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## Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/08/2013

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#Condensed Matter - Superconductivity#Condensed Matter - Materials Science#Condensed Matter - Strongly Correlated Electrons

A quantum critical point (QCP) is a point in a system's phase diagram at
which an order is completely suppressed at absolute zero temperature (T). The
presence of a quantum critical point manifests itself in the finite-T physical
properties, and often gives rise to new states of matter. Superconductivity in
the cuprates and in heavy fermion materials is believed by many to be mediated
by fluctuations associated with a quantum critical point. In the
recently-discovered iron-pnictide high temperature superconductors, it is
unknown whether a QCP exists or not in a carrier-doped system. Here we report
transport and nuclear magnetic resonance (NMR) measurements on
BaFe_{2-x}Ni_xAs_2 (0 =< x =< 0.17). We find two critical points at x_{c1} =
0.10 and x_{c2} = 0.14. The electrical resistivity follows \rho = \rho_0 +
A*T^n, with n = 1 around x_{c1} and another minimal n = 1.1 at x_{c2}. By NMR
measurements, we identity x_{c1} to be a magnetic QCP and suggest that x_{c2}
is a new type of QCP associated with a nematic structural phase transition. Our
results suggest that the superconductivity in carrier-doped pnictides is
closely linked to the quantum criticality.; Comment: 21 pages, 9 figures

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## A molecular dynamics study of chemical gelation in a patchy particle model

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 27/02/2008

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We report event-driven molecular dynamics simulations of the irreversible
gelation of hard ellipsoids of revolution containing several associating
groups, characterizing how the cluster size distribution evolves as a function
of the extent of reaction, both below and above the gel point. We find that in
a very large interval of values of the extent of reaction, parameter-free
mean-field predictions are extremely accurate, providing evidence that in this
model the Ginzburg zone near the gel point, where non-mean field effects are
important, is very limited. We also find that the Flory's hypothesis for the
post-gelation regime properly describes the connectivity of the clusters even
if the long-time limit of the extent of reaction does not reach the fully
reacted state. This study shows that irreversibly aggregating asymmetric
hard-core patchy particles may provide a close realization of the mean-field
model, for which available theoretical predictions may help control the
structure and the connectivity of the gel state. Besides chemical gels, the
model is relevant to network-forming soft materials like systems with
bioselective interactions, functionalized molecules and patchy colloids.; Comment: 6 pages, 4 figures, to be published in Soft Matter

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## Through the Eye of the Needle: Recent Advances in Understanding Biopolymer Translocation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/08/2013

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In recent years polymer translocation, i.e., transport of polymeric molecules
through nanometer-sized pores and channels embedded in membranes, has witnessed
strong advances. It is now possible to observe single-molecule polymer dynamics
during the motion through channels with unprecedented spatial and temporal
resolution. These striking experimental studies have stimulated many
theoretical developments. In this short theory-experiment review, we discuss
recent progress in this field with a strong focus on non-equilibrium aspects of
polymer dynamics during the translocation process.; Comment: 29 pages, 6 figures, 3 tables, to appear in J. Phys.: Condens. Matter
as a Topical Review

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## Oscillatory settling in wormlike-micelle solutions: bursts and a long time scale

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/03/2012

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We study the dynamics of a spherical steel ball falling freely through a
solution of entangled wormlike-micelles. If the sphere diameter is larger than
a threshold value, the settling velocity shows repeated short oscillatory
bursts separated by long periods of relative quiescence. We propose a model
incorporating the interplay of settling-induced flow, viscoelastic stress and,
as in M. E. Cates, D. A. Head and A. Ajdari, Phys. Rev. E, 2002, 66, 025202(R)
and A. Aradian and M. E. Cates, Phys. Rev. E, 2006, 73, 041508, a slow
structural variable for which our experiments offer independent evidence.; Comment: To appear in Soft Matter

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## Recent high-magnetic-field studies of unusual groundstates in quasi-two-dimensional crystalline organic metals and superconductors

