An associative polymer-surfactant system has been used to observe the effects of chain conformation in the entry flow through a microfabricated planar 16:116 contraction-expansion geometry. The well-studied system of the flexible polymer poly(ethylene oxide) (PEO) and anionic surfactant sodium dodecyl sulfate (SDS) was used. Dilute polymer solutions with increasing SDS concentration were characterized in steady and dynamic shear, as well as capillary breakup extensional rheology. Based on this characterization, the primary quantitative difference is an increase in zero-shear viscosity as a result of the PEO chain expansion brought on by association of SDS surfactant micelles. However, these quantitatively similar solutions were observed to exhibit much more qualitatively different flow patterns via fluorescent streak imaging in the entry flow. In contrast to previous work on PEO solutions, the PEO-SDS systems were observed to transition to a steady viscoelastic flow regime characterized by stable lip vortices at much lower elasticity and Weissenberg numbers. The resulting insight gained regarding the utility of microfluidic flows in elucidating effects of subtle conformational changes further illustrates the potential for using microfabricated devices as rheometric tools for measuring the properties of dilute and weakly viscoelastic fluids. (C) 2009 Elsevier B.V. All rights reserved.