8,179 research outputs found
Knudsen pump inspired by Crookes radiometer with a specular wall
A rarefied gas is considered in a channel consisting of two infinite parallel
plates between which an evenly spaced array of smaller plates is arranged
normal to the channel direction. Each of these smaller plates is assumed to
possess one ideally specularly reflective and one ideally diffusively
reflective side. When the temperature of the small plates differs from the
temperature of the sidewalls of the channel, these boundary conditions result
in a temperature profile around the edges of each small plate which breaks the
reflection symmetry along the channel direction. This in turn results in a
force on each plate and a net gas flow along the channel. The situation is
analysed numerically using the direct simulation Monte Carlo (DSMC) method and
compared with analytical results where available. The influence of the ideally
specularly reflective wall is assessed by comparing with simulations using a
finite accommodation coefficient at the corresponding wall. The configuration
bears some similarity with a Crookes radiometer, where a non-symmetric
temperature profile at the radiometer vanes is generated by different
temperatures on each side of the vane, resulting in a motion of the rotor. The
described principle may find applications in pumping gas on small scales driven
by temperature gradients
Dissipative particle dynamics for interacting systems
We introduce a dissipative particle dynamics scheme for the dynamics of
non-ideal fluids. Given a free-energy density that determines the
thermodynamics of the system, we derive consistent conservative forces. The use
of these effective, density dependent forces reduces the local structure as
compared to previously proposed models. This is an important feature in
mesoscopic modeling, since it ensures a realistic length and time scale
separation in coarse-grained models. We consider in detail the behavior of a
van der Waals fluid and a binary mixture with a miscibility gap. We discuss the
physical implications of having a single length scale characterizing the
interaction range, in particular for the interfacial properties.Comment: 25 pages, 12 figure
Advantages and challenges in coupling an ideal gas to atomistic models in adaptive resolution simulations
In adaptive resolution simulations, molecular fluids are modeled employing
different levels of resolution in different subregions of the system. When
traveling from one region to the other, particles change their resolution on
the fly. One of the main advantages of such approaches is the computational
efficiency gained in the coarse-grained region. In this respect the best
coarse-grained system to employ in the low resolution region would be the ideal
gas, making intermolecular force calculations in the coarse-grained subdomain
redundant. In this case, however, a smooth coupling is challenging due to the
high energetic imbalance between typical liquids and a system of
non-interacting particles. In the present work, we investigate this approach,
using as a test case the most biologically relevant fluid, water. We
demonstrate that a successful coupling of water to the ideal gas can be
achieved with current adaptive resolution methods, and discuss the issues that
remain to be addressed
Mesoscale Structures at Complex Fluid-Fluid Interfaces: a Novel Lattice Boltzmann / Molecular Dynamics Coupling
Complex fluid-fluid interfaces featuring mesoscale structures with adsorbed
particles are key components of newly designed materials which are continuously
enriching the field of soft matter. Simulation tools which are able to cope
with the different scales characterizing these systems are fundamental
requirements for efficient theoretical investigations. In this paper we present
a novel simulation method, based on the approach of Ahlrichs and D\"unweg
[Ahlrichs and D\"unweg, Int. J. Mod. Phys. C, 1998, 9, 1429], that couples the
"Shan-Chen" multicomponent Lattice Boltzmann technique to off-lattice molecular
dynamics to simulate efficiently complex fluid-fluid interfaces.
We demonstrate how this approach can be used to study a wide class of
challenging problems. Several examples are given, with an accent on
bicontinuous phases formation in polyelectrolyte solutions and ferrofluid
emulsions. We also show that the introduction of solvation free energies in the
particle-fluid interaction unveils the hidden, multiscale nature of the
particle-fluid coupling, allowing to treat symmetrically (and interchangeably)
the on-lattice and off-lattice components of the system
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