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1.
The mutual effects of the conformations of a star polymer in simple shear flow and the deformation of the solvent flow field
are investigated by a hybrid mesoscale simulation technique. We characterize the flow field near the star polymer as a function
of its functionality (arm number) f . A strong screening of the imposed flow is found inside the star polymer, which increases with increasing f . To elucidate the importance of hydrodynamic screening, we compare results for hydrodynamic and random solvents. The dependence
of the polymer orientation angle on the Weissenberg number shows a power law behavior with super-universal exponent --independent
of hydrodynamic and excluded-volume interactions. In contrast, the polymer rotation frequency changes qualitatively when hydrodynamic
interactions are switched on. 相似文献
2.
B. Spagnolo S. Spezia L. Curcio N. Pizzolato A. Fiasconaro D. Valenti P. Lo Bue E. Peri S. Colazza 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,69(1):133-146
We investigate the role of the colored noise in two
biological systems: (i) adults of Nezara viridula (L.)
(Heteroptera: Pentatomidae), and (ii) polymer translocation. In the
first system we analyze, by directionality tests, the response of
N. viridula individuals to subthreshold signals plus noise
in their mating behaviour. The percentage of insects that react to
the subthreshold signal shows a nonmonotonic behaviour,
characterized by the presence of a maximum, as a function of the
noise intensity. This is the signature of the non-dynamical
stochastic resonance phenomenon. By using a “soft” threshold model
we find that the maximum of the input-output cross correlation
occurs in the same range of noise intensity values for which the
behavioural activation of the insects has a maximum. Moreover this
maximum value is lowered and shifted towards higher noise
intensities, compared to the case of white noise. In the second
biological system the noise driven translocation of short polymers
in crowded solutions is analyzed. An improved version of the Rouse
model for a flexible polymer is adopted to mimic the molecular
dynamics by taking into account both the interactions between
adjacent monomers and the effects of a Lennard-Jones potential
between all beads. The polymer dynamics is simulated in a
two-dimensional domain by numerically solving the Langevin equations
of motion in the presence of thermal fluctuations and a colored
noise source. At low temperatures or for strong colored noise
intensities the translocation process of the polymer chain is
delayed. At low noise intensity, as the polymer length increases, we
find a nonmonotonic behaviour for the mean first translocation time
of the polymer centre of inertia. We show how colored noise
influences the motion of short polymers, by inducing two different
regimes of translocation in the dynamics of molecule transport. 相似文献
3.
We present a study of the dynamics of single polymers colliding with molecular obstacles using Molecular-dynamics simulations.
In concert with these simulations we present a generalized polymer-obstacle collision model which is applicable to a number
of collision scenarios. The work focusses on three specific problems: i) a polymer driven by an external force colliding with
a fixed microscopic post; ii) a polymer driven by a (plug-like) fluid flow colliding with a fixed microscopic post; and iii)
a polymer driven by an external force colliding with a free polymer. In all three cases, we present a study of the length-dependent
dynamics of the polymers involved. The simulation results are compared with calculations based on our generalized collision
model. The generalized model yields analytical results in the first two instances (cases i) and ii)), while in the polymer-polymer
collision example (case iii)) we obtain a series solution for the system dynamics. For the case of a polymer-polymer collision
we find that a distinct V-shaped state exists as seen in experimental systems, though normally associated with collisions
with multiple polymers. We suggest that this V-shaped state occurs due to an effective hydrodynamic counter flow generated
by a net translational motion of the two-chain system. 相似文献
4.
We present an analytical approach of the dynamics of a polymer when it is quenched from a solvent into a good or bad solvent. The dynamics is studied by means of a Langevin equation, first in the absence of hydrodynamic
effect, then taking into account the hydrodynamic interactions with the solvent. The variation of the radius of gyration is
studied as a function of time. In both cases, for the first stage of collapse or swelling, the evolution is described by a
power law with a characteristic time proportional to N
4/3
(N), where N is the number of monomers, without (with) hydrodynamic interactions. At larger times, scaling laws are derived for the diffusive
relaxation time.
Received: 10 March 1998 / Received in final form: 15 September 1998 / Accepted: 25 September 1998 相似文献
5.
For a polyelectrolyte undergoing electrophoretic motion, it is predicted (D. Long, J.L. Viovy, A. Ajdari, Phys. Rev. Lett.