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/03/2005

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After a brief introduction to crystalline organic superconductors and metals,
we shall describe two recently-observed exotic phases that occur only in high
magnetic fields. The first involves measurements of the non-linear electrical
resistance of single crystals of the charge-density-wave (CDW) system
(Per)$_2$Au(mnt)$_2$ in static magnetic fields of up to 45 T and temperatures
as low as 25 mK. The presence of a fully gapped CDW state with typical CDW
electrodynamics at fields higher that the Pauli paramagnetic limit of 34 T
suggests the existence of a modulated CDW phase analogous to the
Fulde-Ferrell-Larkin-Ovchinnikov state. Secondly, measurements of the Hall
potential of single crystals of $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$, made using
a variant of the Corbino geometry in quasistatic magnetic fields, show
persistent current effects that are similar to those observed in conventional
superconductors. The longevity of the currents, large Hall angle, flux
quantization and confinement of the reactive component of the Hall potential to
the edge of the sample are all consistent with the realization of a new state
of matter in CDW systems with significant orbital quantization effects in
strong magnetic fields.; Comment: SNS 2004 Conference presentation

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## Determination of thermal and optical parameters of melanins by photopyroelectric spectroscopy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/04/2005

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Photopyroelectric spectroscopy (PPE) was used to study the thermal and
optical properties of electropolymerized melanins. The photopyroelectric
intensity signal and its phase were independently measured as a function of
wavelength, as well as a function of chopping frequency for a given wavelength
in the saturation part of the PPE spectrum. Equations for both the intensity
and the phase of the PPE signal were used to fit the experimental results. From
the fittings we obtained for the first time, with great accuracy, the thermal
diffusivity coefficient, the thermal conductivity and the specific heat of the
samples, as well as a value for the condensed phase optical gap, which we found
to be 1.70 eV.; Comment: 4 Page Letter

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## Coarse-graining dynamics for convection-diffusion of colloids: Taylor dispersion

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 20/03/2009

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55.53%

By applying a hybrid Molecular dynamics and mesoscopic simulation technique,
we study the classic convection-diffusion problem of Taylor dispersion for
colloidal discs in confined flow. We carefully consider the time and
length-scales of the underlying colloidal system. These are, by computational
necessity, altered in the coarse-grained simulation method, but as long as this
is carefully managed, the underlying physics can be correctly interpreted. We
find that when the disc diameter becomes non-negligible compared to the
diameter of the pipe, there are important corrections to the original Taylor
picture. For example, the colloids can flow more rapidly than the underlying
fluid, and their Taylor dispersion coefficient is decreased. The long-time
tails in the velocity autocorrelation functions are altered by the Poiseuille
flow. Some of the conclusions about coarse-graining the dynamics of colloidal
suspensions are relevant for a wider range of complex fluids.; Comment: submitted to Faraday Discussions for FD144 - Multiscale Modelling of
Soft Matter

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## Optical excitations in electroluminescent polymers: poly(para-phenylenevinylene) family

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.53%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Materials Science#Physics - Chemical Physics#Physics - Optics

Component of photoexcited states with large spatial extent is investigated
for optical absorption spectra of the electroluminescent conjugated polymers by
using the intermediate exciton theory. We calculate the ratio of oscillator
strengths due to long-range excitons with respect to sum of all the oscillator
strengths of the absorption as a function of the monomer number. The oscillator
strengths of the long-range excitons in poly(para-phenylene) are smaller than
those in poly(para-phenylenevinylene), and those of
poly(para-phenylenedivinylene) are larger than those in
poly(para-phenylenevinylene) Such relative variations are explained by the
differences of the number of vinylene units. The oscillator strengths of
long-range excitons in poly(di-para-phenylenevinylene) are much larger than
those of the above three polymers, due to the increase of number of phenyl
rings. We also find that the energy position of the almost localized exciton is
neary the same in the four polymers.; Comment: 15 pages; 9 figures; Accepted for publication in J. Phys.: Condens.
Matter

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## Disentangling the role of structure and friction in shear jamming

