Ph. D. Dissertation Defense by Peter Kottke
Monday, June 28, 2004
( Dr. Ward Winer, Chair)
"Rheological Implications of Tension in Liquids"
This research investigates aspects of the effects of tensile stresses in liquids. Specific areas of application include bearing lubrication and polymer processing situations in which liquids may be subjected to hydrostatic tension, or in which large shear stresses are generated.
The primary thrust of this research concerns the development of a criterion for liquid failure, or cavitation, based upon the general state of stress in the liquid. A variable pressure, rotating inner cylinder, Couette viscometer has been designed and used to test a hypothesized cavitation criterion. The criterion, that cavitation will occur when a principle normal stress in a liquid becomes more tensile than some critical stress, is supported by the results of experiments with the viscometer for a Newtonian liquid. This hypothesis leads predictions of cavitation in simple shear flows that are often used to obtain rheological data (shear cavitation). Based upon experimental observation of shear cavitation, a model for cavitation inception from crevice stabilized gas nuclei, and gaseous, vice vaporous, cavitation is hypothesized.
The cavitation inception model is investigated through numerical simulation, primarily using the boundary element method. Only Newtonian liquids are modeled, and, for simulation purposes, the model is reduced to two dimensions and the limit of negligible inertia (Stokes flow) is considered. The model includes contact line dynamics, and diffusion of dissolved gas through the liquid is considered. The numerical simulations provide important information about the probable nature of cavitation nucleation sites as well as conditions for cavitation inception.
There are several implications of shear cavitation on rheological measurements. It can cause apparent shear thinning and thixotropy. Additionally, there is evidence suggesting a possible link between shear cavitation and extrusion defects such as sharkskin defect. A variable pressure capillary tube viscometer was designed and constructed to investigate a hypothesized relationship between shear cavitation and extrusion defects, and results indicate that despite the occasional coincidence of occurrence of cavitation and sharkskin defects, cavitation can not explain the onset of extrusion defects.
If nuclei are removed, then liquids can withstand tension, which, if uniform, is a negative hydrostatic pressure. A falling body viscometer has been constructed which is used to investigate the effect of negative pressures on viscosity. It is found that current pressure viscosity models can be accurately extrapolated to experimentally achievable negative pressures.