A versatile single-screw-extruder system designed for magnetic resonance imagingmeasurements

Abstract

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-cylinderCouette device (PEEK, length 0.156 m, inner cylinder diameter 0.03 m). A secondchannel in the sample inlet allows two streams of fluid to be pumpedsimultaneously through the system for mixing. The shaft rotation speed can be setbetween 5 and 1200 revolutions per minute (rpm); the barrel and sample feedercan be separately thermostatted to ±0.2 °C in the range of−10 to +60 °Cvia coolant jacket systems; samples with viscosity up to 10 Pa scan be pumped at rates up to 36 l h−1.This enables studies to be conducted with the system configured as a Couettedevice to provide knowledge of the rheological properties of complex fluids beforemore complicated studies of their flow and mixing with the system configured as asingle-screw extruder.Bench and MRI measurements have been carried out to testthe thermostat function of the system. The bench tests showed that the internalvolume of the device reached thermal equilibrium after 1 h of running and could bemaintained at constant temperature (within ±0.2 °C) forperiods of over 6 h. The MRI tests were conducted with the device configured in a Couettegeometry for measurements of the flow velocities of pure glycerol and 1% aqueoussodium carboxymethylcellulose (CMC) in the range of 10–60 °C, and atvarious rotation speeds. Results showed that although the azimuthal velocity distributions versusthe radius (v(r))were independent of temperature for glycerol, there was strong temperaturedependence for the CMC solution. On the latter the power-law index(n) from MRIdata agreed well with the literature values for the same concentrations and temperatures,and showed nvalues increasing with increase of temperature; this demonstratedthat the temperature control was reliable. However, thenvalues increased with decrease of the average shear rate; this is incontrast to the results from conventional rheometry, for which thenvalue is constant, regardless of shear rates at each temperature. Thiswas attributed to the differences between conventional, very-narrow-gapCouette rheometers and the MRI, wide-gap device, and showed theadvantage 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 20 °Cand 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 threedirections were made for the flow of 1% CMC solution at 22 °Cthrough the single-screw extruder using the pulsed-field gradient spin-echo imagingsequence for two-dimensional transverse or coronal images. Two-dimensional MRI formixing of fluids (1% CMC) in the single-screw extruder at 22 °Cis also demonstrated.