A versatile single-screw-extruder system designed for magnetic resonance imaging measurements

A versatile system has been developed for magnetic resonance imaging (MRI) measurements, in which a ceramic barrel/outer cylinder (0.04 m internal diameter) can be configured either as a single-screw extruder (polyetheretherketone (PEEK), length to diameter ratio 4.575, root diameter 0.03 m), or as a concentric-cylinder Couette device (PEEK, length 0.156 m, inner cylinder diameter 0.03 m). A second channel in the sample inlet allows two streams of fluid to be pumped simultaneously through the system for mixing. The shaft rotation speed can be set between 5 and 1200 revolutions per minute (rpm); the barrel and sample feeder can be separately thermostatted to +/-0.2degreesC in the range of -10 to +60degreesC via coolant jacket systems; samples with viscosity up to 10 Pa s can be pumped at rates up to 361 h(-1). This enables studies to be conducted with the system configured as a Couette device to provide knowledge of the rheological properties of complex fluids before more complicated studies of their flow and mixing with the system configured as a single-screw extruder. Bench and MRI measurements have been carried out to test the thennostat function of the system. The bench tests showed that the internal volume of the device reached thermal equilibrium after 1 h of running and could be maintained at constant temperature (within 0.2degreesC for periods of over 6 h. The MRI tests were conducted with the device configured in a Couette geometry for measurements of the flow velocities of pure glycerol and 1% aqueous sodium carboxymethylcellulose (CMC) in the range of 10-60degreesC, and at various rotation speeds. Results showed that although the azimuthal velocity distributions versus the radius (v(r)) were independent of temperature for glycerol, there was strong temperature dependence for the CMC solution. On the latter the power-law index (n) from MRI data agreed well with the literature values for the same concentrations and temperatures, and showed n values increasing with increase of temperature; this demonstrated that the temperature control was reliable. However, the n values increased with decrease of the average shear rate; this is in contrast to the results from conventional rheometry, for which the n value is constant, regardless of shear rates at each temperature. This was attributed to the differences between conventional, very-narrow-gap Couette rheometers and the MRI, wide-gap device, and showed the advantage of using MRI for wide-gap concentric-cylinder rheometry. Couette flow of an aqueous suspension of 35% (w/w) tapioca starch, made of 37% (w/w) sucrose and 1% (w/w) CMC solutions, was measured at 20degreesC and a rotation speed of 20 rpm. This showed that wall slip developed with time, especially at the inner wall of the rotating cylinder. Velocity measurements in three directions were made for the flow of 1% CMC solution at 22degreesC through the single-screw extruder using the pulsed-field gradient spin-echo imaging sequence for two-dimensional transverse or coronal images. Two-dimensional MRI for mixing of fluids (1% CMC) in the single-screw extruder at 22degreesC is also demonstrated.