130 research outputs found
Comparison of the Adsorption Transition for Grafted and Nongrafted Polymers
We compare the thermodynamic behavior of a finite single nongrafted polymer
near an attractive substrate with that of a polymer grafted to that substrate.
After we recently found first-order-like signatures in the microcanonical
entropy at the adsorption transition in the nongrafted case, and given the fact
that many studies on polymer adsorption in the past have been performed for
grafted polymers, the question arises, to what extent and in what way does
grafting change the nature of the adsorption transition? This question is
tackled here using a coarse-grained off-lattice polymer model and covers not
only the adsorption transition but also all other transitions a single polymer
near an attractive substrate of varying strengths undergoes. Because of the
impact of grafting especially on the translational but also on the
conformational entropy of desorbed chains, the adsorption transition is
affected the strongest. Our results are obtained by a combined canonical and
microcanonical analysis of parallel tempering Monte Carlo data.Comment: 8 pages, 6 figure
Adsorption of polymers at nanowires
Low-energy structures of a hybrid system consisting of a polymer and an
attractive nanowire substrate as well as the thermodynamics of the adsorption
transition are studied by means of Monte Carlo computer simulations. Depending
on structural and energetic properties of the substrate, we find different
adsorbed polymer conformations, amongst which are spherical droplets attached
to the wire and monolayer tubes surrounding it. We identify adsorption
temperatures and the type of the transition between adsorbed and desorbed
structures depending on the substrate attraction strength.Comment: Proceedings of the Computational Physics Conference CCP 2010, Jun
23-27, 2010, Trondheim, Norwa
Unique Compact Representation of Magnetic Fields using Truncated Solid Harmonic Expansions
Precise knowledge of magnetic fields is crucial in many medical imaging
applications like magnetic resonance imaging or magnetic particle imaging (MPI)
as they are the foundation of these imaging systems. For the investigation of
the influence of field imperfections on imaging, a compact and unique
representation of the magnetic fields using real solid spherical harmonics,
which can be obtained by measuring a few points of the magnetic field only, is
of great assistance. In this manuscript, we review real solid harmonic
expansions as a general solution of Laplace's equation including an efficient
calculation of their coefficients using spherical t-designs. We also provide a
method to shift the reference point of an expansion by calculating the
coefficients of the shifted expansion from the initial ones. These methods are
used to obtain the magnetic fields of an MPI system. Here, the field-free-point
of the spatial encoding field serves as unique expansion point. Lastly, we
quantify the severity of the distortions of the static and dynamic fields in
MPI by analyzing the expansion coefficients.Comment: 25 page
Monte Carlo simulations of the HP model (the "Ising model" of protein folding)
Using Wang-Landau sampling with suitable Monte Carlo trial moves (pull moves
and bond-rebridging moves combined) we have determined the density of states
and thermodynamic properties for a short sequence of the HP protein model. For
free chains these proteins are known to first undergo a collapse "transition"
to a globule state followed by a second "transition" into a native state. When
placed in the proximity of an attractive surface, there is a competition
between surface adsorption and folding that leads to an intriguing sequence of
"transitions". These transitions depend upon the relative interaction strengths
and are largely inaccessible to "standard" Monte Carlo methods.Comment: 6 pages, 6 figures. Article in press. To be published in Computer
Physics Communications (2011
Systematic Microcanonical Analyses of Polymer Adsorption Transitions
In detailed microcanonical analyses of densities of states obtained by
extensive multicanonical Monte Carlo computer simulations, we investigate the
caloric properties of conformational transitions adsorbing polymers experience
near attractive substrates. For short chains and strong surface attraction, the
microcanonical entropy turns out to be a convex function of energy in the
transition regime, indicating that surface-entropic effects are relevant.
Albeit known to be a continuous transition in the thermodynamic limit of
infinitely long chains, the adsorption transition of nongrafted finite-length
polymers thus exhibits a clear signature of a first-order-like transition, with
coexisting phases of adsorbed and desorbed conformations. Another remarkable
consequence of the convexity of the microcanonical entropy is that the
transition is accompanied by a decrease of the microcanonical temperature with
increasing energy. Since this is a characteristic physical effect it might not
be ignored in analyses of cooperative macrostate transitions in finite systems.Comment: 8 pages, 6 figure
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