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Structure of liquid and glassy methanol confined in cylindrical pores  [PDF]
Denis Morineau,Regis Guegan,Yongde Xia,Christiane Alba-Simionesco
Physics , 2004, DOI: 10.1063/1.1762872
Abstract: We present a neutron scattering analysis of the density and the static structure factor of confined methanol at various temperatures. Confinement is performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24 angstrom and D=35 angstrom. A change of the thermal expansivity of confined methanol at low temperature is the signature of a glass transition, which occurs at higher temperature for the smallest pore. This is an evidence of a surface induced slowing down of the dynamics of the fluid. The structure factor presents a systematic evolution with the pore diameter, which has been analyzed in terms of excluded volume effects and fluid-matrix cross-correlation. Conversely to the case of Van der Waals fluids, it shows that stronger fluid-matrix correlations must be invoked most probably in relation with the H-bonding character of both methanol and silicate surface.
Local order variations in confined hard-sphere fluids  [PDF]
Kim Nyg?rd,Sten Sarman,Roland Kjellander
Physics , 2013, DOI: 10.1063/1.4825176
Abstract: Pair distributions of fluids confined between two surfaces at close distance are of fundamental importance for a variety of physical, chemical, and biological phenomena, such as interactions between macromolecules in solution, surface forces, and diffusion in narrow pores. However, in contrast to bulk fluids, properties of inhomogeneous fluids are seldom studied at the pair-distribution level. Motivated by recent experimental advances in determining anisotropic structure factors of confined fluids, we analyze theoretically the underlying anisotropic pair distributions of the archetypical hard-sphere fluid confined between two parallel hard surfaces using first-principles statistical mechanics of inhomogeneous fluids. For this purpose, we introduce an experimentally accessible ensemble-averaged local density correlation function and study its behavior as a function of confining slit width. Upon increasing the distance between the confining surfaces, we observe an alternating sequence of strongly anisotropic versus more isotropic local order. The latter is due to packing frustration of the spherical particles. This observation highlights the importance of studying inhomogeneous fluids at the pair-distribution level.
Scaling of thermal conductivity of helium confined in pores  [PDF]
Kwangsik Nho,Efstratios Manousakis
Physics , 2001, DOI: 10.1103/PhysRevB.64.144513
Abstract: We have studied the thermal conductivity of confined superfluids on a bar-like geometry. We use the planar magnet lattice model on a lattice $H\times H\times L$ with $L \gg H$. We have applied open boundary conditions on the bar sides (the confined directions of length $H$) and periodic along the long direction. We have adopted a hybrid Monte Carlo algorithm to efficiently deal with the critical slowing down and in order to solve the dynamical equations of motion we use a discretization technique which introduces errors only $O((\delta t)^6)$ in the time step $\delta t$. Our results demonstrate the validity of scaling using known values of the critical exponents and we obtained the scaling function of the thermal resistivity. We find that our results for the thermal resistivity scaling function are in very good agreement with the available experimental results for pores using the temp
Scaling of the specific heat of superfluids confined in pores  [PDF]
Norbert Schultka,Efstratios Manousakis
Physics , 1998,
Abstract: We investigate the scaling properties of the specific heat of the XY model on lattices H x H x L with L >> H (i.e. in a bar-like geometry) with respect to the thickness H of the bar, using the Cluster Monte Carlo method. We study the effect of the geometry and boundary conditions on the shape of the universal scaling function of the specific heat by comparing the scaling functions obtained for cubic, film, and bar-like geometry. In the presence of physical boundary conditions applied along the sides of the bars we find good agreement between our Monte Carlo results and the most recent experimental data for superfluid helium confined in pores.
Density profile of water confined in cylindrical pores in MCM-41 silica  [PDF]
Alan K Soper
Physics , 2011, DOI: 10.1088/0953-8984/24/6/064107
Abstract: It is shown that traditional estimates of the pore diameter in the porous silica material MCM-41-S15 (of order 15\AA) are too small to allow the amount of water that is absorbed by these materials (around 0.5gH2O/g substrate) to occur only inside the pore. Either the additional water is absorbed on the surface of the silica particles and outside the pores, or else the pores are larger than the traditional estimates. In addition the low Q Bragg intensities from a sample of MCM-41-S15 porous silica under different dry and wet conditions and with different hydrogen isotopes are simulated using a simple model of the water and silica density profile across the pore. It is found the best agreement of these intensities with experimental data is shown by assuming the much larger pore diameter of 25\AA (radius 12.5\AA). Qualitative agreement is found between these simulated density profiles and those found in recent empirical potential structure refinement simulations of the same data, even though the latter data did not specifically include the Bragg peaks in the structure refinement. It is shown that the change in the (100) peak intensity on cooling from 300K to 210K, which previously has been ascribed to a change in density of the confined water on cooling, can equally be ascribed to a change in density profile at constant average density. It is further pointed out that, independent of whether the pore diameter really is as large as 25\AA\ or whether a significant amount of water is absorbed outside the pore, the earlier reports of a dynamic cross-over in supercooled confined water could in fact be a crystallisation transition in the larger pore or surface water.
