17,942 research outputs found
Flow transitions in two-dimensional foams
For sufficiently slow rates of strain, flowing foam can exhibit inhomogeneous
flows. The nature of these flows is an area of active study in both
two-dimensional model foams and three dimensional foam. Recent work in
three-dimensional foam has identified three distinct regimes of flow [S. Rodts,
J. C. Baudez, and P. Coussot, Europhys. Lett. {\bf 69}, 636 (2005)]. Two of
these regimes are identified with continuum behavior (full flow and
shear-banding), and the third regime is identified as a discrete regime
exhibiting extreme localization. In this paper, the discrete regime is studied
in more detail using a model two dimensional foam: a bubble raft. We
characterize the behavior of the bubble raft subjected to a constant rate of
strain as a function of time, system size, and applied rate of strain. We
observe localized flow that is consistent with the coexistence of a power-law
fluid with rigid body rotation. As a function of applied rate of strain, there
is a transition from a continuum description of the flow to discrete flow when
the thickness of the flow region is approximately 10 bubbles. This occurs at an
applied rotation rate of approximately
Statistics of Bubble Rearrangements in a Slowly Sheared Two-dimensional Foam
Many physical systems exhibit plastic flow when subjected to slow steady
shear. A unified picture of plastic flow is still lacking; however, there is an
emerging theoretical understanding of such flows based on irreversible motions
of the constituent ``particles'' of the material. Depending on the specific
system, various irreversible events have been studied, such as T1 events in
foam and shear transformation zones (STZ's) in amorphous solids. This paper
presents an experimental study of the T1 events in a model, two-dimensional
foam: bubble rafts. In particular, I report on the connection between the
distribution of T1 events and the behavior of the average stress and average
velocity profiles during both the initial elastic response of the bubble raft
and the subsequent plastic flow at sufficiently high strains
Fibrational induction rules for initial algebras
This paper provides an induction rule that can be used to prove properties of data structures whose types are inductive, i.e., are carriers of initial algebras of functors. Our results are semantic in nature and are inspired by Hermida and Jacobs’ elegant algebraic formulation of induction for polynomial data types. Our contribution is to derive, under slightly different assumptions, an induction rule that is generic over all inductive types, polynomial or not. Our induction rule is generic over the kinds of properties to be proved as well: like Hermida and Jacobs, we work in a general fibrational setting and so can accommodate very general notions of properties on inductive types rather than just those of particular syntactic forms. We establish the correctness of our generic induction rule by reducing induction to iteration. We show how our rule can be instantiated to give induction rules for the data types of rose trees, finite hereditary sets, and hyperfunctions. The former lies outside the scope of Hermida and Jacobs’ work because it is not polynomial; as far as we are aware, no induction rules have been known to exist for the latter two in a general fibrational framework. Our instantiation for hyperfunctions underscores the value of working in the general fibrational setting since this data type cannot be interpreted as a set
Thermodynamics of polymer adsorption to a flexible membrane
We analyze the structural behavior of a single polymer chain grafted to an
attractive, flexible surface. Our model is composed of a coarse-grained
bead-and-spring polymer and a tethered membrane. By means of extensive parallel
tempering Monte Carlo simulations it is shown that the system exhibits a rich
phase behavior ranging from highly ordered, compact to extended random coil
structures and from desorbed to completely adsorbed or even partially embedded
conformations. These findings are summarized in a pseudophase diagram
indicating the predominant class of conformations as a function of the external
parameters temperature and polymer-membrane interaction strength. By comparison
with adsorption to a stiff membrane surface it is shown that the flexibility of
the membrane gives rise to qualitatively new behavior such as stretching of
adsorbed conformations
Transition to turbulence in slowly divergent pipe flow
The results of a combined experimental and numerical study of the flow in
slowly diverging pipes are presented. Interestingly, an axisymmetric conical
recirculation cell has been observed. The conditions for its existence and the
length of the cell are simulated for a range of diverging angles and expansion
ratios. There is a critical velocity for the appearance of this state. When the
flow rate increases further, a subcritical transition for localized turbulence
arises. The transition and relaminarization experiments described here quantify
the extent of turbulence. The findings suggest that the transition scenario in
slowly diverging pipes is a combination of stages similar to those observed in
sudden expansions and in straight circular pipe flow.Comment: 8 pages, 5 figure
Elastic Lennard-Jones Polymers Meet Clusters -- Differences and Similarities
We investigate solid-solid and solid-liquid transitions of elastic flexible
off-lattice polymers with Lennard-Jones monomer-monomer interaction and
anharmonic springs by means of sophisticated variants of multicanonical Monte
Carlo methods. We find that the low-temperature behavior depends strongly and
non-monotonically on the system size and exhibits broad similarities to unbound
atomic clusters. Particular emphasis is dedicated to the classification of
icosahedral and non-icosahedral low-energy polymer morphologies.Comment: 9 pages, 17 figure
Supersymmetry solution for finitely extensible dumbbell model
Exact relaxation times and eigenfunctions for a simple mechanical model of
polymer dynamics are obtained using supersymmetry methods of quantum mechanics.
