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Hydrogels are widely used in myriad applications including those in thebiomedical and personal care fields. It is generally the rheology, i.e., theflow and deformation properties, of hydrogels that define theirfunctionalities. However, the ubiquitous viscoplasticity and associated wallslip behavior of hydrogels handicap the accurate characterization of theirrheological material functions. Here parallel-disk and cone-and-plateviscometers were used to characterize the shear viscosity and wall slipbehavior of a crosslinked poly(acrylic acid), PAA, hydrogel (carbomer,specifically Carbopol). It is demonstrated that parallel-disk viscometry, i.e.,the steady torsional flow in between two parallel disks, but not thecone-and-plate flow can be used to determine the yield stress and otherparameters of viscoplastic constitutive equations and wall slip behaviorunambiguously. Gap-dependent data from parallel-disk viscometry can then beused to characterize the other parameters of the shear viscosity and wall slipbehavior of the hydrogel. The accuracies of the parameters of wall slip andHerschel-Bulkley type viscoplastic constitutive equation in representing theflow and deformation behavior of the hydrogel were tested via the comparisonsof the calculated and experimentally determined velocity distributions andtorques. The excellent agreements found reflect the accuracies of theparameters of shear viscosity and wall slip and indicate that the methodologiesdemonstrated here provide the means necessary to understand in detail thesteady flow and deformation behavior of hydrogels. Such a detailedunderstanding of the viscoplastic nature and wall slip behavior of hydrogelscan then be used to design and develop novel hydrogels with a wider range ofapplications in the medical and other industrial areas, and for finding optimumconditions for their processing and manufacturing.

Parallel-disk and cone-and-plate viscometry of a viscoplastic hydrogel with apparent wall slip