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/10/2015

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55.53%

Amorphous packings of spheres have been intensely investigated in order to
understand the mechanical and flow behaviour of dense granular matter, and to
explore universal aspects of the transition from fluid to structurally arrested
or jammed states. Considerable attention has recently been focussed on
anisotropic packings of frictional grains generated by shear deformation
leading to shear jamming, which occurs below the jamming density for isotropic
packings of frictionless grains. With the aim of disentangling the role of
shear deformation induced structures and friction in generating shear jamming,
we study sheared assemblies of frictionless spheres computationally, over a
wide range of densities, extending far below the isotropic jamming point. We
demonstrate the emergence of a variety of geometric features characteristic of
jammed packings with the increase of shear strain. The average contact number
and the distributions of contact forces suggest the presence of a threshold
density, well below the isotropic jamming point, above which a qualitative
change occurs in the jamming behaviour of sheared configurations. We show that
above this threshold density, friction stabilizes the sheared configurations we
generate. Our results thus reveal the emergence of geometric features
characteristic of jammed states as a result of shear deformation alone...

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## Smectic Polymer Vesicles

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/04/2009

Relevância na Pesquisa

55.53%

Polymer vesicles are stable robust vesicles made from block copolymer
amphiphiles. Recent progress in the chemical design of block copolymers opens
up the exciting possibility of creating a wide variety of polymer vesicles with
varying fine structure, functionality and geometry. Polymer vesicles not only
constitute useful systems for drug delivery and micro/nano-reactors but also
provide an invaluable arena for exploring the ordering of matter on curved
surfaces embedded in three dimensions. By choosing suitable liquid-crystalline
polymers for one of the copolymer components one can create vesicles with
smectic stripes. Smectic order on shapes of spherical topology inevitably
possesses topological defects (disclinations) that are themselves distinguished
regions for potential chemical functionalization and nucleators of vesicle
budding. Here we report on glassy striped polymer vesicles formed from
amphiphilic block copolymers in which the hydrophobic block is a smectic liquid
crystal polymer containing cholesteryl-based mesogens. The vesicles exhibit
two-dimensional smectic order and are ellipsoidal in shape with defects, or
possible additional budding into isotropic vesicles, at the poles.; Comment: 17 pages, 4 figures, pdf

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## Inferring the effective thickness of polyelectrolytes from stretching measurements at various ionic strengths: applications to DNA and RNA

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/01/2006

Relevância na Pesquisa

55.53%

#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Biological Physics#Quantitative Biology - Biomolecules

By resorting to the thick-chain model we discuss how the stretching response
of a polymer is influenced by the self-avoidance entailed by its finite
thickness. The characterization of the force versus extension curve for a thick
chain is carried out through extensive stochastic simulations. The
computational results are captured by an analytic expression that is used to
fit experimental stretching measurements carried out on DNA and single-stranded
RNA (poly-U) in various solutions. This strategy allows us to infer the
apparent diameter of two biologically-relevant polyelectrolytes, namely DNA and
poly-U, for different ionic strengths. Due to the very different degree of
flexibility of the two molecules, the results provide insight into how the
apparent diameter is influenced by the interplay between the
(solution-dependent) Debye screening length and the polymers' ``bare''
thickness. For DNA, the electrostatic contribution to the effective radius,
$\Delta$, is found to be about 5 times larger than the Debye screening length,
consistently with previous theoretical predictions for highly-charged stiff
rods. For the more flexible poly-U chains the electrostatic contribution to
$\Delta$ is found to be significantly smaller than the Debye screening length.; Comment: iopart...

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## Frustrated Polyelectrolyte Bundles and T=0 Josephson-Junction Arrays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.53%

We establish a one-to-one mapping between a model for hexagonal
polyelectrolyte bundles and a model for two-dimensional, frustrated
Josephson-junction arrays. We find that the T=0 insulator-to-superconductor
transition of the {\it quantum} system corresponds to a continuous
liquid-to-solid transition of the condensed charge in the finite temperature
{\it classical} system. We find that the role of the vector potential in the
quantum system is played by elastic strain in the classical system. Exploiting
this correspondence we show that the transition is accompanied by a spontaneous
breaking of chiral symmetry and that at the transition the polyelectrolyte
bundle adopts a universal response to shear.; Comment: 4 pages, 2 figures, 1 table minor changes to text, reference added

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## Melting and freezing of argon in a granular packing of linear mesopore arrays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/03/2008

