669 research outputs found
Effective slip boundary conditions for flows over nanoscale chemical heterogeneities
We study slip boundary conditions for simple fluids at surfaces with
nanoscale chemical heterogeneities. Using a perturbative approach, we examine
the flow of a Newtonian fluid far from a surface described by a heterogeneous
Navier slip boundary condition. In the far-field, we obtain expressions for an
effective slip boundary condition in certain limiting cases. These expressions
are compared to numerical solutions which show they work well when applied in
the appropriate limits. The implications for experimental measurements and for
the design of surfaces that exhibit large slip lengths are discussed.Comment: 14 pages, 3 figure
Nanoscale fluid flows in the vicinity of patterned surfaces
Molecular dynamics simulations of dense and rarefied fluids comprising small
chain molecules in chemically patterned nano-channels predict a novel switching
from Poiseuille to plug flow along the channel. We also demonstrate behavior
akin to the lotus effect for a nanodrop on a chemically patterned substrate.
Our results show that one can control and exploit the behavior of fluids at the
nanoscale using chemical patterning.Comment: Phys. Rev. Lett. in pres
Capillary rise dynamics of liquid hydrocarbons in mesoporous silica as explored by gravimetry, optical and neutron imaging: Nano-rheology and determination of pore size distributions from the shape of imbibition fronts
We present gravimetrical, optical, and neutron imaging measurements of the
capillarity-driven infiltration of mesoporous silica glass by hydrocarbons.
Square-root-of-time Lucas-Washburn invasion kinetics are found for linear
alkanes from n-decane (C10) to n-hexacontane (C60) and for squalane, a branched
alkane, in porous Vycor with 6.5 nm or 10 nm pore diameter, respectively.
Humidity-dependent experiments allow us to study the influence on the
imbibition kinetics of water layers adsorbed on the pore walls. Except for the
longest molecule studied, C60, the invasion kinetics can be described by bulk
fluidity and bulk capillarity, provided we assume a sticking, pore-wall
adsorbed boundary layer, i.e. a monolayer of water covered by a monolayer of
flat-laying hydrocarbons. For C60, however, an enhanced imbibition speed
compared to the value expected in the bulk is found. This suggests the onset of
velocity slippage at the silica walls or a reduced shear viscosity due to the
transition towards a polymer-like flow in confined geometries. Both, light
scattering and neutron imaging indicate a pronounced roughening of the
imbibition fronts. Their overall shape and width can be resolved by neutron
imaging. The fronts can be described by a superposition of independent wetting
fronts moving with pore size-dependent square-root-of-time laws and weighted
according to the pore size distributions obtained from nitrogen gas sorption
isotherms. This finding indicates that the shape of the imbibition front in a
porous medium, such as Vycor glass, with interconnected, elongated pores, is
solely determined by independent movements of liquid menisci. These are
dictated by the Laplace pressure and hydraulic permeability variations and thus
the pore size variation at the invasion front. Our results suggest that pore
size distributions can be derived from the broadening of imbibition fronts.Comment: 28 pages, 12 figures, pre-print, in pres
Fluid transport through heterogeneous pore matrices: Multiscale simulation approaches
Fluids confined in nanopores exhibit several unique structural and dynamical characteristics that affect a number of applications in industry as well as natural phenomena. Understanding and predicting the complex fluid behavior under nano-confinement is therefore of key importance, and both experimental and computational approaches have been employed toward this goal. It is now feasible to employ both simulations and theoretical methods, the results of which can be validated by cutting-edge experimental quantification. Nevertheless, predicting fluid transport through heterogeneous pore networks at a scale large enough to be relevant for practical applications remains elusive because one should account for a variety of fluid–rock interactions, a wide range of confined fluid states, as well as pore-edge effects and the existence of preferential pathways, which, together with many other phenomena, affect the results. The aim of this Review is to overview the significance of molecular phenomena on fluid transport in nanoporous media, the capability and shortcomings of both molecular and continuum fluid modeling approaches, and recent progress in multiscale modeling of fluid transport. In our interpretation, a multiscale approach couples a molecular picture for fluid interactions with solid surfaces at the single nanopore level with hierarchical transport analysis through realistic heterogeneous pore networks to balance physical accuracy with computational expense. When possible, comparison against experiments is provided as a guiding roadmap for selecting the appropriate computational methods. The appropriateness of an approach is certainly related to the final application of interest, as different sectors will require different levels of precision in the predictions
Review of the Synergies Between Computational Modeling and Experimental Characterization of Materials Across Length Scales
With the increasing interplay between experimental and computational
approaches at multiple length scales, new research directions are emerging in
materials science and computational mechanics. Such cooperative interactions
find many applications in the development, characterization and design of
complex material systems. This manuscript provides a broad and comprehensive
overview of recent trends where predictive modeling capabilities are developed
in conjunction with experiments and advanced characterization to gain a greater
insight into structure-properties relationships and study various physical
phenomena and mechanisms. The focus of this review is on the intersections of
multiscale materials experiments and modeling relevant to the materials
mechanics community. After a general discussion on the perspective from various
communities, the article focuses on the latest experimental and theoretical
opportunities. Emphasis is given to the role of experiments in multiscale
models, including insights into how computations can be used as discovery tools
for materials engineering, rather than to "simply" support experimental work.
This is illustrated by examples from several application areas on structural
materials. This manuscript ends with a discussion on some problems and open
scientific questions that are being explored in order to advance this
relatively new field of research.Comment: 25 pages, 11 figures, review article accepted for publication in J.
Mater. Sc
Contact-Line Behavior in Boiling on A Heterogeneous Surface : Physical Insights from Diffuse-Interface Modeling
journal articl
Multiscale Poromechanics of Wet Cement Paste
Capillary effects such as imbibition-drying cycles impact the mechanics of
granular systems over time. A multiscale poromechanics framework was applied to
cement paste, that is the most common building material, experiencing broad
humidity variations over the lifetime of infrastructure. First, the liquid
density distribution at intermediate to high relative humidities is obtained
using a lattice gas density functional method together with a realistic
nano-granular model of cement hydrates. The calculated adsorption/desorption
isotherms and pore size distributions are discussed and compare well to
nitrogen and water experiments. The standard method for pore size distribution
determination from desorption data is evaluated. Then, the integration of the
Korteweg liquid stress field around each cement hydrate particle provided the
capillary forces at the nanoscale. The cement mesoscale structure was relaxed
under the action of the capillary forces. Local irreversible deformations of
the cement nano-grains assembly were identified due to liquid-solid
interactions. The spatial correlations of the nonaffine displacements extend to
a few tens of nm. Finally, the Love-Weber method provided the homogenized
liquid stress at the micronscale. The homogenization length coincided with the
spatial correlation length nonaffine displacements. Our results on the solid
response to capillary stress field suggest that the micronscale texture is not
affected by mild drying, while local irreversible deformations still occur.
These results pave the way towards understanding capillary phenomena induced
stresses in heterogeneous porous media ranging from construction materials,
hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi
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