8,219 research outputs found
Loss of solutions in shear banding fluids in shear banding fluids driven by second normal stress differences
Edge fracture occurs frequently in non-Newtonian fluids. A similar
instability has often been reported at the free surface of fluids undergoing
shear banding, and leads to expulsion of the sample. In this paper the
distortion of the free surface of such a shear banding fluid is calculated by
balancing the surface tension against the second normal stresses induced in the
two shear bands, and simultaneously requiring a continuous and smooth meniscus.
We show that wormlike micelles typically retain meniscus integrity when shear
banding, but in some cases can lose integrity for a range of average applied
shear rates during which one expects shear banding. This meniscus fracture
would lead to ejection of the sample as the shear banding region is swept
through. We further show that entangled polymer solutions are expected to
display a propensity for fracture, because of their much larger second normal
stresses. These calculations are consistent with available data in the
literature. We also estimate the meniscus distortion of a three band
configuration, as has been observed in some wormlike micellar solutions in a
cone and plate geometry.Comment: 23 pages, to be published in Journal of Rheolog
Multiscale modeling and simulation for polymer melt flows between parallel plates
The flow behaviors of polymer melt composed of short chains with ten beads
between parallel plates are simulated by using a hybrid method of molecular
dynamics and computational fluid dynamics. Three problems are solved: creep
motion under a constant shear stress and its recovery motion after removing the
stress, pressure-driven flows, and the flows in rapidly oscillating plates. In
the creep/recovery problem, the delayed elastic deformation in the creep motion
and evident elastic behavior in the recovery motion are demonstrated. The
velocity profiles of the melt in pressure-driven flows are quite different from
those of Newtonian fluid due to shear thinning. Velocity gradients of the melt
become steeper near the plates and flatter at the middle between the plates as
the pressure gradient increases and the temperature decreases. In the rapidly
oscillating plates, the viscous boundary layer of the melt is much thinner than
that of Newtonian fluid due to the shear thinning of the melt. Three different
rheological regimes, i.e., the viscous fluid, visco-elastic liquid, and
visco-elastic solid regimes, form over the oscillating plate according to the
local Deborah numbers. The melt behaves as a viscous fluid in a region for
, and the crossover between the liquid-like and
solid-like regime takes place around (where
is the angular frequency of the plate and and
are Rouse and relaxation time, respectively).Comment: 13pages, 12figure
Dynamics of Strongly Deformed Polymers in Solution
Bead spring models for polymers in solution are nonlinear if either the
finite extensibility of the polymer, excluded volume effects or hydrodynamic
interactions between polymer segments are taken into account. For such models
we use a powerful method for the determination of the complete relaxation
spectrum of fluctuations at {\it steady state}. In general, the spectrum and
modes differ significantly from those of the linear Rouse model. For a tethered
polymer in uniform flow the differences are mainly caused by an inhomogeneous
distribution of tension along the chain and are most pronounced due to the
finite chain extensibility. Beyond the dynamics of steady state fluctuations we
also investigate the nonlinear response of the polymer to a {\em large sudden
change} in the flow. This response exhibits several distinct regimes with
characteristic decay laws and shows features which are beyond the scope of
single mode theories such as the dumbbell model.Comment: 7 pages, 3 figure
Self-stabilised fractality of sea-coasts through damped erosion
Erosion of rocky coasts spontaneously creates irregular seashores. But the
geometrical irregularity, in turn, damps the sea-waves, decreasing the average
wave amplitude. There may then exist a mutual self-stabilisation of the waves
amplitude together with the irregular morphology of the coast. A simple model
of such stabilisation is studied. It leads, through a complex dynamics of the
earth-sea interface, to the appearance of a stationary fractal seacoast with
dimension close to 4/3. Fractal geometry plays here the role of a morphological
attractor directly related to percolation geometry.Comment: 4 pages, 5 figure
OpenEssayist: a supply and demand learning analytics tool for drafting academic essays
This paper focuses on the use of a natural language analytics engine to provide feedback to students when preparing an essay for summative assessment. OpenEssayist is a real-time learning analytics tool, which operates through the combination of a linguistic analysis engine that processes the text in the essay, and a web application that uses the output of the linguistic analysis engine to generate the feedback. We outline the system itself and present analysis of observed patterns of activity as a cohort of students engaged with the system for their module assignments. We report a significant positive correlation between the number of drafts submitted to the system and the grades awarded for the first assignment. We can also report that this cohort of students gained significantly higher overall grades than the students in the previous cohort, who had no access to OpenEssayist. As a system that is content free, OpenEssayist can be used to support students working in any domain that requires the writing of essays
Beating patterns of filaments in viscoelastic fluids
Many swimming microorganisms, such as bacteria and sperm, use flexible
flagella to move through viscoelastic media in their natural environments. In
this paper we address the effects a viscoelastic fluid has on the motion and
beating patterns of elastic filaments. We treat both a passive filament which
is actuated at one end, and an active filament with bending forces arising from
internal motors distributed along its length. We describe how viscoelasticity
modifies the hydrodynamic forces exerted on the filaments, and how these
modified forces affect the beating patterns. We show how high viscosity of
purely viscous or viscoelastic solutions can lead to the experimentally
observed beating patterns of sperm flagella, in which motion is concentrated at
the distal end of the flagella
Taxing the Informal Economy: The Current State of Knowledge and Agendas for Future Research
This paper reviews the literature on taxation of the informal economy, taking stock of key debates
and drawing attention to recent innovations. Conventionally, the debate on whether to tax has frequently focused
on the limited revenue potential, high cost of collection, and potentially adverse impact on small firms. Recent
arguments have increasingly emphasised the more indirect benefits of informal taxation in relation to economic
growth, broader tax compliance, and governance. More research is needed, we argue, into the relevant costs and
benefits for all, including quasi-voluntary compliance, political and administrative incentives for reform, and
citizen-state bargaining over taxation
Finite-temperature simulations of the scissors mode in Bose-Einstein condensed gases
The dynamics of a trapped Bose-condensed gas at finite temperatures is
described by a generalized Gross-Pitaevskii equation for the condensate order
parameter and a semi-classical kinetic equation for the thermal cloud, solved
using -body simulations. The two components are coupled by mean fields as
well as collisional processes that transfer atoms between the two. We use this
scheme to investigate scissors modes in anisotropic traps as a function of
temperature. Frequency shifts and damping rates of the condensate mode are
extracted, and are found to be in good agreement with recent experiments.Comment: 4 pages, 3 figure
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