1,362 research outputs found
Emergence of rheological properties in lattice Boltzmann simulations of gyroid mesophases
We use a lattice Boltzmann (LB) kinetic scheme for modelling amphiphilic
fluids that correctly predicts rheological effects in flow. No macroscopic
parameters are included in the model. Instead, three-dimensional hydrodynamic
and rheological effects are emergent from the underlying particulate
conservation laws and interactions. We report evidence of shear thinning and
viscoelastic flow for a self-assembled gyroid mesophase. This purely kinetic
approach is of general importance for the modelling and simulation of complex
fluid flows in situations when rheological properties cannot be predicted {\em
a priori}.Comment: 7 pages, 5 figure
Large-scale lattice Boltzmann simulations of complex fluids: advances through the advent of computational grids
During the last two years the RealityGrid project has allowed us to be one of
the few scientific groups involved in the development of computational grids.
Since smoothly working production grids are not yet available, we have been
able to substantially influence the direction of software development and grid
deployment within the project. In this paper we review our results from large
scale three-dimensional lattice Boltzmann simulations performed over the last
two years. We describe how the proactive use of computational steering and
advanced job migration and visualization techniques enabled us to do our
scientific work more efficiently. The projects reported on in this paper are
studies of complex fluid flows under shear or in porous media, as well as
large-scale parameter searches, and studies of the self-organisation of liquid
cubic mesophases.
Movies are available at
http://www.ica1.uni-stuttgart.de/~jens/pub/05/05-PhilTransReview.htmlComment: 18 pages, 9 figures, 4 movies available, accepted for publication in
Phil. Trans. R. Soc. London Series
Massively parallel molecular-continuum simulations with the macro-micro-coupling tool
Efficient implementations of hybrid molecular-continuum flow solvers are required to allow for fast and massively parallel simulations of large complex systems. Several coupling strategies have been proposed over the last years for 2D/ 3D, time-dependent/ steady-state or compressible/incompressible scenarios. Despite their different application areas, most of these schemes comprise the same or similar building blocks. Still, to the authors’ knowledge, no common implementation of these building blocks is available yet. In this contribution, the Macro-Micro-Coupling tool is presented which is meant to support developers in coupling mesh-based methods with molecular dynamics. It is written in C++ and supports two- and three-dimensional scenarios. Its design is reviewed, and aspects for massively parallel coupled scenarios are addressed. Afterwards, scaling results are presented for a hybrid simulation which couples a molecular dynamics code to the Lattice Boltzmann application of the Peano framework
Closed formula for the transport of micro-nano-particle across model porous media
In the last decade the Fick-Jacobs approximation has been exploited to
capture the transport across constrictions. Here, we review the derivation of
the Fick-Jacobs equation with particular emphasis on its linear response
regime. We show that for fore-aft symmetric channels the flux of
non-interacting systems is fully captured by its linear response regime. For
this case we derive a very simple formula that captures the correct trends and
that can be exploited as a simple tool to design experiments or simulations.
Finally, we show that higher order corrections in the flux may appear for
non-symmetric channels
Slip flow over structured surfaces with entrapped microbubbles
On hydrophobic surfaces, roughness may lead to a transition to a
superhydrophobic state, where gas bubbles at the surface can have a strong
impact on a detected slip. We present two-phase lattice Boltzmann simulations
of a Couette flow over structured surfaces with attached gas bubbles. Even
though the bubbles add slippery surfaces to the channel, they can cause
negative slip to appear due to the increased roughness. The simulation method
used allows the bubbles to deform due to viscous stresses. We find a decrease
of the detected slip with increasing shear rate which is in contrast to some
recent experimental results implicating that bubble deformation cannot account
for these experiments. Possible applications of bubble surfaces in microfluidic
devices are discussed.Comment: 4 pages, 4 figures. v2: revised version, to appear in Phys. Rev. Let
Electroneutrality breakdown for electrolytes embedded in varying-section nanopores
We determine the local charge dynamics of a electrolyte embedded in a
varying-section channel. By means of an expansion based on the length scale
separation between the axial and transverse direction of the channel, we derive
closed formulas for the local excess charge for both, dielectric and conducting
walls, in (planar geometry) as well as in (cylindrical geometry). Our
results show that, even at equilibrium, the local charge electroneutrality is
broken whenever the section of the channel is not homogeneous for both
dielectric and conducting walls as well as for and channels.
