19 research outputs found
Self-assembled porous media from particle-stabilized emulsions
We propose a new mechanism to create self-assembled porous media with highly
tunable geometrical properties and permeabilities: We first allow a
particle-stabilized emulsion to form from a mixture of two fluids and colloidal
particles. Then, either one fluid phase or the particle layer is solidified,
which can be achieved by techniques such as polymerization or freezing. Based
on computer simulations we demonstrate that modifying only the particle
wettability or concentration results in porous structures with a wide range of
pore sizes and a permeability that can be varied by up to three orders of
magnitude. We then discuss optimization of these properties for self-assembled
filters or reactors and conclude that structures based on so-called "bijels"
are most suitable candidates.Comment: 4 pages, 4 figure
Domain and droplet sizes in emulsions stabilized by colloidal particles
Particle-stabilized emulsions are commonly used in various industrial
applications. These emulsions can present in different forms, such as Pickering
emulsions or bijels, which can be distinguished by their different topologies
and rheology. We numerically investigate the effect of the volume fraction and
the uniform wettability of the stabilizing spherical particles in mixtures of
two fluids. For this, we use the well-established three-dimensional lattice
Boltzmann method, extended to allow for the added colloidal particles with
non-neutral wetting properties. We obtain data on the domain sizes in the
emulsions by using both structure functions and the Hoshen-Kopelman (HK)
algorithm, and demonstrate that both methods have their own (dis-)advantages.
We confirm an inverse dependence between the concentration of particles and the
average radius of the stabilized droplets. Furthermore, we demonstrate the
effect of particles detaching from interfaces on the emulsion properties and
domain size measurements.Comment: 9 pages, 9 figure
Timescales of emulsion formation caused by anisotropic particles
Particle stabilized emulsions have received an enormous interest in the
recent past, but our understanding of the dynamics of emulsion formation is
still limited. For simple spherical particles, the time dependent growth of
fluid domains is dominated by the formation of droplets, particle adsorption
and coalescence of droplets (Ostwald ripening), which eventually can be almost
fully blocked due to the presence of the particles. Ellipsoidal particles are
known to be more efficient stabilizers of fluid interfaces than spherical
particles and their anisotropic shape and the related additional rotational
degrees of freedom have an impact on the dynamics of emulsion formation. In
this paper, we investigate this point by means of simple model systems
consisting of a single ellipsoidal particle or a particle ensemble at a flat
interface as well as a particle ensemble at a spherical interface. By applying
combined multicomponent lattice Boltzmann and molecular dynamics simulations we
demonstrate that the anisotropic shape of ellipsoidal particles causes two
additional timescales to be of relevance in the dynamics of emulsion formation:
a relatively short timescale can be attributed to the adsorption of single
particles and the involved rotation of particles towards the interface. As soon
as the interface is jammed, however, capillary interactions between the
particles cause a local reordering on very long timescales leading to a
continuous change in the interface configuration and increase of interfacial
area. This effect can be utilized to counteract the thermodynamic instability
of particle stabilized emulsions and thus offers the possibility to produce
emulsions with exceptional stability.Comment: 14 pages, 14 figure
Recent advances in the simulation of particle-laden flows
A substantial number of algorithms exists for the simulation of moving
particles suspended in fluids. However, finding the best method to address a
particular physical problem is often highly non-trivial and depends on the
properties of the particles and the involved fluid(s) together. In this report
we provide a short overview on a number of existing simulation methods and
provide two state of the art examples in more detail. In both cases, the
particles are described using a Discrete Element Method (DEM). The DEM solver
is usually coupled to a fluid-solver, which can be classified as grid-based or
mesh-free (one example for each is given). Fluid solvers feature different
resolutions relative to the particle size and separation. First, a
multicomponent lattice Boltzmann algorithm (mesh-based and with rather fine
resolution) is presented to study the behavior of particle stabilized fluid
interfaces and second, a Smoothed Particle Hydrodynamics implementation
(mesh-free, meso-scale resolution, similar to the particle size) is introduced
to highlight a new player in the field, which is expected to be particularly
suited for flows including free surfaces.Comment: 16 pages, 4 figure
Effects of nanoparticles and surfactant on droplets in shear flow
We present three-dimensional numerical simulations, employing the
well-established lattice Boltzmann method, and investigate similarities and
differences between surfactants and nanoparticles as additives at a fluid-fluid
interface. We report on their respective effects on the surface tension of such
an interface. Next, we subject a fluid droplet to shear and explore the
