804 research outputs found
Attractive forces between anisotropic inclusions in the membrane of a vesicle
The fluctuation-induced interaction between two rod-like, rigid inclusions in
a fluid vesicle is studied by means of canonical ensemble Monte Carlo
simulations. The vesicle membrane is represented by a triangulated network of
hard spheres. Five rigidly connected hard spheres form rod-like inclusions that
can leap between sites of the triangular network. Their effective interaction
potential is computed as a function of mutual distance and angle of the
inclusions. On account of the hard-core potential among these, the nature of
the potential is purely entropic. Special precaution is taken to reduce lattice
artifacts and the influence of finite-size effects due to the spherical
geometry. Our results show that the effective potential is attractive and
short-range compared with the rod length L. Its well depth is of the order of
\kappa/10, where \kappa is the bending modulus.Comment: 7 pages, 5 eps + 3 latex figures. REVTeX. Submitted to Eur. Phys. J.
Absence of simulation evidence for critical depletion in slit-pores
Recent Monte Carlo simulation studies of a Lennard-Jones fluid confined to a
mesoscopic slit-pore have reported evidence for ``critical depletion'' in the
pore local number density near the liquid-vapour critical point. In this note
we demonstrate that the observed depletion effect is in fact a simulation
artifact arising from small systematic errors associated with the use of long
range corrections for the potential truncation. Owing to the large
near-critical compressibility, these errors lead to significant changes in the
pore local number density. We suggest ways of avoiding similar problems in
future studies of confined fluids.Comment: 4 pages Revtex. Submitted to J. Chem. Phy
Mean-field density functional theory of a nanoconfined classical, three-dimensional Heisenberg fluid. II. The interplay between molecular packing and orientational order
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 149, 054704 (2018) and may be found at https://doi.org/10.1063/1.5040934.As in Paper I of this series of papers [S. M. Cattes et al., J. Chem. Phys. 144, 194704 (2016)], we study a Heisenberg fluid confined to a nanoscopic slit pore with smooth walls. The pore walls can either energetically discriminate specific orientations of the molecules next to them or are indifferent to molecular orientations. Unlike in Paper I, we employ a version of classical density functional theory that allows us to explicitly account for the stratification of the fluid (i.e., the formation of molecular layers) as a consequence of the symmetry-breaking presence of the pore walls. We treat this stratification within the White Bear version (Mark I) of fundamental measure theory. Thus, in this work, we focus on the interplay between local packing of the molecules and orientational features. In particular, we demonstrate why a critical end point can only exist if the pore walls are not energetically discriminating specific molecular orientations. We analyze in detail the positional and orientational order of the confined fluid and show that reorienting molecules across the pore space can be a two-dimensional process. Last but not least, we propose an algorithm based upon a series expansion of Bessel functions of the first kind with which we can solve certain types of integrals in a very efficient manner.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface
Mean-field density functional theory of ananoconfined classical, three-dimensional Heisenberg fluid. I. The role of molecularanchoring
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 144, 194704 (2016) and may be found at https://doi.org/10.1063/1.4949330.In this work, we employ classical density functional theory (DFT) to investigate for the first time equilibrium properties of a Heisenberg fluid confined to nanoscopic slit pores of variable width. Within DFT pair correlations are treated at modified mean-field level. We consider three types of walls: hard ones, where the fluid-wall potential becomes infinite upon molecular contact but vanishes otherwise, and hard walls with superimposed short-range attraction with and without explicit orientation dependence. To model the distance dependence of the attractions, we employ a Yukawa potential. The orientation dependence is realized through anchoring of molecules at the substrates, i.e., an energetic discrimination of specific molecular orientations. If the walls are hard or attractive without specific anchoring, the results are âquasi-bulkâ-like in that they can be linked to a confinement-induced reduction of the bulk mean field. In these cases, the precise nature of the walls is completely irrelevant at coexistence. Only for specific anchoring nontrivial features arise, because then the fluid-wall interaction potential affects the orientation distribution function in a nontrivial way and thus appears explicitly in the Euler-Lagrange equations to be solved for minima of the grand potential of coexisting phases.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface
Structure and Dynamics of Solvated Polymers near a Silica Surface: On the Different Roles Played by Solvent
Whereas it is experimentally known that the inclusion of nanoparticles in
hydrogels can lead to a mechanical reinforcement, a detailed molecular
understanding of the adhesion mechanism is still lacking. Here we use
coarse-grained molecular dynamics simulations to investigate the nature of the
interface between silica surfaces and solvated polymers. We show how
differences in the nature of the polymer and the polymer--solvent interactions
can lead to drastically different behavior of the polymer--surface adhesion.
Comparing explicit and implicit solvent models, we conclude that this effect
cannot be fully described in an implicit solvent. We highlight the crucial role
of polymer solvation for the adsorption of the polymer chain on the silica
surface, the significant dynamics of polymer chains on the surface, and details
of the modifications in the structure solvated polymer close to the interface
Defect topologies in chiral liquid crystals confined to mesoscopic channels
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 142, 194704 (2015) and may be found at https://doi.org/10.1063/1.4920979.We present Monte Carlo simulations in the grand canonical and canonical ensembles of a chiral liquid crystal confined to mesochannels of variable sizes and geometries. The mesochannels are taken to be quasi-infinite in one dimension but finite in the two other directions. Under thermodynamic conditions chosen and for a selected value of the chirality coupling constant, the bulk liquid crystal exhibits structural characteristics of a blue phase II. This is established through the tetrahedral symmetry of disclination lines and the characteristic simple-cubic arrangement of double-twist helices formed by the liquid-crystal molecules along all three axes of a Cartesian coordinate system. If the blue phase II is then exposed to confinement, the interplay between its helical structure, various anchoring conditions at the walls of the mesochannels, and the shape of the mesochannels gives rise to a broad variety of novel, qualitative disclination-line structures that are reported here for the first time.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface
Entropy-driven enhanced self-diffusion in confined reentrant supernematics
We present a molecular dynamics study of reentrant nematic phases using the
Gay-Berne-Kihara model of a liquid crystal in nanoconfinement. At densities
above those characteristic of smectic A phases, reentrant nematic phases form
that are characterized by a large value of the nematic order parameter
. Along the nematic director these "supernematic" phases exhibit a
remarkably high self-diffusivity which exceeds that for ordinary, lower-density
nematic phases by an order of magnitude. Enhancement of self-diffusivity is
attributed to a decrease of rotational configurational entropy in confinement.
Recent developments in the pulsed field gradient NMR technique are shown to
provide favorable conditions for an experimental confirmation of our
simulations.Comment: 10 pages, 5 figure
On the dynamics of exotic matter: towards creation of Perpetuum Mobile of third kind
The one-dimensional dynamics of a classical ideal 'exotic' fluid with
equation of state violating the weak energy condition is
discussed. Under certain assumptions it is shown that the well-known
Hwa-Bjorken exact solution of one-dimensional relativistic hydrodynamics is
confined within the future/past light cone. It is also demonstrated that the
total energy of such a solution is equal to zero and that there are regions
within the light cone with negative and positive total energies.
For certain equations of state there is a continuous energy transfer from the
-regions to the -regions resulting in indefinite growth of energy in
the regions with time, which may be interpreted as action of a specific
'Perpetuum Mobile' (Perpetuum Motion). It is speculated that if it is possible
to construct a three-dimensional non-stationary flow of an exotic fluid having
a finite negative value of energy such a situation would also occur. Such a
flow may continuously transfer positive energy to gravitational waves,
resulting in a runaway. It is conjectured that theories plagued by such
solutions should be discarded as inherently unstable.Comment: accepted to Physics Letters
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