The fluid mechanics of gels and of highly viscous fluids: Investigation of slip phenomena

Traditionally, the assumption of a no-slip boundary condition is applied to most fluid flow analyses. For high viscosity fluids and for gelled systems which are commonly encountered in the food, petroleum, and plastics industries, however, this boundary condition may be expected to be invalid.; It has been suggested by Ramamurthy (1986) that any fluid will eventually slip as its viscosity increases progressively, providing only physical forces of adhesion hold the fluid to the wall. To determine the validity of this statement and to characterize the slip once it is initiated, two types of polymers were subjected to capillary flow experiment. In the first part of this study, the flow characteristics and subsequently the slip velocities of chemically cross-linked Hydroxypropyl guar (HPG) gels were quantified. Secondly, for a study of slip at high stress levels with minimal complications due to viscous heating and viscoelasticity, a series of highly viscous polydimethylsiloxanes (silicone oils) was chosen.; Being soft solids, the HPG gels slip easily under shear, thus they are conveniently used in the measurement of slip velocity. These gels provide a good example of the behavior of the materials in which internal resistance to flow is much greater than the interfacial friction. The slip mechanism of the gels is observed to be an oscillatory one, alternating between adhesion and slip, at low stress levels, and close to total slip beyond a stress of about 7000 dynes/cm{dollar}sp2{dollar}. The slip velocity is a function of stress at the wall at least over the stress range studied.; In contrast, silicone oils are liquid and the critical shear stresses at which these fluids start to slip are of the order of 10{dollar}sp6{dollar} dynes/cm{dollar}sp2{dollar}. The precise stress level of a particular fluid at the onset of slip increases as the viscosity decreases. The slip induced irregular extrudates of the silicone liquids resemble those observed in many polymer melt fractures. In addition, numerical simulations of Poiseuille flows with steady slip at the boundary were generated from Polyflow, a commercial finite element method. For the 5000 poise silicone oil, the predictions of the numerical program follow the trend of the experimental results very closely.