deformation properties of the droplet, its inclination angle relative to the
shear flow, the dynamics of the particles at the interface, and the possibility
of breakup. Particles are seen not to affect the surface tension of the
interface, although they do change the overall interfacial free energy. The
particles do not remain homogeneously distributed over the interface, but form
clusters in preferred regions that are stable for as long as the shear is
applied. However, although the overall structure remains stable, individual
nanoparticles roam the droplet interface, with a frequency of revolution that
is highest in the middle of the droplet interface, normal to the shear flow,
and increases with capillary number. We recover Taylor's law for small
deformation of droplets when surfactant or particles are added to the droplet
interface. The effect of surfactant is captured in the capillary number, but
the inertia of adsorbed massive particles increases deformation at higher
capillary number and eventually leads to easier breakup of the droplet.Comment: 17 pages, 17 figures. The figure quality was reduced to fulfill
arXiv's file size restriction
Effects of nanoparticles and surfactant on droplets in shear flow
We present three-dimensional numerical simulations, employing the well-established lattice Boltzmann method, and investigate similarities and differences between surfactants and nanoparticles as additives at a fluid-fluid interface. We report on their respective effects on the surface tension of such an interface. Next, we subject a fluid droplet to shear and explore the deformation properties of the droplet, its inclination angle relative to the shear flow, the dynamics of the particles at the interface, and the possibility of breakup. Particles are seen not to affect the surface tension of the interface, although they do change the overall interfacial free energy. The particles do not remain homogeneously distributed over the interface, but form clusters in preferred regions that are stable for as long as the shear is applied. However, although the overall structure remains stable, individual nanoparticles roam the droplet interface, with a frequency of revolution that is highest in the middle of the droplet interface, normal to the shear flow, and increases with capillary number. We recover Taylor's law for small deformation of droplets when surfactant or particles are added to the droplet interface. The effect of surfactant is captured in the capillary number, but the inertia of adsorbed massive particles increases deformation at higher capillary number and eventually leads to easier breakup of the droplet
Hydraulic properties of porous sintered glass bead systems
n this paper, porous sintered glass bead packings are studied, using X-ray Computed Tomography (XRCT) images at 16 m voxel resolution, to obtain not only the porosity field, but also other properties like tortuosity, particle sizes, pore throat, particle sphericity, specific surface area and the permeability. The influence of the sintering procedure and the original particle size distributions on the microstructure, and thus on the hydraulical properties, is analyzed in detail. The XRCT data are visualized and studied by advanced image filtering and analysis algorithms on to the extracted sub-systems (cubes of different sizes) to determine the correlations between the microstructure and the measured macroscopic hydraulic parameters. Since accurate permeability measurements are not simple, special focus lies on the experimental set up and procedure, for which a new innovative multi-purpose cell based on a modular concept is presented. Furthermore, segmented voxel-based images (defining the microstructure) are used for 3D (three-dimensional) lattice Boltzmann simulations to directly compute some of the properties in the creeping flow regime. A very good agreement between experimental and numerical porosity and permeability could be achieved, validating the numerical model and results. Porosity and permeability gradients along the sample height could be related to gravity acting during sintering. Furthermore, porosity increases in the outer zones of the samples due to the different contact geometry between the beads and the confining cylinder wall during sintering (which is replaced by a membrane during permeability testing to close these pores at the surface of the sample). The influence of different filters on the gray scale distributions and the impact of the segmentation procedure on porosity and permeability is systematically studied. The complex relationships and dependencies between numerical determined permeabilities and hydraulical influence parameters are investigated carefully. In accordance to the well-known Kozeny-Carman model, a similar trend for local permeability values in dependance on porosity and particle diameter is obtained. From the XRCT analysis two distinct peaks in pore throat distributions could be identified, which can be clearly assigned to typical pore throat areas occurring in slightly polydisperse granular systems. Moreover, a linear dependency between average pore throat diameter and porosity as well as permeability is reported. Furthermore, almost identical mean values for porosity and permeability are found from conventional Representative Volume Element (RVE) analysis. For sintered granular systems, the empirical constant in the classical Kozeny-Carman model is determined to be 131, while a value of 180 is expected for perfect mono-disperse sphere packings