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We examine the effect of polymer concentration and chain architecture on the steady state displacement of polymeric fluids by air in between two infinitely long closely spaced parallel plates, i.e., Hele-Shaw flow. A stabilized finite element method coupled with a pseudosolid domain mapping technique is used for carrying out the computations. The constitutive equations employed in this study are the Finitely Extensible Nonlinear Elastic-Chilcott Rallison ͑FENE-CR͒ and the Finitely Extensible Nonlinear Elastic-Peterlin ͑FENE-P͒ models for dilute solutions, the Giesekus constitutive equation for dilute, semidilute and concentrated solutions, and the Extended Pom-Pom ͑XPP͒ constitutive equation for linear and branched polymeric melts. Our study indicates the presence of a recirculation flow at low Ca and a bypass flow at high Ca irrespective of polymer concentration and chain architecture. We show that the interfacial dynamics in both the recirculation and the bypass flow depend on extensional hardening and shear thinning characteristics of the fluids. In the recirculation flow, we observe the formation of normal elastic stress boundary layers in the capillary transition region, an accompanying increase in the film thickness and a compression of the bubble in the capillary transition region, at moderate Wi. In the bypass flow, in addition to the elastic stress boundary layer in the capillary transition region, an additional stress boundary layer is observed at the tip of the bubble. The amount of film thickening, the magnitude of the stress in the stress boundary layer and the amount of bubble compression are largest for the most extensional hardening fluids and reduce with decreasing extensional hardening and increasing shear thinning. We show that the film thickness is determined by two competing forces, i.e., normal stress gradients in the flow direction, in the capillary transition region ͑recirculation flow͒ and the tip region ͑bypass flow͒ and shear stress gradients a͒

We have used a quartz crystal microbalance (QCM) to study the solution-phase adsorption of the perfluoropolyether lubricants Fomblin ZDOL and Demnum SA from perfluorohexane and perfluorobutylmethyl ether onto model carbon-coated hard-disk surfaces. The validity of the QCM results and the applicability of the well-known Sauerbrey equation have been verified through combined QCM and surface plasmon resonance experiments. The adsorption isotherms are relatively simple and exhibit a Langmuirlike behavior. However, the plateau region is sometimes ill-defined, and the adsorption process is strongly dependent on the history of the system. The adsorbed mass at a particular concentration shows a pronounced dependence on the equilibration time between successive concentration changes, even though the adsorption kinetics appears to have equilibrated. In the first few seconds, the adsorption process is diffusion limited, with the adsorbed mass quickly increasing to a maximum before decaying to the equilibrium value. To interpret these results, we present an adsorption model involving a site-specific adsorption process, fast initial adsorption, and surface reorganization that is able to qualitatively reproduce the observed kinetic and equilibrium behavior.

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