Boger Fluids

A Boger fluid is an elastic liquid with a constant viscosity. Because the viscosity is independent of shear rate or nearly so, elastic effects can be separated from viscous effects in viscoelastic flows because the latter effects can be determined with Newtonian fluids. Boger fluids are dilute polymer solutions generally made witha solvent sufficiently viscous that stresses due to elasticity are measurable. This article closely examines rheological characteristics and reviews flows with Boger fluids, flows for which investigators have found the onset and magnitude of elastic effects through the use of these fluids. Constitutive equations for viscoelastic fluids are evaluated through comparisons to experimental data, including the simple Oldroyd-B equation, which is physically sensible but only marginally accurate. A significant benefit of the introduction of Boger fluids has been the clear identification of a flow instability clue to elasticity alone.