15,986 research outputs found
Stability of monolayers and bilayers in a copolymer-homopolymer blend model
We study the stability of layered structures in a variational model for
diblock copolymer-homopolymer blends. The main step consists of calculating the
first and second derivative of a sharp-interface Ohta-Kawasaki energy for
straight mono- and bilayers. By developing the interface perturbations in a
Fourier series we fully characterise the stability of the structures in terms
of the energy parameters.
In the course of our computations we also give the Green's function for the
Laplacian on a periodic strip and explain the heuristic method by which we
found it.Comment: 40 pages, 34 Postscript figures; second version has some minor
corrections; to appear in "Interfaces and Free Boundaries
Copolymer-homopolymer blends: global energy minimisation and global energy bounds
We study a variational model for a diblock-copolymer/homopolymer blend. The
energy functional is a sharp-interface limit of a generalisation of the
Ohta-Kawasaki energy. In one dimension, on the real line and on the torus, we
prove existence of minimisers of this functional and we describe in complete
detail the structure and energy of stationary points. Furthermore we
characterise the conditions under which the minimisers may be non-unique.
In higher dimensions we construct lower and upper bounds on the energy of
minimisers, and explicitly compute the energy of spherically symmetric
configurations.Comment: 31 pages, 6 Postscript figures; to be published in Calc. Var. Partial
Differential Equations. Version history: Changes in v2 w.r.t v1 only concern
metadata. V3 contains some minor revisions and additions w.r.t. v2. V4
corrects a confusing typo in one of the formulas of the appendix. V5 is the
definitive version that will appear in prin
The weakening relationship between the Impact Factor and papers' citations in the digital age
Historically, papers have been physically bound to the journal in which they
were published but in the electronic age papers are available individually, no
longer tied to their respective journals. Hence, papers now can be read and
cited based on their own merits, independently of the journal's physical
availability, reputation, or Impact Factor. We compare the strength of the
relationship between journals' Impact Factors and the actual citations received
by their respective papers from 1902 to 2009. Throughout most of the 20th
century, papers' citation rates were increasingly linked to their respective
journals' Impact Factors. However, since 1990, the advent of the digital age,
the strength of the relation between Impact Factors and paper citations has
been decreasing. This decrease began sooner in physics, a field that was
quicker to make the transition into the electronic domain. Furthermore, since
1990, the proportion of highly cited papers coming from highly cited journals
has been decreasing, and accordingly, the proportion of highly cited papers not
coming from highly cited journals has also been increasing. Should this pattern
continue, it might bring an end to the use of the Impact Factor as a way to
evaluate the quality of journals, papers and researchers.Comment: 14 pages, 5 figure
Coarse-grained simulation of polymer translocation through an artificial nanopore
The translocation of a macromolecule through a nanometer-sized pore is an
interesting process with important applications in the development of
biosensors for single--molecule analysis and in drug delivery and gene therapy.
We have carried out a molecular dynamics simulation study of electrophoretic
translocation of a charged polymer through an artificial nanopore to explore
the feasibility of semiconductor--based nanopore devices for ultra--fast DNA
sequencing. The polymer is represented by a simple bead--spring model designed
to yield an appropriate coarse-grained description of the phosphate backbone of
DNA in salt--free aqueous solution. A detailed analysis of single translocation
event is presented to assess whether the passage of individual ions through the
pore can be detected by a nanoscale field--effect transistor by measuring
variations in electrostatic potential during polymer translocation. We find
that it is possible to identify single events corresponding to the passage of
counterions through the pore, but that discrimination of individual ions on the
polymer chain based on variations in electrostatic potential is problematic.
Several distinct stages in the translocation process are identified,
characterized by changes in polymer conformation and by variations in the
magnitude and direction of the internal electric field induced by the
fluctuating charge distribution. The dependence of the condensed fraction of
counterions on Bjerrum length leads to significant changes in polymer
conformation, which profoundly affect the dynamics of electrophoresis and
translocation.Comment: 37 pages Revtex, 11 postscript figure
The effect of dark strings on semilocal strings
Dark strings have recently been suggested to exist in new models of dark
matter that explain the excessive electronic production in the galaxy. We study
the interaction of these dark strings with semilocal strings which are
solutions of the bosonic sector of the Standard Model in the limit
, where is the Weinberg angle. While
embedded Abelian-Higgs strings exist for generic values of the coupling
constants, we show that semilocal solutions with non-vanishing condensate
inside the string core exist only above a critical value of the Higgs to gauge
boson mass ratio when interacting with dark strings. Above this critical value,
which is greater than unity, the energy per unit length of the semilocal-dark
string solutions is always smaller than that of the embedded Abelian-Higgs-dark
string solutions and we show that Abelian-Higgs-dark strings become unstable
above this critical value. Different from the non-interacting case, we would
thus expect semilocal strings to be stable for values of the Higgs to gauge
boson mass ratio larger than unity. Moreover, the one-parameter family of
solutions present in the non-interacting case ceases to exist when semilocal
strings interact with dark strings.Comment: 16 pages including 6 figures; stability analysis adde
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