9,527 research outputs found
Fractures in complex fluids: the case of transient networks
We present a comprehensive review of the current state of fracture phenomena
in transient networks, a wide class of viscoelastic fluids. We will first
define what is a fracture in a complex fluid, and recall the main structural
and rheological properties of transient networks. Secondly, we review
experimental reports on fractures of transient networks in several
configurations: shear-induced fractures, fractures in Hele-Shaw cells and
fracture in extensional geometries (filament stretching rheometry and pendant
drop experiments), including fracture propagation. The tentative extension of
the concepts of brittleness and ductility to the fracture mechanisms in
transient networks is also discussed. Finally, the different and apparently
contradictory theoretical approaches developed to interpret fracture nucleation
will be addressed and confronted to experimental results. Rationalized criteria
to discriminate the relevance of these different models will be proposed.Comment: Review; Rheologica Acta 2013 published on lin
Systematic coarse-graining of the dynamics of entangled polymer melts: the road from chemistry to rheology
For optimal processing and design of entangled polymeric materials it is
important to establish a rigorous link between the detailed molecular
composition of the polymer and the viscoelastic properties of the macroscopic
melt. We review current and past computer simulation techniques and critically
assess their ability to provide such a link between chemistry and rheology. We
distinguish between two classes of coarse-graining levels, which we term
coarse-grained molecular dynamics (CGMD) and coarse-grained stochastic dynamics
(CGSD). In CGMD the coarse-grained beads are still relatively hard, thus
automatically preventing bond crossing. This also implies an upper limit on the
number of atoms that can be lumped together and therefore on the longest chain
lengths that can be studied. To reach a higher degree of coarse-graining, in
CGSD many more atoms are lumped together, leading to relatively soft beads. In
that case friction and stochastic forces dominate the interactions, and actions
must be undertaken to prevent bond crossing. We also review alternative methods
that make use of the tube model of polymer dynamics, by obtaining the
entanglement characteristics through a primitive path analysis and by
simulation of a primitive chain network. We finally review super-coarse-grained
methods in which an entire polymer is represented by a single particle, and
comment on ways to include memory effects and transient forces.Comment: Topical review, 31 pages, 10 figure
Interfacial layering in a three-component polymer system
We study theoretically the temporal evolution and the spatial structure of
the interface between two polymer melts involving three different species (A,
A* and B). The first melt is composed of two different polymer species A and A*
which are fairly indifferent to one another (Flory parameter chi_AA* ~ 0). The
second melt is made of a pure polymer B which is strongly attracted to species
A (chi_AB 0). We then show
that, due to these contradictory tendencies, interesting properties arise
during the evolution of the interface after the melts are put into contact: as
diffusion proceeds, the interface structures into several adjacent
"compartments", or layers, of differing chemical compositions, and in addition,
the central mixing layer grows in a very asymmetric fashion. Such unusual
behaviour might lead to interesting mechanical properties, and demonstrates on
a specific case the potential richness of multi-component polymer interfaces
(as compared to conventional two-component interfaces) for various
applications.Comment: Revised version, to appear in Macromolecule
Perspectives on the viscoelasticity and flow behavior of entangled linear and branched polymers
We briefly review the recent advances in the rheology of entangled polymers
and identify emerging research trends and outstanding challenges, especially
with respect to branched polymers. Emphasis is placed on the role of
well-characterized model systems, as well as the synergy of
synthesis-characterization, rheometry and modeling/simulations. The theoretical
framework for understanding the observed linear and nonlinear rheological
phenomena is the tube model which is critically assessed in view of its
successes and shortcomings, whereas alternative approaches are briefly
discussed. Finally, intriguing experimental findings and controversial issues
that merit consistent explanation, such as shear banding instabilities,
multiple stress overshoots in transient simple shear and enhanced steady-state
elongational viscosity in polymer solutions, are discussed, whereas future
directions such as branch point dynamics and anisotropic monomeric friction are
outlined.Comment: 25 pages, accepted for publication in Journal of Physics Condensed
Matter (August 2015
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