Shear viscosity and wall slip behavior of a viscoplastic hydrogel

The coupled apparent slip and viscoplastic behavior of a hydrogel consisting of 0.2 wt. % aqueous solution of poly(acrylic acid) was analyzed employing steady torsional and circular tube (capillary) flows. Transparent disks and capillaries fabricated out of borosilicate glass were used to allow velocity measurements. The steady torsional flow of the hydrogel was dominated by wall slip which gave rise to plug flow over the apparent shear rate range of 0.1 to 1 s^-1, in agreement with the plug flow observed in the capillary under similar shear stresses. The transition from plug flow provided the yield stress of the hydrogel, which was found to be consistent with velocity data collected over the 0.1-200 s^-1 apparent shear rate range of steady torsional and capillary flows. The availability of both pressure drop versus flow rate and wall slip velocity data enabled the validation of correction procedures proposed earlier for the determination of the slip-corrected wall shear rate [Yilmazer and Kalyon, "Dilatancy of concentrated suspensions with Newtonian matrices," Polym. Compos. 12, 226-232 (1991); D. Kalyon, "Apparent slip and viscoplasticity of concentrated suspensions," J. Rheol. 49, 621-640 (2005)] and the determination of the shear viscosity parameters. The apparent slip layer thicknesses were found to be consistent with the elastohydrodynamic mechanism proposed by Martin et al. ["Wetting transitions at soft, sliding interfaces," Phys. Rev. E 65, 031605 (2002)] and Meeker et al. ["Slip and flow in pastes of soft particles: Direct observation and rheology," J. Rheol. 48, 1295-1320 (2004)] for steady torsional flow but not for capillary flow, emphasizing the role the applied pressure plays in shaping the apparent slip behavior.