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Well, this is humbling...
Introduction:
Xanthan gum is a polysccharide, derived from the bacterial coat of Xanthomonas campestris. It is also the source of dismay for young scientists, who's only feeble desire is to obtain useful data. See Figure 1 below for an illustration of this phenomenon.
Methods:
Xanthan solutions were prepared with 500mL of deinoized water, 500mg of xanthan, and 292mg of pure table salt. The xanthan comes as a powder and is mixed in the DI water with the NaCl using a magnetic stir bar for 2 hours. Rheology experiments were conducted no more than 6 days after mixing.
Shear thinning behavior was investigated using a luer lock tip syringe attached to a 1.5 meter long tube with an ID of 0.381mm. A syringe pump controlled with labview was used to establish the different shear rates within the fluid as it flows through the tube. Pressure was measured at the tube entrance using a 0-1PSIG pressure transducer.
Each experiment consists of measuring the relative pressure drop along the tube during different flowrates for one solution concentration. Flow rates were maintained for a period of 90 minutes to ensure the system has reached steady state. Following each run interval, the system was allowed to rest for at least 90 minutes to establish steady conditions for the zero flow regime. The absolute pressure drop was calculated by subtracting the pressure during steady state zero flow conditions from the previous run steady state pressure.
Data are analyzed using excel. The apparent viscosity is calculated from the ratio of the maximum shear stress in the tube and the shear rate. Results from one experiment are outlined in Figure 1.
Results:
Figure 1 shows collected data for the 1 g/L xanthan with 0.01M NaCl solution compared to the cross model (where we want to be). The blue diamonds are data collected on thursday evening, whereas the 'x's were collected during the weekend. Each experiment was conducted using different syringe sizes, and therefore different pump speeds.
Figure 1, Apparent viscosity as a function of shear rate for a .1% xanthan
0.01M NaCl solution
Results for the two experiments widely disagree. Blue diamond data points follow the cross model trend more closely with less scatter. The x data points span nearly three orders of magnitude on the ordinate.
The very low pressures observe during the SGE test can explain most of the chaos exhibited by measured data. The transducer has a difficult time because the gauge pressure in the tube can reach as low as 0.002PSI (scaling with the expected errors of most transducers). Furthermore, the meassured pressure values during the zero flow and low flow intervals are almost indistinguishable. Observed shear stresses developed in the tubing, therefore, are very similar for lower flow rate and zero flow rate regimes.
We propose two solutions that will hopefully abate this issue:
- Increasing the length of the tubing to increase headloss along streamlines to make it easier for the transducer to measure the pressure drop.
- Changing the syringe and extending the testing duration to ensure steady states are achieved and to reduce noise in our measurements.
Conclusion:
My experience so far with CMOP has shed light on the reality of conducting research. This is not easy. Despite the difficulties associated with working on this project I am having fun. Who knew six little points on a graph could bare so much weight?
These struggles could not have come a momment too soon. On Friday Veronika Megler gave a seminar discussing her perspectives of working as a professional scientist/engineer. She explained that we cannot be dissapointed with life's unexpected outcomes. Veronika also demonstrated that meaningful life experience does not correlate with external impact or status. Her talk was very informative and entertaining. I will keep those lessons in mind as a continue hacking through this project.
Thanks.
