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Belated reflections on Week 6: Mad scientist is not just a figure of speech

One thing I can say with confidence is that conducting research demands a certain level of patience and dedication that can breed frenzy in even the most sensible people.

Once again, business as usual in the lab. I employ a special concoction of lucky-guess and brute-force methods to measure shear thinning behavior of xanthan solutions. I announce with great excitement that the capillary experiments have, for the most part, reached a conclusion. The soil column, however, continues to illude my greatest efforts to obtain any sensible data.

To recap; we measured shear thinning behavior of 1g/L and 0.2 g/L xanthan solutions with 0.01M NaCl in 0.01" and 0.03" diameter capillary tubes. The tubes are an idealized physical representation of fluid pathways in porous media. The two xanthan concentrations establish an envelope within which we can predict shear thinning behavior of intermediate solution concentrations using the cross model. Results from our experiment suggest that both solutions follow the cross model in each of the capillary tubes.

Measuring shear thinning in the soil column consists of a slightly different approach. Darcy's Law is used to calculate fluid viscosity as a function of specific discharge, the intrinsic permeability of the soil, and the hydraulic gradient in the column. Literature disagrees on the approach to calculate shear rate; in each case, however, it is a function of the specific discharge and the porosity of the soil media.

The 120-140 graded sand column was measured to have an intrinsic permeability of 6.686(10-8) cm2. Intrinsic permeability is calculated from the specific weight and dynamic viscosity of the fluid, and the hydraulic conductivity (which can be viewed as the potential for fluid flow in the porous media). Hydraulic conductivity (K) is defined as the slope of the line relating specific discharge to the hydraulic gradient. In order to determine K, I established different flow rates of water in the soil column. During which I recorded the pressure drop along the column. Once the system had reached steady state I stopped the experiment, calculated the hydraulic gradient, and moved onto a different flow rate.

We have successfully measured the viscosity of water as a function of shear rate in the column. As expected, the viscosity remains constant at around 0.001Pa*s at any given shear rate. Xanthan solutions thus far have failed to adhere to the cross model's predictions. I suspect an error exists in my method of calculating the shear rate. The results show my estimation of viscosity to be in error by a factor of ten. This discrepency screams a mistake in converting units. Either that, or we have discovered a new type of sand that has a time-variable permeability. I must deterime the source of this error, and when I find it there will be no mercy.

After the experimental results are sorted out, I will begin working on a computer model to predict non-newtonian fluid flow characteristics in porous media.