  Hydrogels are widely used in myriad applications including those in the
biomedical and personal care fields. It is generally the rheology, i.e., the
flow and deformation properties, of hydrogels that define their
functionalities. However, the ubiquitous viscoplasticity and associated wall
slip behavior of hydrogels handicap the accurate characterization of their
rheological material functions. Here parallel-disk and cone-and-plate
viscometers were used to characterize the shear viscosity and wall slip
behavior 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 the
cone-and-plate flow can be used to determine the yield stress and other
parameters of viscoplastic constitutive equations and wall slip behavior
unambiguously. Gap-dependent data from parallel-disk viscometry can then be
used to characterize the other parameters of the shear viscosity and wall slip
behavior of the hydrogel. The accuracies of the parameters of wall slip and
Herschel-Bulkley type viscoplastic constitutive equation in representing the
flow and deformation behavior of the hydrogel were tested via the comparisons
of the calculated and experimentally determined velocity distributions and
torques. The excellent agreements found reflect the accuracies of the
parameters of shear viscosity and wall slip and indicate that the methodologies
demonstrated here provide the means necessary to understand in detail the
steady flow and deformation behavior of hydrogels. Such a detailed
understanding of the viscoplastic nature and wall slip behavior of hydrogels
can then be used to design and develop novel hydrogels with a wider range of
applications in the medical and other industrial areas, and for finding optimum
conditions for their processing and manufacturing.

Fluid Dynamics;Soft Condensed Matter