Direct measurement techniques are employed to quantify the kinematics of λ-DNA flows in three microfluidic components with abruptly changing geometries. Flow through these geometries subjects the fluid to large spatial gradients in velocity, leading to viscoelastic effects. The effects on DNA transport in microscale flows are significant because flow behavior may be influenced by elasticity of individual molecules, interactions between molecules, both viscous and elastic forces dominate inertial forces at this length scale, and the fully extended length of the molecule L approaches the characteristic channel length Wc
This study investigates the flow of semi-dilute DNA solutions in a 2:1 planar abrupt micro-contraction, a planar gradual micro-contraction, and a planar 90 degree micro-bend at low Reynolds (Re) numbers and high Weissenberg (Wi) numbers. We thus access high elasticity number flows, El = Wi/Re ~ 10^5, that have been previously unexplored. For flows in the 2:1 abrupt and gradual micro-contraction, video microscopy and streak images reveal highly elastic behavior evidenced by the presence of large, stable vortices symmetric about the centerline in the corners of the abrupt contraction and about the centerline upstream of the channel entrance in the gradual contraction. Micro-particle image velocimetry (μPIV) measurements are used to obtain high resolution, quantitative velocity measurements of the vortex growth. The kinematics of flows of dilute DNA solutions of modest El (El ~ 10^2) in the gradual micro-contraction were also characterized. This DNA concentration is sufficiently low such that the fluid behaves as a Newtonian fluid; the flow fields appear undisturbed by the presence of the DNA over the parameter range probed. An elastic instability is found for flows of semi-dilute DNA solutions in abrupt 90 degree micro-bends. Flow visualization and μPIV experiments indicate that the vortex, which is present in the inside, upstream corner of the bend, grows with increasing Re and Wi.
We believe these are among the first μPIV measurements in a viscoelastic microflow and the first involving DNA solutions. These direct measurements provide a deeper understanding of the underlying physics of macromolecular transport in microfluidic flow which will enable the realization of enhanced designs of lab-on-a-chip systems.
December 31, 2006
Gulati, S. (2006). Effects of Abrupt Changes in Microfluidic Geometry on Complex Biological Fluid Flows. (n.p.): University of California, Berkeley with the University of California, San Francisco.