Extensional
Rheometry on a Chip: Flows of Dilute Polymer
Solutions in Microfluidic Geometries
Hatsopoulos Microfluids Laboratory,
Department of
Mechanical Engineering, M.I.T.,
Applications as diverse as DNA separation and ink-jet printing involve microfluidic geometries which generate strong elongational flows of dilute polymer solutions. In this work, we investigate the non-Newtonian flow of dilute aqueous polyethylene oxide (PEO) solutions through microfabricated planar abrupt contraction-expansions. The contraction geometries are fabricated from a high-resolution chrome mask and cross-linked PDMS gels using the tools of soft-lithography. The small length scales and high deformation rates near the contraction plane lead to significant extensional flow effects even with dilute polymer solutions having time constants on the order of milliseconds. Conventional rheological characterization of such fluids even in simple homogeneous flows is challenging because of the need to generate large deformation rates; under such conditions the low levels of viscoelasticity can easily be swamped by inertial effects. To determine the relaxation time we therefore use capillary break-up rheometry and high-speed video imaging. The measured viscometric properties can then be used to quantify the dynamics of the extensional flows arising in microfluidic devices such as planar contractions. Our ultimate goal is to construct a microfluidic-based extensional rheometer on-a-chip. Measurements show that the dimensionless extra pressure drop associated with non-Newtonian flow across the contraction plane increases by more than 200% and is accompanied by significant upstream vortex growth. Streak photography and video-microscopy using epifluorescent particles shows that the flow ultimately becomes unstable and three-dimensional. The moderate Reynolds numbers (0.03 ≤ Re ≤ 44) associated with these high Deborah number (0 ≤ De ≤ 600) microfluidic flows results in the exploration of new regions of the Re-De parameter space in which the effects of both elasticity and inertia can be observed. Understanding such interactions will be increasingly important in microfluidic applications involving complex fluids and can best be interpreted in terms of the Elasticity number, El = De/Re.