Interfacial effects in visco-plastic lubrication flows

Poiseuille flows with yield stress fluids produce an unyielded central plug which can act as a solid conduit surrounding central (core) flows of Newtonian or power law fluids. Effectively, the annular yield stress fluid acts as a lubricant that isolates the core flow from wall friction. Stable flows with a yield stress annular fluid and a Newtonian or power law core fluid are termed viso-plastic lubrication (VPL) flows. This study examined interfacial effects in vertical VPL Poiseuille flows using a carbopol solution as the annular (yield stress) fluid and xanthan (inelastic shear thinning fluid) or polyethlyeneoxide (PEO; an elastic shear thinning fluid) as the core fluids. Experiments with the inelastic core fluid (xanthan) involved introducing stepped (high to low) or pulsed (high to low to high) changes in the core flow to an established stable VPL flow. Step changes produced a "yield front" (narrowing of the core flow or "interfacial radius") that propagated upward at a velocity considerably greater than the velocity of the annular carbopol plug but close to the average velocity of the xanthan core flow following the step change. Pulsed changes in the core flow produced one of three outcomes depending on the magnitude of the flows preceding and following the step change: (1) a stable ("frozen in") deformation in the carbopol/xanthan interface that moved upward at the velocity of the carbopol plug,(2) no persistent deformation of the interface, or (3) a breakdown of the stable VPL flow characterized by extensive mixing of the core and annular flows. Experiments with the elastic core fluid (PEO) involved introducing multiple pulsed changes (high/low/high, high/low/high, ...) in the core flow to an established VPL flow. These pulsed changes typically produced linked multiple diamond shaped stable deformations ("diamond necklace") in the interface that moved upwards at the velocity of the carbopol plug. The frequency and amplitude (maximum radius) of the diamond deformations could be controlled by the timing of pulses and the respective flow rates, but not the diamond shape itself which appears to be a consequence of the complex rheology of the fluids.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat