87 research outputs found
Deformation and break-up of viscoelastic droplets Using Lattice Boltzmann Models
We investigate the break-up of Newtonian/viscoelastic droplets in a
viscoelastic/Newtonian matrix under the hydrodynamic conditions of a confined
shear flow. Our numerical approach is based on a combination of
Lattice-Boltzmann models (LBM) and Finite Difference (FD) schemes. LBM are used
to model two immiscible fluids with variable viscosity ratio (i.e. the ratio of
the droplet to matrix viscosity); FD schemes are used to model viscoelasticity,
and the kinetics of the polymers is introduced using constitutive equations for
viscoelastic fluids with finitely extensible non-linear elastic dumbbells with
Peterlin's closure (FENE-P). We study both strongly and weakly confined cases
to highlight the role of matrix and droplet viscoelasticity in changing the
droplet dynamics after the startup of a shear flow. Simulations provide easy
access to quantities such as droplet deformation and orientation and will be
used to quantitatively predict the critical Capillary number at which the
droplet breaks, the latter being strongly correlated to the formation of
multiple neckings at break-up. This study complements our previous
investigation on the role of droplet viscoelasticity (A. Gupta \& M.
Sbragaglia, {\it Phys. Rev. E} {\bf 90}, 023305 (2014)), and is here further
extended to the case of matrix viscoelasticity.Comment: 8 pages, 5 figures, IUTAM Symposium on Multiphase flows with phase
change: challenges and opportunities, Hyderabad, India 201
Analytical calculation of slip flow in lattice Boltzmann models with kinetic boundary conditions
We present a mathematical formulation of kinetic boundary conditions for
Lattice Boltzmann schemes in terms of reflection, slip, and accommodation
coefficients. It is analytically and numerically shown that, in the presence of
a non-zero slip coefficient, the Lattice Boltzmann flow develops a physical
slip flow component at the wall. Moreover, it is shown that the slip
coefficient can be tuned in such a way to recover quantitative agreement with
analytical and experimental results up to second order in the Knudsen number.Comment: 27 pages, 4 figure
A Lattice Boltzmann study of the effects of viscoelasticity on droplet formation in microfluidic cross-junctions
Based on mesoscale lattice Boltzmann (LB) numerical simulations, we
investigate the effects of viscoelasticity on the break-up of liquid threads in
microfluidic cross-junctions, where droplets are formed by focusing a liquid
thread of a dispersed (d) phase into another co-flowing continuous (c)
immiscible phase. Working at small Capillary numbers, we investigate the
effects of non-Newtonian phases in the transition from droplet formation at the
cross-junction (DCJ) to droplet formation downstream of the cross-junction (DC)
(Liu Zhang, , 082101 (2011)). We will
analyze cases with (DV), where viscoelastic
properties are confined in the dispersed phase, as well as cases with (MV), where viscoelastic properties are confined in
the continuous phase. Moderate flow-rate ratios of the
two phases are considered in the present study. Overall, we find that the
effects are more pronounced in the case with MV, where viscoelasticity is found
to influence the break-up point of the threads, which moves closer to the
cross-junction and stabilizes. This is attributed to an increase of the polymer
feedback stress forming in the corner flows, where the side channels of the
device meet the main channel. Quantitative predictions on the break-up point of
the threads are provided as a function of the Deborah number, i.e. the
dimensionless number measuring the importance of viscoelasticity with respect
to Capillary forces.Comment: 15 pages, 14 figures. This Work applies the Numerical Methodology
described in arXiv:1406.2686 to the Problem of Droplet Generation in
Microfluidic Cross Junctions. arXiv admin note: substantial text overlap with
arXiv:1508.0014
Effects of viscoelasticity on droplet dynamics and break-up in microfluidic T-Junctions: a lattice Boltzmann study
The effects of viscoelasticity on the dynamics and break-up of fluid threads
in microfluidic T-junctions are investigated using numerical simulations of
dilute polymer solutions at changing the Capillary number (\mbox {Ca}), i.e.
