Gas-assisted displacement of viscoelastic fluids: flow dynamics at the bubble front

The penetration of a long inviscid bubble through ideal elastic fluids in a capillary tube geometry is studied using Particle Tracking Velocimetry (PTV). Experiments are performed to characterize the fluid kinematics around the bubble tip. A plane of polarized light from a 10 mW HeNe laser source is used to illuminate the center of the capillary tube. The recording camera (CCD) and the laser optics are mounted on a hydraulic translating stage to track the penetrating bubble. PTV is used to quantitatively describe the flow field with an emphasis on the tip region. Streamlines and contour plots of constant sheer rates and extension rates are presented for both the Newtonian and Boger fluids in the recirculation and complete bypass flow regimes. The residual coating thickness for Newtonian fluids is a function only of the Capillary number. At a Deborah number of De = (gamma) over dot approximate to 1, the coating thickness deviates from Newtonian behavior. The maximum local shear rates in the flow increased from 0.25 s(-1) at De = 0.023 to 8.0 s(-1) at De = 2.56 and the local extension rates increased from 0.09 s(-1) at De = 0.023 to 4.5 s(-1) at De = 2.56. Since the fluid extensionally thickens at (epsilon) over dot approximate to 1, this suggests that the extensionally thickening response of the Boger fluids has a significant effect on the fractional coverage. Flow field simulations for the Newtonian fluids are conducted using POLYFLOW. A comparison of the results from experiments and simulations is presented in order to validate experimental techniques. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.