I n this chapter suggestions will b e given for possible future experiments that would complement the work presented earlier. In most cases the following suggestions were intended to be an integral part of the study presented in this thesis, but time restrictions prevented this. The following additional comments can be classified into two broad categories; revisiting experiments presented in this thesis and new experiments.
9.1 Revisiting Experiments
Experiments similar to those presented in this thesis, could be performed with improvements which would enable additional investigations. Following is a revisi tation of experiments found in the various previous chapters.
Included in the velocity measurements of polyethylene oxide ( PEO) solutions (Ch. 4) was a study investigating the center-of-mass diffusion of P EO for various concentrations. In particular, the critical concentration, c* , at which the polymers became entangled was sought . This process manifested itself as an abrupt decrease in diffusion magnitude as the concentration was increased. A more conclusive investigation of this 'entanglement concentration' could be achieved if more data points were obtained around the 1-2 % (wjv) concentration. An ideal study could include more diffusion data well below 1 % concentration.
The chemical selective imaging of both P EO and water (Sec. 4.2) revealed a similar normalised signal amplitude when the sample was sheared at 30 S-l .
It may be more fruitful to search for the migration of PEO and water molecules at larger shear rates. This study will require the use of smaller capillary pipes, thus higher resolution images will have to be employed. These experiments could take advantage of the quadrupole gradient coil, described in chapter 8, which will allow a tube diameter of 1 .0 mm or smaller. Close attention should be made to
CHAPTER 9. FUTURE DIRECTIONS 167 acquiring images of the boundary of the tube wall. It must be mentioned that similar preliminary experiments have been completed but not. included in this thesis because of poor data quality.
A question arises with the rheological data presented in this thesis as to the difference, if any, between the slip and spurt phenomena. As seen in chapter 6, at higher flow rates the enhanced shear rate of the C PyCl/N aSal wormlike mi celle, close to the capillary wall, developed into a flow profile akin to the slip phenomenon. As a result of this development, the question emerges as to whether Xanthan gum solutions also follow a similar flow development. The slip experi ments, performed on Xanthan solutions, could be repeated with the emphasis of the study toward collecting low shear rate velocity data. Perhaps an intermediate flow behavior could be found. It must be noted that a clear difference between the slip and spurt phenomena has been postulated in this thesis, stating bound ary and bulk fluid properties respectively. The low shear rate experiments would hopefully support the above postulates.
The study of the amount of slippage of Xanthan solutions (Sec. 5.4.2), and its dependence upon reservoir/capillary diameters, is not conclusive and requires further velocity measurements with more W
/ D
combinations.The hysteresis effect of wormlike micelles, mentioned in section 2.3.2, could be examined utilising velocity profiles in capillary geometry. Careful flow rate adjust ments would have to be made approaching the unstable flow rate from below and above. Also, further analysis is needed of the behavior of CPyCl/N aSal wormlike micelles under the influence of higher shear rates. The theory of the 'shear band's' spatial increase with increasing shear rate could be verified. Again, as with the proposed higher shear rate study of Xanthan solutions, smaller capillary pipes, in conjunction with the quadrupole gradient coil, could be used.
Although extensive research has been completed on the echo instability of the double phase encoding pulse sequence, further exploration is required to find the cause for such anomalies. In particular, careful mapping of the susceptibility fields is needed, as the orientation of the glass cylinder/acetone sample is maneuvered from a position perpendicular to Bo , to parallel. The changes to image's attenua tion as a result to the rod's orientation may prove an interesting clue to the source of the instabilities.
9 .2 New Experiments
The different rheometer geometries presented in this thesis offer a variety of stress variations. For example, the linear variation provided with the pipe and the more
CHAPTER 9. FUTURE DIRECTIONS 1 68 'gentle' stress variation
(�)
observed with the cylindrical-Couette. Throughout this thesis, various assumptions have been incorporated into the research. One in particular is the shear rate homogeneity with the cone-and-plate rheometer. Because of the unique orientation and cone-plate arrangement, it is postulated that shear rate of the sample will have no radial dependence. This postulate should be verified using careful velocity measurements of the combined cylindrical Couette-cone-and-plate rheometer described in section 3.4.3. Naturally measure ments should be concentrated on the cone-and-plate region of the rheometer.Other geometries for rheometers should also be investigated, including exten sional flow. The research in this thesis has concentrated on shear type stresses and would be complimented with further study with extensional stresses. The rheometer could be assembled using finely bored jets, each propelling its fluid to ward the other. This unique arrangement would provide a topographical map of stress components.
Finally, as mentioned in chapter 8, measurement of other order parameters, which are sensitive to molecular alignment, would be beneficial to the studies presented here. These include proton dipolar and deuterium quadrupolar interac tions which could be incorporated while the sample molecules are being subjected to a stress or strain-controlled experiment . Again, the quadrupole gradient coil would be useful to obtain the gradient strength required for spatial localisation of a deuterium image.