.
3) Rotational viscometers (simpler and cheaper version of a rheometer).
4) Capillary viscometer (as John mentioned), but might be difficult with your sample.
Also, why not use a rheometer? You can buy disposable measurement geometries for most rheometers if messing them up is your concern.
If the solutions are diluted, can be measured easily, inexpensively and with enough accuracy the viscosity of such samples using capillary viscometers by gravity.
Undoubtedly, the most suitable are rotational viscometers, allowing us to obtain flow curves, which are like rheological "fingerprint" of the tested sample
There are several theoretical approaches regarding the rheology of suspensions of spherical particles, which are the commonly used ferrofluids. Einstein's theory is only sufficient for high diluted systems, Rosensweig gives a good analytic model which fits also for higher volume concentrations. For the general behaviour of colloidal suspension in dependency of the volume fraction and particle shape Mueller et al. (Proc. R. Soc. A 2010) gives a very good overview.
Use of capillary rheometer is another option to measure viscosity of ferrofluid. The proper capillary diameter should be chosen for different carrier (depending on viscosity) so as to minimize measurement time scale. If capillary diameter is very big then you will not get significant change in measurement time. There are number of papers in the literature mentioning this.
Thank you Eric. Yes I am using Rosensweig's approximations. I think his model is a taylor series expansion of the Einstein model
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