The model includes the finite extensibility of the molecule and does not make
use of the self-consistently averaging approximation. The finite extensibility
reduces the relaxation times when compared to a linear force. The linear
viscoelastic behaviour is obtained in the form of the ``generalized Maxwell
model''. Using these results, a numerical integration scheme is proposed in the
presence of a given flow kinematics.Comment: 5 pages, 2 figure
Magnetorotational-type instability in Couette-Taylor flow of a viscoelastic polymer liquid
We describe an instability of viscoelastic Couette-Taylor flow that is
directly analogous to the magnetorotational instability (MRI) in astrophysical
magnetohydrodynamics, with polymer molecules playing the role of magnetic field
lines. By determining the conditions required for the onset of instability and
the properties of the preferred modes, we distinguish it from the centrifugal
and elastic instabilities studied previously. Experimental demonstration and
investigation should be much easier for the viscoelastic instability than for
the MRI in a liquid metal. The analogy holds with the case of a predominantly
toroidal magnetic field such as is expected in an accretion disk and it may be
possible to access a turbulent regime in which many modes are unstable.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Are Local Filters Blind to Provenance? Ant Seed Predation Suppresses Exotic Plants More than Natives
The question of whether species’ origins influence invasion outcomes has been a point of substantial debate in invasion ecology. Theoretically, colonization outcomes can be predicted based on how species’ traits interact with community filters, a process presumably blind to species’ origins. Yet, exotic plant introductions commonly result in monospecific plant densities not commonly seen in native assemblages, suggesting that exotic species may respond to community filters differently than natives. Here, we tested whether exotic and native species differed in their responses to a local community filter by examining how ant seed predation affected recruitment of eighteen native and exotic plant species in central Argentina. Ant seed predation proved to be an important local filter that strongly suppressed plant recruitment, but ants suppressed exotic recruitment far more than natives (89% of exotic species vs. 22% of natives). Seed size predicted ant impacts on recruitment independent of origins, with ant preference for smaller seeds resulting in smaller seeded plant species being heavily suppressed. The disproportionate effects of provenance arose because exotics had generally smaller seeds than natives. Exotics also exhibited greater emergence and earlier peak emergence than natives in the absence of ants. However, when ants had access to seeds, these potential advantages of exotics were negated due to the filtering bias against exotics. The differences in traits we observed between exotics and natives suggest that higher-order introduction filters or regional processes preselected for certain exotic traits that then interacted with the local seed predation filter. Our results suggest that the interactions between local filters and species traits can predict invasion outcomes, but understanding the role of provenance will require quantifying filtering processes at multiple hierarchical scales and evaluating interactions between filters.Fil: Pearson, Dean. University of Montana; Estados Unidos. United States Department of Agriculture; Estados UnidosFil: Icasatti, Nadia Soledad. United States Department of Agriculture; Estados Unidos. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Hierro, Jose Luis. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; ArgentinaFil: Bird, Benjamin B.. United States Department of Agriculture; Estados Unido
Interfacial friction between semiflexible polymers and crystalline surfaces
The results obtained from molecular dynamics simulations of the friction at
an interface between polymer melts and weakly attractive crystalline surfaces
are reported. We consider a coarse-grained bead-spring model of linear chains
with adjustable intrinsic stiffness. The structure and relaxation dynamics of
polymer chains near interfaces are quantified by the radius of gyration and
decay of the time autocorrelation function of the first normal mode. We found
that the friction coefficient at small slip velocities exhibits a distinct
maximum which appears due to shear-induced alignment of semiflexible chain
segments in contact with solid walls. At large slip velocities the decay of the
friction coefficient is independent of the chain stiffness. The data for the
friction coefficient and shear viscosity are used to elucidate main trends in
the nonlinear shear rate dependence of the slip length. The influence of chain
stiffness on the relationship between the friction coefficient and the
structure factor in the first fluid layer is discussed.Comment: 31 pages, 12 figure
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