701 research outputs found
An analytical analysis of vesicle tumbling under a shear flow
Vesicles under a shear flow exhibit a tank-treading motion of their membrane,
while their long axis points with an angle < 45 degrees with respect to the
shear stress if the viscosity contrast between the interior and the exterior is
not large enough. Above a certain viscosity contrast, the vesicle undergoes a
tumbling bifurcation, a bifurcation which is known for red blood cells. We have
recently presented the full numerical analysis of this transition. In this
paper, we introduce an analytical model that has the advantage of being both
simple enough and capturing the essential features found numerically. The model
is based on general considerations and does not resort to the explicit
computation of the full hydrodynamic field inside and outside the vesicle.Comment: 19 pages, 9 figures, to be published in Phys. Rev.
Influence of shear flow on vesicles near a wall: a numerical study
We describe the dynamics of three-dimensional fluid vesicles in steady shear
flow in the vicinity of a wall. This is analyzed numerically at low Reynolds
numbers using a boundary element method. The area-incompressible vesicle
exhibits bending elasticity. Forces due to adhesion or gravity oppose the
hydrodynamic lift force driving the vesicle away from a wall. We investigate
three cases. First, a neutrally buoyant vesicle is placed in the vicinity of a
wall which acts only as a geometrical constraint. We find that the lift
velocity is linearly proportional to shear rate and decreases with increasing
distance between the vesicle and the wall. Second, with a vesicle filled with a
denser fluid, we find a stationary hovering state. We present an estimate of
the viscous lift force which seems to agree with recent experiments of Lorz et
al. [Europhys. Lett., vol. 51, 468 (2000)]. Third, if the wall exerts an
additional adhesive force, we investigate the dynamical unbinding transition
which occurs at an adhesion strength linearly proportional to the shear rate.Comment: 17 pages (incl. 10 figures), RevTeX (figures in PostScript
Can we avoid high coupling?
It is considered good software design practice to organize source code into modules and to favour within-module connections (cohesion) over between-module connections (coupling), leading to the oft-repeated maxim "low coupling/high cohesion". Prior research into network theory and its application to software systems has found evidence that many important properties in real software systems exhibit approximately scale-free structure, including coupling; researchers have claimed that such scale-free structures are ubiquitous. This implies that high coupling must be unavoidable, statistically speaking, apparently contradicting standard ideas about software structure. We present a model that leads to the simple predictions that approximately scale-free structures ought to arise both for between-module connectivity and overall connectivity, and not as the result of poor design or optimization shortcuts. These predictions are borne out by our large-scale empirical study. Hence we conclude that high coupling is not avoidable--and that this is in fact quite reasonable
Transition to an Insulating Phase Induced by Attractive Interactions in the Disordered Three-Dimensional Hubbard Model
We study numerically the interplay of disorder and attractive interactions
for spin-1/2 fermions in the three-dimensional Hubbard model. The results
obtained by projector quantum Monte Carlo simulations show that at moderate
disorder, increasing the attractive interaction leads to a transition from
delocalized superconducting states to the insulating phase of localized pairs.
This transition takes place well within the metallic phase of the
single-particle Anderson model.Comment: revtex, 4 pages, 3 figure
What makes implementation intentions (in)effective for physical activity among older adults?
Objectives: For most populations, implementation intentions (IIs) facilitate physical activity (PA). However, for older adults, previous studies found mixed evidence for the effectiveness of this behaviour change technique (BCT). To examine which characteristics of IIs predict successful enactment, the content of older participantsâ IIs formed within a self-regulatory intervention to prompt PA was analysed. Design: A sample of N = 126 German speaking adults aged 64 and older formed up to six IIs for PA and reported their enactment five weeks later. Methods: Controlling for age and sex, multilevel models tested associations between characteristics of IIs (e.g., chronological rank of II, hetero- and homogeneity, specificity, presence of certain cues) and enactment. Results: Significantly related to enactment were: the chronological rank of an II (first IIs superior to last IIs), greater heterogeneity in activities, greater specificity of when-cues, and greater use of pre-existing routines. Conclusions: Participants were more likely to enact their IIs five weeks later if they planned different (heterogeneous) activities, created IIs with more specific when-cues (e.g., on Monday at 9 am), and in particular a routine (e.g., after breakfast). They also enacted the first three IIs (chronological rank of II) more often than the last three IIs. Future experimental studies should test whether providing instructions to create IIs based on the above significant characteristics lead to more effective health behaviour change among older adults
Effective swimming strategies in low Reynolds number flows
The optimal strategy for a microscopic swimmer to migrate across a linear
shear flow is discussed. The two cases, in which the swimmer is located at
large distance, and in the proximity of a solid wall, are taken into account.