Star Polymers Confined in a Nanoslit: A Simulation Test of Scaling and Self-Consistent Field Theories  [PDF]
J. Paturej,A. Milchev,S. A. Egorov,K. Binder
Physics , 2013,
Abstract: The free energy cost of confining a star polymer where $f$ flexible polymer chains containing $N$ monomeric units are tethered to a central unit in a slit with two parallel repulsive walls a distance $D$ apart is considered, for good solvent conditions. Also the parallel and perpendicular components of the gyration radius of the star polymer, and the monomer density profile across the slit are obtained. Theoretical descriptions via Flory theory and scaling treatments are outlined, and compared to numerical self-consistent field calculations (applying the Scheutjens-Fleer lattice theory) and to Molecular Dynamics results for a bead-spring model. It is shown that Flory theory and self-consistent field (SCF) theory yield the correct scaling of the parallel linear dimension of the star with $N$, $f$ and $D$, but cannot be used for estimating the free energy cost reliably. We demonstrate that the same problem occurs already for the confinement of chains in cylindrical tubes. We also briefly discuss the problem of a free or grafted star polymer interacting with a single wall, and show that the dependence of confining force on the functionality of the star is different for a star confined in a nanoslit and a star interacting with a single wall, which is due to the absence of a symmetry plane in the latter case.
Phase equilibrium calculations for confined fluids, including surface tension prediction models
Espósito, R. O.;Tavares, F. W.;Castier, M.;
Brazilian Journal of Chemical Engineering , 2005, DOI: 10.1590/S0104-66322005000100010
Abstract: phase equilibrium calculations for fluids confined inside capillary tubes or porous media are formulated using the isofugacity equations. in this situation, the phase pressures are not equal and it is assumed that they are related by the laplace equation. with this formulation, existing procedures for phase equilibrium calculations can be readily modified to include capillary effects. in this paper, we review some of the main authors who have studied the behavior of fluids inside porous media and perform bubble- and dew-point calculations for pure components and mixtures, using the peng-robinson equation of state to model the coexisting phases and several planar surface tension models. comparisons with results from the literature indicate that the proposed formulation is adequate for representing phase equilibrium inside capillary tubes.
Comb-like polymers inside nanoscale pores  [PDF]
Cyprien Gay,Elie Raphael
Physics , 2000,
Abstract: A new method of polymer characterization, based on permeation studies using nanoscale pores, was recently proposed by Brochard and de Gennes. In the present paper, we study how this method, initially developped for star polymers can be extended to comb-like polymers.
Effects of substrate relaxation on adsorption in pores  [PDF]
Hye-Young Kim,Silvina M. Gatica,George Stan,Milton W. Cole
Physics , 2008, DOI: 10.1007/s10909-009-9884-0
Abstract: Fluids in porous media are commonly studied with analytical or simulation methods, usually assuming that the host medium is rigid. By evaluating the substrate's response (relaxation) to the presence of the fluid we assess the error inherent in that assumption. One application is a determination of the ground state of 3He in slit and cylindrical pores. With the relaxation, there results a much stronger cohesion than would be found for a rigid host. Similar increased binding effects of relaxation are found for classical fluids confined within slit pores or nanotube bundles.
Hydrodynamics of confined active fluids  [PDF]
Tommaso Brotto,Jean-Baptiste Caussin,Eric Lauga,Denis Bartolo
Physics , 2012, DOI: 10.1103/PhysRevLett.110.038101
Abstract: We theoretically describe the dynamics of swimmer populations confined in thin liquid films. We first demonstrate that hydrodynamic interactions between confined swimmers only depend on their shape and are independent of their specific swimming mechanism. We also show that due to friction with the walls, confined swimmers do not reorient due to flow gradients but the flow field itself. We then quantify the consequences of these microscopic interaction rules on the large-scale hydrodynamics of isotropic populations. We investigate in details their stability and the resulting phase behavior, highlighting the differences with conventional active, three-dimensional suspensions. Two classes of polar swimmers are distinguished depending on their geometrical polarity. The first class gives rise to coherent directed motion at all scales whereas for the second class we predict the spontaneous formation of coherent clusters (swarms).
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