84 research outputs found
Core-shell structures in single flexible-semiflexible block copolymers: Finding the free energy minimum for the folding transition
We investigate the folding transition of a single diblock copolymer
consisting of a semiflexible and a flexible block. We obtain a {\it
Saturn-shaped} core-shell conformation in the folded state, in which the
flexible block forms a core and the semiflexible block wraps around it. We
demonstrate two distinctive features of the core-shell structures: (i) The
kinetics of the folding transition in the copolymer are significantly more
efficient than those of a semiflexible homopolymer. (ii) The core-shell
structure does not depend on the transition pathway
Collapse or Swelling Dynamics of Homopolymer Rings: Self-consistent Hartree approach
We investigate by the use of the Martin - Siggia - Rose generating functional
technique and the self - consistent Hartree approximation, the dynamics of the
ring homopolymer collapse (swelling) following an instantaneous change into a
poor (good) solvent conditions.The equation of motion for the time dependent
monomer - to - monomer correlation function is systematically derived. It is
argued that for describing of the coarse - graining process (which neglects the
capillary instability and the coalescence of ``pearls'') the Rouse mode
representation is very helpful, so that the resulting equations of motion can
be simply solved numerically. In the case of the collapse this solution is
analyzed in the framework of the hierarchically crumpled fractal picture, with
crumples of successively growing scale along the chain. The presented numerical
results are in line with the corresponding simple scaling argumentation which
in particular shows that the characteristic collapse time of a segment of
length scales as (where is a bare
friction coefficient and is a depth of quench). In contrast to the
collapse the globule swelling can be seen (in the case that topological effects
are neglected) as a homogeneous expansion of the globule interior. The swelling
of each Rouse mode as well as gyration radius is discussed.Comment: 20 pages, 7 figures, submitted to Phys. Rev.
Bouncing or sticky droplets: impalement transitions on superhydrophobic micropatterned surfaces
When a liquid drops impinges a hydrophobic rough surface it can either bounce
off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel
droplets). The analysis of drop impact and quasi static ''loading'' experiments
on model microfabricated surfaces allows to clearly identify the forces
hindering the impalement transitions. A simple semi-quantitative model is
proposed to account for the observed relation between the surface topography
and the robustness of fakir non-wetting states. Motivated by potential
applications in microfluidics and in the fabrication of self cleaning surfaces,
we finally propose some guidelines to design robust superhydrophobic surfaces.Comment: 7 pages, 5 figure
How long does it take to pull an ideal polymer into a small hole?
We present scaling estimates for characteristic times and
of pulling ideal linear and randomly branched polymers of
monomers into a small hole by a force . We show that the absorbtion process
develops as sequential straightening of folds of the initial polymer
configuration. By estimating the typical size of the fold involved into the
motion, we arrive at the following predictions: and , and we also confirm them by
the molecular dynamics experiment.Comment: 4 pages, 3 figure
Collapse Dynamics of a Homopolymer: Theory and Simulation
We present a scaling theory describing the collapse of a homopolymer chain in
poor solvent. At time t after the beginning of the collapse, the original
Gaussian chain of length N is streamlined to form N/g segments of length R(t),
each containing g ~ t monomers. These segments are statistical quantities
representing cylinders of length R ~ t^{1/2} and diameter d ~ t^{1/4}, but
structured out of stretched arrays of spherical globules. This prescription
incorporates the capillary instability. We compare the time-dependent structure
factor derived for our theory with that obtained from ultra-large-scale
molecular dynamics simulation with explicit solvent. This is the first time
such a detailed comparison of theoretical and simulation predictions of
collapsing chain structure has been attempted. The favorable agreement between
the theoretical and computed structure factors supports the picture of the
coarse-graining process during polymer collapse.Comment: 4 pages, 3 figure
Gravitational oscillations of a liquid column
We report gravity oscillations of a liquid column partially immersed in a
bath of liquid. We stress in particular some peculiarities of this system,
namely (i) the fact that the mass of this oscillator constantly changes with
time; (ii) the singular character of the beginning of the rise, for which the
mass of the oscillator is zero; (iii) the sources of dissipation in this
system, which is found to be dominated at low viscosity by the entrance (or
exit) effects, leading to a long-range damping of the oscillations. We conclude
with some qualitative description of a second-order phenomenon, namely the
eruption of a jet at the beginning of the rise.Comment: 22 pages, pdf. Submitted to Physics of Fluid
Folding and unfolding kinetics of a single semiflexible polymer
We theoretically investigate the kinetics of the folding transition of a
single semiflexible polymer. In the folding transition, the growth rate
decrease with an increase in the number of monomers in a collapsed domain,
suggesting that the main contribution to dissipation is from the motion of the
domain. In the unfolding transition, dynamic scaling exponents, 1/8 and 1/4,
were determined for disentanglement and relaxation steps, respectively. We
performed Langevin dynamics simulations to test our theory. It is found that
our theory is in good agreement with simulations. We also propose the kinetics
of the transitions in the presence of the hydrodynamic interaction.Comment: 12 pages, 10 figure
Glassy behavior of a homopolymer from molecular dynamics simulations
We study at- and out-of-equilibrium dynamics of a single homopolymer chain at
low temperature using molecular dynamics simulations. The main quantities of
interest are the average root mean square displacement of the monomers below
the theta point, and the structure factor, as a function of time. The
observation of these quantities show a close resemblance to those measured in
structural glasses and suggest that the polymer chain in its low temperature
phase is in a glassy phase, with its dynamics dominated by traps. In
equilibrium, at low temperature, we observe the trapping of the monomers and a
slowing down of the overall motion of the polymer as well as non-exponential
relaxation of the structure factor. In out-of-equilibrium, at low temperatures,
we compute the two-time quantities and observe breaking of ergodicity in a
range of waiting times, with the onset of aging.Comment: 11 pages, 4 figure
Early Stages of Homopolymer Collapse
Interest in the protein folding problem has motivated a wide range of
theoretical and experimental studies of the kinetics of the collapse of
flexible homopolymers. In this Paper a phenomenological model is proposed for
the kinetics of the early stages of homopolymer collapse following a quench
from temperatures above to below the theta temperature. In the first stage,
nascent droplets of the dense phase are formed, with little effect on the
configurations of the bridges that join them. The droplets then grow by
accreting monomers from the bridges, thus causing the bridges to stretch.
During these two stages the overall dimensions of the chain decrease only
weakly. Further growth of the droplets is accomplished by the shortening of the
bridges, which causes the shrinking of the overall dimensions of the chain. The
characteristic times of the three stages respectively scale as the zeroth, 1/5
and 6/5 power of the the degree of polymerization of the chain.Comment: 11 pages, 3 figure
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