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This article describes the technical performance and initial results for a new, two-piston, "multipass rheometer." Fluid is contained within a capillary by two servo-hydraulically controlled pistons. Both steady flow and oscillatory data can be obtained from the device with mean pressure as an independent adjustable variable. Steady and oscillatory data are presented for a silicone oil and a viscoelastic solution and certain data are compared with results obtained from a Rheometrics RDS-II mechanical spectrometer or with literature data. The results show that the multipass rheometer is capable of extending the experimental ranges of many conventional rheometers, particularly in terms of pressure control, high shear rates, and multiple, successive flow measurements.

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The rheology and microstructure of aqueous dispersions containing Mg-A1 layered double hydroxides (LDHs) are considered. The thickening and gelling of such dispersions can be controlled by the addition of electrolyte or, within a certain composition range, by the application of shear. In addition to electrolyte and LDH concentration, the degree of thickening is shown to depend on both the primary crystal size of the LDH and the crystallinity of the material. XRD, particle size measurements, and direct observation by optical and electron microscopy were used to probe the microstructure of unthickened and thickened dispersions. Thickening appears to result from the interaction of predominantly rod-shaped aggregates formed by the face-to-face association of the primary LDH crystals. No interlayer swelling or delamination of the LDH crystals was observed.