Interestingly, even within our expansion, the local excess charge in the fluid
can be comparable to the net charge on the walls. We critically discuss the
onset of such local electroneutrality breakdown in particular with respect to
the correction that it induces on the effective free energy profile experienced
by tracer ions
Turning catalytically active pores into active pumps
We develop a semi-analytical model of self-diffusioosmotic transport in
active pores, which includes advective transport and the inverse chemical
reaction which consumes solute. In previous work (Phys. Rev. Lett. 129, 188003,
2022), we have demonstrated the existence of a spontaneous symmetry breaking in
fore-aft symmetric pores that enables them to function as a micropump. We now
show that this pumping transition is controlled by three timescales. Two
timescales characterize advective and diffusive transport. The third timescale
corresponds to how long a solute molecule resides in the pore before being
consumed. Introducing asymmetry to the pore (either via the shape or the
catalytic coating) reveals a second type of advection-enabled transitions. In
asymmetric pores, the flow rate exhibits discontinuous jumps and hysteresis
loops upon tuning the parameters that control the asymmetry. This work
demonstrates the interconnected roles of shape and catalytic patterning in the
dynamics of active pores, and shows how to design a pump for optimum
performance
Order-disorder transition in nanoscopic semiconductor quantum rings
Using the path integral Monte Carlo technique we show that semiconductor
quantum rings with up to six electrons exhibit a temperature, ring diameter,
and particle number dependent transition between spin ordered and disordered
Wigner crystals. Due to the small number of particles the transition extends
over a broad temperature range and is clearly identifiable from the electron
pair correlation functions.Comment: 4 pages, 5 figures, For recent information on physics of small
systems see http://www.smallsystems.d
Simulations of slip flow on nanobubble-laden surfaces
On microstructured hydrophobic surfaces, geometrical patterns may lead to the
appearance of a superhydrophobic state, where gas bubbles at the surface can
have a strong impact on the fluid flow along such surfaces. In particular, they
can strongly influence a detected slip at the surface. We present two-phase
lattice Boltzmann simulations of a flow over structured surfaces with attached
gas bubbles and demonstrate how the detected slip depends on the pattern
geometry, the bulk pressure, or the shear rate. Since a large slip leads to
reduced friction, our results allow to assist in the optimization of
microchannel flows for large throughput.Comment: 22 pages, 12 figure
Emulsification in binary liquids containing colloidal particles: a structure-factor analysis
We present a quantitative confocal-microscopy study of the transient and
final microstructure of particle-stabilised emulsions formed via demixing in a
binary liquid. To this end, we have developed an image-analysis method that
relies on structure factors obtained from discrete Fourier transforms of
individual frames in confocal image sequences. Radially averaging the squared
modulus of these Fourier transforms before peak fitting allows extraction of
dominant length scales over the entire temperature range of the quench. Our
procedure even yields information just after droplet nucleation, when the
(fluorescence) contrast between the two separating phases is scarcely
discernable in the images. We find that our emulsions are stabilised on
experimental time scales by interfacial particles and that they are likely to
have bimodal droplet-size distributions. We attribute the latter to coalescence
together with creaming being the main coarsening mechanism during the late
stages of emulsification and we support this claim with (direct)
confocal-microscopy observations. In addition, our results imply that the
observed droplets emerge from particle-promoted nucleation, possibly followed
by a free-growth regime. Finally, we argue that creaming strongly affects
droplet growth during the early stages of emulsification. Future investigations
could clarify the link between quench conditions and resulting microstructure,
paving the way for tailor-made particle-stabilised emulsions from binary
liquids.Comment: http://iopscience.iop.org/0953-8984/22/45/455102
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