Levich Institute Seminar Announcement, 08/31/2004

Tuesday, 08/31/2004
2:00 PM
Steinman Hall, Room #312
(Chemical Engineering Conference Room)

Professor Gareth McKinley
Massachusetts Institute of Technology
Department of Mechanical Engineering

"Extensional Flows of Dilute Polymer Solutions in Microfluidic Geometries "


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. Measurements show that the dimensionless extra pressure drop 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.


Gareth McKinley obtained his Bachelors and Masters degrees from Downing College at the University of Cambridge. His thesis work was performed in the Polymer Science and Technology (PPST) at M.I.T. with Bob Brown and Bob Armstrong and focused on experimental studies of nonlinear dynamics in viscoelastic fluid systems. From 1991 to 1997 he held teaching positions as an Assistant Professor and then as an Associate Professor of Natural Sciences in the Division of Engineering & Applied Sciences at Harvard University. In 1994 he won the Annual Award of the British Society of Rheology, and in September 1995 he was awarded a National Science Foundation Presidential Faculty Fellowship at the White House. In 1996 he was the Rosenbaum Visiting Fellow at the Isaac Newton Institute for the Mathematical Sciences at Cambridge University and in January 1998 he returned to M.I.T. as Professor of Mechanical Engineering. In 2002 he was a Visiting Professor at Monash University and a Miegunyah Fellow at the University of Melbourne. He is currently director of the Hatsopoulos Microfluids Laboratory and Joint leader of the Processing Team within the Institute for Soldier Nanotechnology (ISN) at MIT. He is also the Executive Editor of J. Non-Newt. Fluid Mechanics.


Current research interests center around extensional rheology and microrheology of complex fluids, energy dissipation mechanisms in field responsive fluids, polymer nanocomposites, nanowetting and contact line dynamics.