8 research outputs found
Simulations of extensional flow in microrheometric devices
We present a detailed numerical study of the flow of a Newtonian fluid through microrheometric devices featuring a sudden contraction–expansion. This flow configuration is typically used to generate extensional deformations and high strain rates. The excess pressure drop resulting from the converging and diverging flow is an important dynamic measure to quantify if the device is intended to be used as a microfluidic extensional rheometer. To explore this idea, we examine the effect of the contraction length, aspect ratio and Reynolds number on the flow kinematics and resulting pressure field. Analysis of the computed velocity and pressure fields show that, for typical experimental conditions used in microfluidic devices, the steady flow is highly three-dimensional with open spiraling vortical structures in the stagnant corner regions. The numerical simulations of the local kinematics and global pressure drop are in good agreement with experimental results. The device aspect ratio is shown to have a strong impact on the flow and consequently on the excess pressure drop, which is quantified in terms of the dimensionless Couette and Bagley correction factors. We suggest an approach for calculating the Bagley correction which may be especially appropriate for planar microchannels
The Inertio-Elastic Planar Entry Flow of Low-Viscosity Elastic Fluids in Micro-fabricated Geometries
The non-Newtonian flow of dilute aqueous polyethylene oxide (PEO) solutions through
microfabricated planar abrupt contraction-expansions is investigated. 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 in the contraction throat lead to significant extensional flow effects even with dilute polymer solutions having time constants on the order of milliseconds. The dimensionless
extra pressure drop across the contraction increases by more than 200% and is
accompanied by significant upstream vortex growth. Streak photography and videomicroscopy
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, which is independent of the
flow kinematics and depends only on the fluid rheology and the characteristic size of the
device.NS
Worm-like surfactant solutions, flow induced gelation and effective slip in microchannels
The unique rheological properties of worm-like surfactant solutions lead to a flow-induced phase transition from a “liquid-like” to “gel-like” phase when the fluid is subjected to shearing and extensional flows in microdevices. We investigate the complex flow behaviour that occurs upstream of a 4:1 planar contraction. Even during slow flows (Re < 0.009), we observe the formation of an extended micellar network, resulting in shear bands, large elastic vortices and extended upstream jetting consisting of an inviscid core bounded by stagnant fluid. Such systems provide great potential for generating shear-free transport zones even in straight channels within microfluidic devices
Extensional flow of dilute polymer solutions through microfabricated hyperbolic contractions
Extensional rheometry on a chip : Flows of dilute polymer solutions in microfluidic contractions
The importance of downstream events in microfluidic viscoelastic entry flows: Consequences of increasing the constriction length
Extensional flows of polymer solutions in microfluidic converging/diverging geometries
The effects of fluid elasticity in the flow of non-Newtonian fluids in microfluidic converging/diverging geometries are investigated. We investigate the structure and dynamics of inertio-elastic flow instabilities and elastic corner vortices which develop upstream of the contraction plane, and explore their dependence on the relative magnitudes of inertia and elastic stress generated by the high deformation rates in the contraction geometry. The results show that the shape, size and evolution of these flow structures varies with the elasticity number, which is independent of the flow kinematics and is only dependent on fluid properties (viscosity, density and polymer relaxation time) and the characteristic size of the channel