76, 3858 (1996); D. Long, A. Ajdari, Electrophoresis 17, 1161 (1996)) that the mechanical force necessary to stall the molecule is substantially smaller than the sum of electrical
forces applied on all monomers. In fact, it should be proportional to its hydrodynamic friction coefficient and therefore
to the size of its conformation. In our work we examine this prediction using coarse-grained molecular-dynamics simulations
in which we explicitly include the polymer, the solvent, the counterions and salt. The electrophoretic mobility of polyelectrolytes
is evaluated, the mechanical force necessary to keep the molecules tethered is measured and the resulting anisotropic polymer
conformations are observed and quantified. Our results corroborate Long et al.'s prediction. 相似文献
6.
Mader MA Ez-Zahraouy H Misbah C Podgorski T 《The European physical journal. E, Soft matter》2007,22(4):275-280
A simple 2D model of deformable vesicles tumbling in a shear under flow is introduced in order to
account for the main qualitative features observed experimentally as shear rates are increased.
The simplicity of the model allows for a full analytical
tractability while retaining the essential physical ingredients. The model reveals that the main axes of the vesicle
undergo oscillations which are coupled to the vesicle orientation in
the flow. The model reproduces and sheds light on the main novel features reported in
recent experiments [M. Mader et al., Eur. Phys. J. E. 19, 389
(2006)], namely that both coefficients A and B that enter the
Keller-Skalak equation, dψ/dt = A+Bcos(2 ψ) (ψ is
the vesicle orientation angle in the shear flow), undergo a collapse
upon increasing shear rate. 相似文献
7.
U. Seifert 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(3):405-415
The dynamics of a single fluid bilayer membrane in an external hydrodynamic flow field is considered. The deterministic equation
of motion for the configuration is derived taking into account both viscous dissipation in the surrounding liquid and local
incompressibility of the membrane. For quasi-spherical vesicles in shear flow, thermal fluctuations can be incorporated in
a Langevin-type equation of motion for the deformation amplitudes. The solution to this equation shows an overdamped oscillatory
approach to a stationary tanktreading shape. Inclination angle and ellipticity of the contour are determined as a function
of excess area and shear rate. Comparisons to numerical results and experiments are discussed.
Received 20 August 1998 相似文献
8.
Juan D. Olarte-Plata 《Molecular physics》2018,116(15-16):2032-2040
ABSTRACTWe investigate the dependence of the shear viscosity of suspensions of spherical colloids as a function of the volume fraction of the suspension, the colloid–colloid interactions and the shear rate. We couple molecular dynamics to describe the motion of the colloids with stochastic rotation dynamics (MD–SRD) for the fluid environment by means of stochastic collisions, in order to incorporate hydrodynamics effects leading to non-newtonian responses. The shear viscosity is computed using non-equilibrium simulations by imposing explicit velocity gradients. The impact of the colloid–colloid interactions is examined by modelling the inter-colloid pair potential with a repulsive power law, that allows interpolating different degrees of colloidal softness. The general rheological behaviour of our suspensions can be described with a Krieger–Dougherty like equation, which must be corrected to account for the variations in the maximum packing fraction and non-equilibrium effects arising from the flux of momentum imposed to the suspension, which appear when varying the softness of the inter-colloidal interactions. We further show evidence for non-newtonian behaviour at high Péclet numbers, characterised both by shear thinning and shear thickening, and thus demonstrate these effects can be successfully captured using MD–SRD methods. 相似文献
9.
Dynamics of a tethered polymer in shear flow 总被引:1,自引:0,他引:1
The dynamics of a single polymer tethered to a solid surface in a shear flow was observed using fluorescently labeled DNA chains. Dramatic shear enhanced temporal fluctuations in the chain extension were observed. The rate of these fluctuations initially decreased for increasing shear rate gamma; and increased above a critical gamma;. Simulations revealed that these anomalous dynamics arise from a continual recirculating motion of the chain or cyclic dynamics. These dynamics arise from a coupling of the chain velocity in the flow direction to thermally driven fluctuations of the chain in the shear gradient direction. 相似文献
10.
Faezeh Pousaneh 《Molecular physics》2020,118(4)
We present molecular dynamics simulations of pseudo hard sphere fluid (generalized WCA potential with exponents (50, 49) proposed by Jover et al. [J. Chem. Phys 137, (2012)] using GROMACS package. The equation of state and radial distribution functions at contact are obtained from simulations and compared to the available theory of true hard spheres (HS) and available data on pseudo hard spheres. The comparison shows agreements with data by Jover et al. and the Carnahan–Starling equation of HS. The shear viscosity is obtained from the simulations and compared to the Enskog expression and previous HS simulations. It is demonstrated that the PHS potential reproduces the HS shear viscosity accurately. 相似文献
11.