Relevância na Pesquisa

55.53%

#Physics - Chemical Physics#Condensed Matter - Soft Condensed Matter#Condensed Matter - Statistical Mechanics#Physics - Atomic and Molecular Clusters

Freezing and melting of Ar condensed in a granular packing of template-grown
arrays of linear mesopores (SBA-15, mean pore diameter 8 nanometer) has been
studied by specific heat measurements C as a function of fractional filling of
the pores. While interfacial melting leads to a single melting peak in C,
homogeneous and heterogeneous freezing along with a delayering transition for
partial fillings of the pores result in a complex freezing mechanism
explainable only by a consideration of regular adsorption sites (in the
cylindrical mesopores) and irregular adsorption sites (in niches of the rough
external surfaces of the grains, and at points of mutual contact of the powder
grains). The tensile pressure release upon reaching bulk liquid/vapor
coexistence quantitatively accounts for an upward shift of the
melting/freeezing temperature observed while overfilling the mesopores.; Comment: 4 pages, 4 figures, to appear as a Letter in Physical Review Letters

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## Nonlinear Elasticity, Fluctuations and Heterogeneity of Nematic Elastomers

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/09/2007

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Liquid crystal elastomers realize a fascinating new form of soft matter that
is a composite of a conventional crosslinked polymer gel (rubber) and a liquid
crystal. These {\em solid} liquid crystal amalgams, quite similarly to their
(conventional, fluid) liquid crystal counterparts, can spontaneously partially
break translational and/or orientational symmetries, accompanied by novel soft
Goldstone modes. As a consequence, these materials can exhibit unconventional
elasticity characterized by symmetry-enforced vanishing of some elastic moduli.
Thus, a proper description of such solids requires an essential modification of
the classical elasticity theory. In this work, we develop a {\em rotationally
invariant}, {\em nonlinear} theory of elasticity for the nematic phase of ideal
liquid crystal elastomers. We show that it is characterized by soft modes,
corresponding to a combination of long wavelength shear deformations of the
solid network and rotations of the nematic director field. We study thermal
fluctuations of these soft modes in the presence of network heterogeneities and
show that they lead to a large variety of anomalous elastic properties, such as
singular length-scale dependent shear elastic moduli, a divergent elastic
constant for splay distortion of the nematic director...

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## First-order phase transitions in two-dimensional off-lattice liquid crystals

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/09/2007

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We consider an off-lattice liquid crystal pair potential in strictly two
dimensions. The potential is purely repulsive and short-ranged. Nevertheless,
by means of a single parameter in the potential, the system is shown to undergo
a first-order phase transition. The transition is studied using mean-field
density functional theory, and shown to be of the isotropic-to-nematic kind. In
addition, the theory predicts a large density gap between the two coexisting
phases. The first-order nature of the transition is confirmed using computer
simulation and finite-size scaling. Also presented is an analysis of the
interface between the coexisting domains, including estimates of the line
tension, as well as an investigation of anchoring effects.; Comment: 12 pages, 17 figures, submitted to J. Phys.: Condens. Matter

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## Transport properties controlled by a thermostat: An extended dissipative particle dynamics thermostat

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/09/2007

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55.53%

We introduce a variation of the dissipative particle dynamics (DPD)
thermostat that allows for controlling transport properties of molecular
fluids. The standard DPD thermostat acts only on a relative velocity along the
interatomic axis. Our extension includes the damping of the perpendicular
components of the relative velocity, yet keeping the advantages of conserving
Galilei invariance and within our error bar also hydrodynamics. This leads to a
second friction parameter for tuning the transport properties of the system.
Numerical simulations of a simple Lennard-Jones fluid and liquid water
demonstrate a very sensitive behaviour of the transport properties, e.g.,
viscosity, on the strength of the new friction parameter. We envisage that the
new thermostat will be very useful for the coarse-grained and adaptive
resolution simulations of soft matter, where the diffusion constants and
viscosity of the coarse-grained models are typically too high/low,
respectively, compared to all-atom simulations.; Comment: 6 pages, 4 figures

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## Stress phase space for static granular matter