It is shown that migration can be achieved by means of a combination of sailing
through the flow and swimming, where the swimming strokes are induced by the
external flow without need of internal energy sources or external drives. The
structural dynamics required for the swimmer to move in the desired direction
is discussed and two simple models, based respectively on the presence of an
elastic structure, and on an orientation dependent friction, to control the
deformations induced by the external flow, are analyzed. In all cases, the
deformation sequence is a generalization of the tank-treading motion regimes
observed in vesicles in shear flows. Analytic expressions for the migration
velocity as a function of the deformation pattern and amplitude are provided.
The effects of thermal fluctuations on propulsion have been discussed and the
possibility that noise be exploited to overcome the limitations imposed on the
microswimmer by the scallop theorem have been discussed.Comment: 14 pages, 5 figure
A Mathematical Model of Liver Cell Aggregation In Vitro
The behavior of mammalian cells within three-dimensional structures is an area of intense biological research and underpins the efforts of tissue engineers to regenerate human tissues for clinical applications. In the particular case of hepatocytes (liver cells), the formation of spheroidal multicellular aggregates has been shown to improve cell viability and functionality compared to traditional monolayer culture techniques. We propose a simple mathematical model for the early stages of this aggregation process, when cell clusters form on the surface of the extracellular matrix (ECM) layer on which they are seeded. We focus on interactions between the cells and the viscoelastic ECM substrate. Governing equations for the cells, culture medium, and ECM are derived using the principles of mass and momentum balance. The model is then reduced to a system of four partial differential equations, which are investigated analytically and numerically. The model predicts that provided cells are seeded at a suitable density, aggregates with clearly defined boundaries and a spatially uniform cell density on the interior will form. While the mechanical properties of the ECM do not appear to have a significant effect, strong cell-ECM interactions can inhibit, or possibly prevent, the formation of aggregates. The paper concludes with a discussion of our key findings and suggestions for future work
A new twist on dS/CFT
We stress that the dS/CFT correspondence should be formulated using unitary
principal series representations of the de Sitter isometry group/conformal
group, rather than highest-weight representations as originally proposed. These
representations, however, are infinite-dimensional, and so do not account for
the finite gravitational entropy of de Sitter space in a natural way. We then
propose to replace the classical isometry group by a q-deformed version. This
is carried out in detail for two-dimensional de Sitter and we find that the
unitary principal series representations deform to finite-dimensional unitary
representations of the quantum group. We believe this provides a promising
microscopic framework to account for the Bekenstein-Hawking entropy of de
Sitter space.Comment: 21 pages, revtex, v2 references adde
Quantum Chaos in Open versus Closed Quantum Dots: Signatures of Interacting Particles
This paper reviews recent studies of mesoscopic fluctuations in transport
through ballistic quantum dots, emphasizing differences between conduction
through open dots and tunneling through nearly isolated dots. Both the open
dots and the tunnel-contacted dots show random, repeatable conductance
fluctuations with universal statistical proper-ties that are accurately
characterized by a variety of theoretical models including random matrix
theory, semiclassical methods and nonlinear sigma model calculations. We apply
these results in open dots to extract the dephasing rate of electrons within
the dot. In the tunneling regime, electron interaction dominates transport
since the tunneling of a single electron onto a small dot may be sufficiently
energetically costly (due to the small capacitance) that conduction is
suppressed altogether. How interactions combine with quantum interference are
best seen in this regime.Comment: 15 pages, 11 figures, PDF 2.1 format, to appear in "Chaos, Solitons &
Fractals
An approximate renormalization-group transformation for Hamiltonian systems with three degrees of freedom
We construct an approximate renormalization transformation that combines
Kolmogorov-Arnold-Moser (KAM)and renormalization-group techniques, to analyze
instabilities in Hamiltonian systems with three degrees of freedom. This scheme
is implemented both for isoenergetically nondegenerate and for degenerate
Hamiltonians. For the spiral mean frequency vector, we find numerically that
the iterations of the transformation on nondegenerate Hamiltonians tend to
degenerate ones on the critical surface. As a consequence, isoenergetically
degenerate and nondegenerate Hamiltonians belong to the same universality
class, and thus the corresponding critical invariant tori have the same type of
scaling properties. We numerically investigate the structure of the attracting
set on the critical surface and find that it is a strange nonchaotic attractor.
We compute exponents that characterize its universality class.Comment: 10 pages typeset using REVTeX, 7 PS figure
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