M. Cosentino Lagomarsino B. Bassetti P. Jona 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(1):81-88
We introduce a model system of stochastic entities, called rowers which include some essentials of the behavior of real cilia. We introduce and discuss the problem of symmetry breaking for
these objects and its connection with the onset of macroscopic, directed flow in the fluid. We perform a mean field-like calculation
showing that hydrodynamic interaction may provide for the symmetry breaking mechanism and the onset of fluid flow. Finally,
we discuss the problem of the metachronal wave in a stochastic context through an analytical calculation based on a path integral
representation of our model equation.
Received 12 June 2001 and Received in final form 9 January 2002 相似文献
12.
The detection of linear polymers translocating through a nanoscopic pore is a promising idea for the development of new DNA
analysis techniques. However, the physics of constrained macromolecules and the fluid that surrounds them at the nanoscopic
scale is still not well understood. In fact, many theoretical models of polymer translocation neglect both excluded-volume
and hydrodynamic effects. We use Molecular Dynamics simulations with explicit solvent to study the impact of hydrodynamic
interactions on the translocation time of a polymer. The translocation time τ that we examine is the unbiased (no charge on
the chain and no driving force) escape time of a polymer that is initially placed halfway through a pore perforated in a monolayer
wall. In particular, we look at the effect of increasing the pore radius when only a small number of fluid particles can be
located in the pore as the polymer undergoes translocation, and we compare our results to the theoretical predictions of Chuang
et al. (Phys. Rev. E 65, 011802 (2001)). We observe that the scaling of the translocation time varies from τ ∼ N
11/5 to τ ∼ N
9/5 as the pore size increases (N is the number of monomers that goes up to 31 monomers). However, the scaling of the polymer relaxation time remains consistent
with the 9/5 power law for all pore radii. 相似文献
13.
We investigate stationary nonequilibrium states of systems of particles moving according to Hamiltonian dynamics with specified potentials. The systems are driven away from equilibrium by Maxwell-demon reflection rules at the walls. These deterministic rules conserve energy but not phase space volume, and the resulting global dynamics may or may not be time reversible (or even invertible). Using rules designed to simulate moving walls, we can obtain a stationary shear flow. Assuming that for macroscopic systems this flow satisfies the Navier-Stokes equations, we compare the hydrodynamic entropy production with the average rate of phase-space volume compression. We find that they are equalwhen the velocity distribution of particles incident on the walls is a local Maxwellian. An argument for a general equality of this kind, based on the assumption of local thermodynamic equilibrium, is given. Molecular dynamic simulations of hard disks in a channel produce a steady shear flow with the predicted behavior. 相似文献
14.
V. Fourmaux-Demange A. Brûlet F. Boué P. Davidson P. Keller J.P. Cotton 《The European physical journal. E, Soft matter》2000,1(4):301-317
We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of
the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose,
two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic
phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization
N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation
processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase,
the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order
to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t
r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine:
is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t
r
1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t
r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are
produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly,
the relaxation is the same at all scales of the whole polymer chain.
Received 5 May 1999 and Received in final form 18 October 1999 相似文献
15.
Mader MA Vitkova V Abkarian M Viallat A Podgorski T 《The European physical journal. E, Soft matter》2006,19(4):389-397
The dynamics of giant lipid vesicles under shear flow is experimentally investigated. Consistent with previous theoretical
and numerical studies, two flow regimes are identified depending on the viscosity ratio between the interior and the exterior
of the vesicle, and its reduced volume or excess surface. At low viscosity ratios, a tank-treading motion of the membrane
takes place, the vesicle assuming a constant orientation with respect to the flow direction. At higher viscosity ratios, a
tumbling motion is observed in which the whole vesicle rotates with a periodically modulated velocity. When the shear rate
increases, this tumbling motion becomes increasingly sensitive to vesicle deformation due to the elongational component of
the flow and significant deviations from simpler models are observed. A good characterization of these various flow regimes
is essential for the validation of analytical and numerical models, and to relate microscopic dynamics to macroscopic rheology
of suspensions of deformable particles, such as blood. 相似文献
16.
