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SQUEEZING FLOWS OF VISCOELASTIC FLUIDS
The isothermal squeezing flow of viscoelastic fluids under constant squeezing speed is analyzed using lubrication approximations and a memory integral rheological model. The kinematics are described by a power law velocity profile. Wagner's model was chosen for its success in analyzing transient viscometric flows. Squeezing flow experiments were performed on well characterized aqueous solutions of polyacrylamide. The squeezing flow apparatus is designed around the INSTRON machine. The crosshead provided a constant downward motion to a circular disk attached to it, while the bottom plate contained a cavity where the test fluid was squeezed. The squeezing force was measured by a sensitive load cell attached to the top disk.^ Experiments were conducted for the Newtonian and viscoelastic fluids at different squeezing speeds and initial gap thickness. The Newtonian data was quite accurately described by Stefan's equation, suggesting the validity of the experimental design. In the case of viscoelastic fluids, the experimental squeezing force overshot the purely viscous predictions of the power law model. This behavior resembles the stress overshoot observed during onset of constant shear rate flow. The mathematical model presented in this thesis was able to describe the experimental squeezing force for viscoelastic fluids quite adequately, especially in the low to moderate shear rate region. The model was also able to predict the time at which the normalized force (normalized with respect to the power law predictions of that instant) reached its maximum value. The success of the model was limited by the range of validity of Wagner's rheological model, especially at higher shear rates. In agreement with earlier work, these experiments suggest an increased load bearing capacity of polymer loaded lubricants, due to stress overshoot. The range of shear rates that can be studied with this apparatus could be increased by using a more sensitive displacement transducer.^ A qualitative understanding of the dynamics of viscoelastic squeezing flow with slip at the wall, was obtained by analyzing the flow using Wagner's rheological model and Newtonian kinematics. Even for viscoelastic fluids, boundary slip reduces the squeezing force substantially, suggesting that the introduction of slip, due to hot wall temperatures, may affect adversely the load bearing capacity of lubricants.^ The growth of bubbles in a viscoelastic medium presents a situation where there is squeezing of interstitial fluid with slip at the bubble-fluid interface. This flow was analyzed by numerical solution of a modified Rayleigh equation. The analysis suggests that elasticity enhances the growth of the bubbles during both isolated growth and strong interaction.^ One important area of future work should involve a detailed study of the observed stress relaxation of viscoelastic fluids after cessation of squeezing flow. This would have ramifications in characterizing the elastic strain recovery of polymer melts. The finite element analysis of the hydrodynamics of viscoelastic squeezing flow should also be pursued so that the assumptions made in the present analysis can be relaxed. In addition, the method could be used to describe the shape of the bubble interfaces during bubble-bubble interaction. ^
PRADEEP PANDURANG SHIRODKAR,
"SQUEEZING FLOWS OF VISCOELASTIC FLUIDS"
(January 1, 1981).
Doctoral Dissertations Available from Proquest.