Jai A. Pathak *, David J. Ross ** and Kalman D. Migler **
* Polymers Division, MSEL, NIST, Gaithersburg, MD 20899-8544
** Process Measurements Division, CSTL, NIST, Gaithersburg, MD 20899-8362
Abstract
Flow instabilities are well known to occur in macroscopic flows when
elastic fluids flow along curved streamlines. In this work we use flow
visualization to study the mechanism underlying a purely elastic flow instability
for Poiseuille flow in a micro (m)-channel
having a zigzag path (curved streamlines), and quantitatively investigate
its implications for fluid mixing (studied by fluorescence microscopy)
in the m-channel. We find that the instability
enhances mixing over the range of applied flow rates. For Newtonian streams,
mixing occurs by molecular diffusion, and as expected, mixing worsens with
increasing flow rate because of decreasing residence time. However, for
elastic fluid streams, we find substantial enhancement of mixing at sufficiently
high
throughputs, which indicates a strategy to counter the loss of diffusive
mixing at high throughputs by exciting an elastic flow instability. Flow
visualization is done using neutrally buoyant non-Brownian tracer particles
added to the elastic fluids and also to the Newtonian fluids. In the Newtonian
fluids, the tracer particles follow the streamlines. In the elastic fluids,
the particles are radially displaced while flowing around bends
in the zigzag m-channel, revealing the
presence of secondary flow. This radial secondary flow motivates us to
draw an analogy between the instability observed here for the elastic fluids
in the m-channel and the elastic instability
that occurs in systems with curved streamlines, e.g., in the viscoelastic
(non-inertial) Taylor-Couette, Dean and Taylor-Dean instabilities.
Name: Dr. Jai A. Pathak
Division: Polymers (854)
Laboratory: MSEL
Room and Building Address: A 213 Bldg. 224
Mail Stop: 8544
Telephone: (301) 975-4295
FAX: (301) 975-4924
email: Jai.Pathak@nist.gov
Sigma Xi member: Yes (since 2003)
Category: Materials Science