2,708 research outputs found
Orientation dependent elastic stress concentration at tips of slender objects translating in viscoelastic fluids
Elastic stress concentration at tips of long slender objects moving in
viscoelastic fluids has been observed in numerical simulations, but despite the
prevalence of flagellated motion in complex fluids in many biological
functions, the physics of stress accumulation near tips has not been analyzed.
Here we theoretically investigate elastic stress development at tips of slender
objects by computing the leading order viscoelastic correction to the
equilibrium viscous flow around long cylinders, using the weak-coupling limit.
In this limit nonlinearities in the fluid are retained allowing us to study the
biologically relevant parameter regime of high Weissenberg number. We calculate
a stretch rate from the viscous flow around cylinders to predict when large
elastic stress develops at tips, find thresholds for large stress development
depending on orientation, and calculate greater stress accumulation near tips
of cylinders oriented parallel to motion over perpendicular.Comment: Supplementary information include
Solitary coherent structures in viscoelastic shear flow: computation and mechanism
Starting from stationary bifurcations in Couette-Dean flow, we compute
nontrivial stationary solutions in inertialess viscoelastic circular Couette
flow. These solutions are strongly localized vortex pairs, exist at arbitrarily
large wavelengths, and show hysteresis in the Weissenberg number, similar to
experimentally observed ``diwhirl'' patterns. Based on the computed velocity
and stress fields, we elucidate a heuristic, fully nonlinear mechanism for
these flows. We propose that these localized, fully nonlinear structures
comprise fundamental building blocks for complex spatiotemporal dynamics in the
flow of elastic liquids.Comment: 5 pages text and 4 figures. Submitted to Physical Review Letter
On the relation between viscoelastic and magnetohydrodynamic flows and their instabilities
We demonstrate a close analogy between a viscoelastic medium and an
electrically conducting fluid containing a magnetic field. Specifically, the
dynamics of the Oldroyd-B fluid in the limit of large Deborah number
corresponds to that of a magnetohydrodynamic (MHD) fluid in the limit of large
magnetic Reynolds number. As a definite example of this analogy, we compare the
stability properties of differentially rotating viscoelastic and MHD flows. We
show that there is an instability of the Oldroyd-B fluid that is physically
distinct from both the inertial and elastic instabilities described previously
in the literature, but is directly equivalent to the magnetorotational
instability in MHD. It occurs even when the specific angular momentum increases
outwards, provided that the angular velocity decreases outwards; it derives
from the kinetic energy of the shear flow and does not depend on the curvature
of the streamlines. However, we argue that the elastic instability of
viscoelastic Couette flow has no direct equivalent in MHD.Comment: 21 pages, 3 figures, to be published in J. Fluid Mec
Recommended from our members
The matching of a "one-dimensional" numerical simulation and experiment results for low viscosity Newtonian and non-Newtonian fluids during fast filament stretching and subsequent break-up
Novel Experimentally Observed Phenomena in Soft Matter
Soft materials such as colloidal suspensions, polymer solutions and liquid
crystals are constituted by mesoscopic entities held together by weak forces.
Their mechanical moduli are several orders of magnitude lower than those of
atomic solids. The application of small to moderate stresses to these materials
results in the disruption of their microstructures. The resulting flow is
non-Newtonian and is characterised by features such as shear rate-dependent
viscosities and non-zero normal stresses. This article begins with an
introduction to some unusual flow properties displayed by soft matter.
Experiments that report a spectrum of novel phenomena exhibited by these
materials, such as turbulent drag reduction, elastic turbulence, the formation
of shear bands and the existence of rheological chaos, flow-induced
birefringence and the unusual rheology of soft glassy materials, are reviewed.
The focus then shifts to observations of the liquid-like response of granular
media that have been subjected to external forces. The article concludes with
examples of the patterns that emerge when certain soft materials are vibrated,
or when they are displaced with Newtonian fluids of lower viscosities.Comment: 30 pages, 11 figures, invited review article, supplementary videos
may be obtained from the journal websit
- …