624 research outputs found
Unzipping flux lines from extended defects in type-II superconductors
With magnetic force microscopy in mind, we study the unbinding transition of
individual flux lines from extended defects like columnar pins and twin planes
in type II superconductors. In the presence of point disorder, the transition
is universal with an exponent which depends only on the dimensionality of the
extended defect. We also consider the unbinding transition of a single vortex
line from a twin plane occupied by other vortices. We show that the critical
properties of this transition depend strongly on the Luttinger liquid parameter
which describes the long distance physics of the two-dimensional flux line
array.Comment: 5 pages, 4 figure
Melting and unzipping of DNA
Experimental studies of the thermal denaturation of DNA yield a strong
indication that the transition is first order. This transition has been
theoretically studied since the early sixties, mostly within an approach in
which the microscopic configurations of a DNA molecule are given by an
alternating sequence of non-interacting bound segments and denaturated loops.
Studies of these models neglect the repulsive, self-avoiding, interaction
between different loops and segments and have invariably yielded continuous
denaturation transitions. In this study we exploit recent results on scaling
properties of polymer networks of arbitrary topology in order to take into
account the excluded-volume interaction between denaturated loops and the rest
of the chain. We thus obtain a first-order phase transition in d=2 dimensions
and above, in agreement with experiments. We also consider within our approach
the unzipping transition, which takes place when the two DNA strands are pulled
apart by an external force acting on one end. We find that the unzipping
transition is also first order. Although the denaturation and unzipping
transitions are thermodynamically first order, they do exhibit critical
fluctuations in some of their properties. For instance, the loop size
distribution decays algebraically at the transition and the length of the
denaturated end segment diverges as the transition is approached. We evaluate
these critical properties within our approach.Comment: 12pages,8 figures, REVTEX
Tug-of-war in motility assay experiments
The dynamics of two groups of molecular motors pulling in opposite directions
on a rigid filament is studied theoretically. To this end we first consider the
behavior of one set of motors pulling in a single direction against an external
force using a new mean-field approach. Based on these results we analyze a
similar setup with two sets of motors pulling in opposite directions in a
tug-of-war in the presence of an external force. In both cases we find that the
interplay of fluid friction and protein friction leads to a complex phase
diagram where the force-velocity relations can exhibit regions of bistability
and spontaneous symmetry breaking. Finally, motivated by recent work, we turn
to the case of motility assay experiments where motors bound to a surface push
on a bundle of filaments. We find that, depending on the absence or the
presence of a bistability in the force-velocity curve at zero force, the bundle
exhibits anomalous or biased diffusion on long-time and large-length scales
Dinotefuran and Piperonyl Butoxide Mixture for The Extermination and Prevention of Ctenocephalides Canis and Ctenocephalides Felis Felis In Dogs And Cats
Ctenocephalides canis and Ctenocephalides felis felis are insects which are among the most common ectoparasites of common household dogs and cats. Flea killers have been developed for decades to counter this worldwide pest. It is a never-ending battle because of the continuous genetic resistance presented by the pest. In the present study, we applied a mixture of the pesticide Dinotefuran (26% w/v) and the synergist Piperonyl butoxide (6% w/v) to dogs (Solpreme Dogtrade and cats (Solpreme Cattrade) infested with the fleas via ldquoSpot onrdquo method. The treatment exterminated all of the fleas within 3 days (100%, plt 0.001). Re-infestation after 10 days caused an increase in the number of fleas, yet most of the effect lasted for at least 30 days. Further tests showed that the treatment is safe, water resistant, and has a long shelf life. The development of this novel mixture of substances provides an effective, safe means in the struggle with the continuous development of resistance to pesticides among major skin parasites
Structure factor and dynamics of the helix-coil transition
Thermodynamical properties of the helix-coil transition were successfully
described in the past by the model of Lifson, Poland and Sheraga. Here we
compute the corresponding structure factor and show that it possesses a
universal scaling behavior near the transition point, even when the transition
is of first order. Moreover, we introduce a dynamical version of this model,
that we solve numerically. A Langevin equation is also proposed to describe the
dynamics of the density of hydrogen bonds. Analytical solution of this equation
shows dynamical scaling near the critical temperature and predicts a gelation
phenomenon above the critical temperature. In the case when comparison of the
two dynamical approaches is possible, the predictions of our phenomenological
theory agree with the results of the Monte Carlo simulations.Comment: 11 pages, 7 figure
Phase transition in a non-conserving driven diffusive system
An asymmetric exclusion process comprising positive particles, negative
particles and vacancies is introduced. The model is defined on a ring and the
dynamics does not conserve the number of particles. We solve the steady state
exactly and show that it can exhibit a continuous phase transition in which the
density of vacancies decreases to zero. The model has no absorbing state and
furnishes an example of a one-dimensional phase transition in a homogeneous
non-conserving system which does not belong to the absorbing state universality
classes
Phase diagram for unzipping DNA with long-range interactions
We present a critique and extension of the mean-field approach to the
mechanical pulling transition in bound polymer systems. Our model is motivated
by the theoretically and experimentally important examples of adsorbed polymers
and double-stranded DNA, and we focus on the case in which quenched disorder in
the sequence of monomers is unimportant for the statistical mechanics. We show
how including excluded volume interactions in the model affects the phase
diagram for the critical pulling force, and we predict a re-entrancy phase at
low temperatures which has not been previously discussed. We also consider the
case of non-equilibrium pulling, in which the external force probes the local,
rather than the global structure of the dsDNA or adsorbed polymer. The dynamics
of the pulling transition in such experiments could illuminate the polymer's
loop structure, which depends on the nature of excluded volume interactions.Comment: 4 pages, 2 figures; this version clarifies Eq. 8, and corrects errors
in Fig.
Sequence heterogeneity and the dynamics of molecular motors
The effect of sequence heterogeneity on the dynamics of molecular motors is
reviewed and analyzed using a set of recently introduced lattice models. First,
we review results for the influence of heterogenous tracks such as a
single-strand of DNA or RNA on the dynamics of the motors. We stress how the
predicted behavior might be observed experimentally in anomalous drift and
diffusion of motors over a wide range of parameters near the stall force and
discuss the extreme limit of strongly biased motors with one-way hopping. We
then consider the dynamics in an environment containing a variety of different
fuels which supply chemical energy for the motor motion, either on a
heterogeneous or on a periodic track. The results for motion along a periodic
track are relevant to kinesin motors in a solution with a mixture of different
nucleotide triphosphate fuel sources.Comment: To appear in a JPhys special issue on molecular motor
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