at changing the balance between the viscous forces and the surface tension at
the interface, up to \mbox{Ca} \approx 3 \times 10^{-2}. A Navier-Stokes (NS)
description of the solvent based on the lattice Boltzmann models (LBM) is here
coupled to constitutive equations for finite extensible non-linear elastic
dumbbells with the closure proposed by Peterlin (FENE-P model). We present the
results of three-dimensional simulations in a range of \mbox{Ca} which is
broad enough to characterize all the three characteristic mechanisms of breakup
in the confined T-junction, i.e. , and regimes. The various model parameters of the FENE-P constitutive
equations, including the polymer relaxation time and the finite
extensibility parameter , are changed to provide quantitative details on
how the dynamics and break-up properties are affected by viscoelasticity. We
will analyze cases with (DV), where
viscoelastic properties are confined in the dispersed (d) phase, as well as
cases with (MV), where viscoelastic properties
are confined in the continuous (c) phase. Moderate flow-rate ratios of the two phases are considered in the present study. Overall, we
find that the effects are more pronounced in the case with MV, as the flow
driving the break-up process upstream of the emerging thread can be sensibly
perturbed by the polymer stresses.Comment: 16 pages, 14 figures; This Work applies the Numerical Methodology
described in arXiv:1406.2686 to the Problem of Droplet Generation in
Microfluidic T-Junctions. arXiv admin note: substantial text overlap with
arXiv:1508.0055
Avalanche statistics during coarsening dynamics
We study the coarsening dynamics of a two dimensional system via lattice
Boltzmann numerical simulations. The system under consideration is a biphasic
system consisting of domains of a dispersed phase closely packed together in a
continuous phase and separated by thin interfaces. Such system is elastic and
typically out of equilibrium. The equilibrium state is attained via the
coarsening dynamics, wherein the dispersed phase slowly diffuses through the
interfaces, causing domains to change in size and eventually rearrange
abruptly. The effect of rearrangements is propagated throughout the system via
the intrinsic elastic interactions and may cause rearrangements elsewhere,
resulting in intermittent bursts of activity and avalanche behaviour. Here we
aim at quantitatively characterizing the corresponding avalanche statistics
(i.e. size, duration, inter-avalanche time). Despite the coarsening dynamics is
triggered by an internal driving mechanism, we find quantitative indications
that such avalanche statistics displays scaling-laws very similar to those
observed in the response of disordered materials to external loads
Metastability at the Yield-Stress Transition in Soft Glasses
We study the solid-to-liquid transition in a two-dimensional fully periodic
soft-glassy model with an imposed spatially heterogeneous stress. The model we
consider consists of droplets of a dispersed phase jammed together in a
continuous phase. When the peak value of the stress gets close to the yield
stress of the material, we find that the whole system intermittently tunnels to
a metastable "fluidized" state, which relaxes back to a metastable "solid"
state by means of an elastic-wave dissipation. This macroscopic scenario is
studied through the microscopic displacement field of the droplets, whose time
statistics displays a remarkable bimodality. Metastability is rooted in the
existence, in a given stress range, of two distinct stable rheological branches
as well as long-range correlations (e.g., large dynamic heterogeneity)
developed in the system. Finally, we show that a similar behavior holds for a
pressure-driven flow, thus suggesting possible experimental tests.Comment: 13 pages, 11 figure
Mesoscopic simulation study of wall roughness effects in micro-channel flows of dense emulsions
We study the Poiseuille flow of a soft-glassy material above the jamming
point, where the material flows like a complex fluid with Herschel- Bulkley
rheology. Microscopic plastic rearrangements and the emergence of their spatial
correlations induce cooperativity flow behavior whose effect is pronounced in
presence of confinement. With the help of lattice Boltzmann numerical
simulations of confined dense emulsions, we explore the role of geometrical
roughness in providing activation of plastic events close to the boundaries. We
probe also the spatial configuration of the fluidity field, a continuum
quantity which can be related to the rate of plastic events, thereby allowing
us to establish a link between the mesoscopic plastic dynamics of the jammed
material and the macroscopic flow behaviour
Fluidisation and plastic activity in a model soft-glassy material flowing in micro-channels with rough walls
By means of mesoscopic numerical simulations of a model soft-glassy material,
we investigate the role of boundary roughness on the flow behaviour of the
material, probing the bulk/wall and global/local rheologies. We show that the
roughness reduces the wall slip induced by wettability properties and acts as a
source of fluidisation for the material. A direct inspection of the plastic
events suggests that their rate of occurrence grows with the fluidity field,
reconciling our simulations with kinetic elasto-plastic descriptions of jammed
materials. Notwithstanding, we observe qualitative and quantitative differences
in the scaling, depending on the distance from the rough wall and on the
imposed shear. The impact of roughness on the orientational statistics is also
studied
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