Obermayer B Hallatschek O Frey E Kroy K 《The European physical journal. E, Soft matter》2007,23(4):375-388
We analyze the nonequilibrium dynamics of single inextensible semiflexible biopolymers as stretching forces are applied at
the ends. Based on different (contradicting) heuristic arguments, various scaling laws have been proposed for the propagation
speed of the backbone tension which is induced in response to stretching. Here, we employ a newly developed unified theory
to systematically substantiate, restrict, and extend these approaches. Introducing the practically relevant scenario of a
chain equilibrated under some prestretching force f
pre that is suddenly exposed to a different external force f
ext at the ends, we give a concise physical explanation of the underlying relaxation processes by means of an intuitive blob
picture. We discuss the corresponding intermediate asymptotics, derive results for experimentally relevant observables, and
support our conclusions by numerical solutions of the coarse-grained equations of motion for the tension. 相似文献
17.
Mattioni L Wittmer JP Baschnagel J Barrat JL Luijten E 《The European physical journal. E, Soft matter》2003,10(4):369-385
Correlations in the motion of reptating polymers in a melt are investigated by means of Monte Carlo simulations of the three-dimensional
slithering-snake version of the bond-fluctuation model. Surprisingly, the slithering-snake dynamics becomes inconsistent with
classical reptation predictions at high chain overlap (created either by chain length N or by the volume fraction φ of occupied lattice sites), where the relaxation times increase much faster than expected. This
is due to the anomalous curvilinear diffusion in a finite time window whose upper bound (N) is set by the density of chain ends φ/N. Density fluctuations created by passing chain ends allow a reference polymer to break out of the local cage of immobile
obstacles created by neighboring chains. The dynamics of dense solutions of “snakes” at t ≪ is identical to that of a benchmark system where all chains but one are frozen. We demonstrate that the subdiffusive dynamical
regime is caused by the slow creeping of a chain out of its correlation hole. Our results are in good qualitative agreement
with the activated-reptation scheme proposed recently by Semenov and Rubinstein (Eur. Phys. J. B, 1 (1998) 87). Additionally, we briefly comment on the relevance of local relaxation pathways within a slithering-snake scheme.
Our preliminary results suggest that a judicious choice of the ratio of local to slithering-snake moves is crucial to equilibrate
a melt of long chains efficiently.
Received: 18 December 2002 / Accepted: 3 April 2003 / Published online: 12 May 2003
RID="a"
ID="a"e-mail: jwittmer@dpm.univ-lyon1.fr
RID="b"
ID="b"Current address: University of Illinois at Urbana-Champaign. 相似文献
18.
Reverse non-equilibrium molecular dynamics was applied for the calculation of the viscosity for different chain lengths. Each chain consisted of m tangent spherical sites, where m was 1, 2, 4, 8 or 16, respectively. From these results, shear thinning was observed at high shear rates. The normal stress forces were also estimated via the calculation of the total stress tensor, and they were related to the shear thinning effect depending on the length of the chain. Furthermore, a power law equation was used to fit the rheological curves of each chain, making possible the calculation of the viscoelasticity as a function of the sites involved in the chains. 相似文献
19.
Based on a recently established formalism [U. Ebert, J. Stat. Phys.
82:183 (1996)], we analyze the diffusive motion of a long polymer in a quenched random medium. The medium is modeled by a frozen semidilute polymer system. In the framework of standard renormalization group (RG) theory we present a systematic perturbative approach to handle such a many-chain system. In contrast to the work cited above, here we deal with long-range correlated disorder and find an attractive RG fixed point. Unlike in polymer statics, the semidilute limit here yields new nontrivial power laws for dynamic quantities. The exponents are intermediate between the Rouse and reptation results. An explicit one-loop calculation for the center-of-mass motion is given. 相似文献
20.
A. Milchev J.P. Wittmer D.P. Landau 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):241-251
We use an off-lattice microscopic model for solutions of equilibrium polymers (EP) in a lamellar shear flow generated by means
of a self-consistent external field between parallel hard walls. The individual conformations of the chains are found to elongate
in flow direction and shrink perpendicular to it while the average polymer length decreases with increasing shear rate. The
Molecular Weight Distribution of the chain lengths retains largely its exponential form in dense solutions whereas in dilute
solutions it changes from a power-exponential Schwartz distribution to a purely exponential one upon an increase of the shear
rate. With growing shear rate the system becomes increasingly inhomogeneous so that a characteristic variation of the total
monomer density, the diffusion coefficient, and the center-of-mass distribution of polymer chains of different contour length
with the velocity of flow is observed. At higher temperature, as the average chain length decreases significantly, the system is shown to undergo an order-disorder transition
into a state of nematic liquid crystalline order with an easy direction parallel to the hard walls. The influence of shear
flow on this state is briefly examined.
Received 22 October 1998 and Received in final form 12 April 1999 相似文献