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.53%

This paper proposes a phase space to compare the static packings of a
granular system compatible to a macrostate that is set by the external stress.
The nature of this phase space is analyzed, showing that the consideration of
the allowed and forbidden regions and the internal degrees of freedom of every
configuration (i.e. geometrical pattern) could be a relevant factor for the
establishment of its probability and, therefore, of the expected properties of
the sample. This is due to the fact that many combinations of forces acting on
a particle can keep it in static equilibrium. Every set of forces can be
considered equivalent to a microscopic stress field, but the kind of
interaction and the geometrical restrictions mean that not all stress states
can be represented by any set, whereas others can be represented by many sets.
Consequently the points of the phase space are degenerate, and the density of
states of each configuration strongly determines the most probable statistical
distribution. It is shown how these functions just depend on the deviatoric
stress. A first analysis of two-dimensional (2D) arrangements is included to
clarify this assertion.; Comment: 8 pages, 10 figures

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## Shear-induced structuration of confined carbon black gels: Steady-state features of vorticity-aligned flocs

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 20/12/2010

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Various dispersions of attractive particles are known to aggregate into
patterns of vorticity-aligned stripes when sheared in confined geometries. We
report a thorough experimental investigation of such shear-induced vorticity
alignment through direct visualization of carbon black gels in both simple
plane shear and rotational shear cells. Control parameters such as the gap
width, the strain rate, and the gel concentration are systematically varied. It
is shown that in steady states the wavelength of the striped pattern depends
linearly on the gap width $h$ while being insensitive to both the gel
concentration $C$ and the shear rate $\gp$. The width of the vorticity-aligned
flocs coincides with the gap width and is also independent of $C$ and $\gp$,
which hints to a simple picture in terms of compressible cylindrical flocs.
Finally, we show that there exists a critical shear rate $\gp_c$ above which
structuration does not occur and that $\gp_c$ scales as $h^{-\alpha}$ with
$\alpha=1.4\pm 0.1$ independently of $C$. This extensive data set should open
the way to quantitative modelling of the vorticity alignment phenomenon in
attractive colloidal systems.; Comment: 10 pages, 14 figures, submitted to Soft Matter

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## Theory for the phase behaviour of a colloidal fluid with competing interactions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/08/2008

Relevância na Pesquisa

55.53%

We study the phase behaviour of a fluid composed of particles which interact
via a pair potential that is repulsive for large inter-particle distances, is
attractive at intermediate distances and is strongly repulsive at short
distances (the particles have a hard core). As well as exhibiting gas-liquid
phase separation, this system also exhibits phase transitions from the uniform
fluid phases to modulated inhomogeneous fluid phases. Starting from a
microscopic density functional theory, we develop an order parameter theory for
the phase transition in order to examine in detail the phase behaviour. The
amplitude of the density modulations is the order parameter in our theory. The
theory predicts that the phase transition from the uniform to the modulated
fluid phase can be either first order or second order (continuous). The phase
diagram exhibits two tricritical points, joined to one another by the line of
second order transitions.; Comment: 12 pages, 9 figures. Accepted for publication in J. Phys. Condens.
Matter

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## Non-Equilibrium Dynamics of Single polymer Adsorption to Solid Surfaces

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

55.53%

Adsorption of polymers to surfaces is crucial for understanding many
fundamental processes in nature. Recent experimental studies indicate that the
adsorption dynamics is dominated by non-equilibrium effects. We investigate the
adsorption of a single polymer of length $N$ to a planar solid surface in the
absence of hydrodynamic interactions. We find that for weak adsorption energies
the adsorption time scales $ \sim N^{(1+2\nu)/(1+\nu)}$, where $\nu$ is the
Flory exponent for the polymer. We argue that in this regime the single chain
adsorption is closely related to a field-driven polymer translocation through
narrow pores. Surprisingly, for high adsorption energies the adsorption time
becomes longer, as it scales $\sim N^{(1+\nu)}$, which is explained by strong
stretching of the unadsorbed part of the polymer close to the adsorbing
surface. These two dynamic regimes are separated by an energy scale that is
characterised by non-equilibrium contributions during the adsorption process.; Comment: 11 pages, 3 figures, 1 table, substantial added material, to appear
in J. Phys.: Condens. Matter as a Fast Track Communication

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