401 research outputs found
Effect of Marangoni stress on the bulk rheology of a dilute emulsion of surfactant-laden deformable droplets in linear flows
In the present study we analytically investigate the deformation and bulk
rheology of a dilute emulsion of surfactant-laden droplets suspended in a
linear flow. We use an asymptotic approach to predict the effect of surfactant
distribution on the deformation of a single droplet as well as the effective
shear and extensional viscosity for the dilute emulsion. The non-uniform
distribution of surfactants due to the bulk flow results in the generation of a
Marangoni stress which affects both the deformation as well as the bulk
rheology of the suspension. The present analysis is done for the limiting case
when the surfactant transport is dominated by the surface diffusion relative to
surface convection. As an example, we have used two commonly encountered bulk
flows, namely, uniaxial extensional flow and simple shear flow. With the
assumption of negligible inertial forces present in either of the phases, we
are able to show that both the surfactant concentration on the droplet surface
as well as the ratio of viscosity of the droplet phase with respect to the
suspending fluid has a significant effect on the droplet deformation as well as
the bulk rheology. It is seen that increase in the non-uniformity in surfactant
distribution on the droplet surface results in a higher droplet deformation and
a higher effective viscosity for either of linear flows considered. For the
case of simple shear flow, surfactant distribution is found to have no effect
on the inclination angle, however, a higher viscosity ratio predicts the
droplet to be more aligned towards the direction of flow
Complex Fluid-Fluid Interface may Non Trivially Dictate Droplet Deformation in an Incipient Flow
The present study theoretically predicts the effect of interfacial viscosity
on the deformation of a compound drop as well as on the bulk rheology. The
system at hand comprises of a dilute emulsion of concentric compound drops,
laden with surfactants and suspended in a linear flow. Two types of linear
flows are considered in this study, namely, a uniaxial extensional flow and a
simple shear flow. Presence of surfactants along the drop surface leads to the
generation of an interfacial viscosity, which is different from the bulk. This
interfacial viscosity generates a viscous drag that along with bulk
flow-induced nonuniform surfactant distribution on the drop surface
significantly alters drop dynamics. For the present study an asymptotic
approach is used to solve the flow field under the limiting case of
diffusion-dominated-surfactant transport. Assuming the surfactants to be
bulk-insoluble and negligible inertia to be present in fluid flow, it is shown
that presence of interfacial viscosity reduces the deformation of a compound
drop and enhances the stability of a dilute double emulsion. At the same time
the effective viscosity of the emulsion also increases with rise in interfacial
viscosity. For large values of interfacial dilatational viscosity the drop
deformation is seen to increase and hence the stability of the double emulsion
is questionable
Cross-stream migration of a surfactant-laden deformable droplet in a Poiseuille flow
The motion of a viscous deformable droplet suspended in an unbounded
Poiseuille flow in the presence of bulk-insoluble surfactants is studied
analytically. Assuming the convective transport of fluid and heat to be
negligible, we perform a small-deformation perturbation analysis to obtain the
droplet migration velocity. The droplet dynamics strongly depends on the
distribution of surfactants along the droplet interface, which is governed by
the relative strength of convective transport of surfactants as compared with
the diffusive transport of surfactants. The present study is focused on the
following two limits: (i) when the surfactant transport is dominated by surface
diffusion, and (ii) when the surfactant transport is dominated by surface
convection. In the first limiting case, it is seen that the axial velocity of
the droplet decreases with increase in the advection of the surfactants along
the surface. The variation of cross-stream migration velocity, on the other
hand, is analyzed over three different regimes based on the ratio of the
viscosity of the droplet phase to that of the carrier phase. In the first
regime the migration velocity decreases with increase in surface advection of
the surfactants although there is no change in direction of droplet migration.
For the second regime, the direction of the cross-stream migration of the
droplet changes depending on different parameters. In the third regime, the
migration velocity is merely affected by any change in the surfactant
distribution. For the other limit of higher surface advection in comparison to
surface diffusion of the surfactants, the axial velocity of the droplet is
found to be independent of the surfactant distribution. However, the
cross-stream velocity is found to decrease with increase in non-uniformity in
surfactant distribution
Persistent 1-Cycles: Definition, Computation, and Its Application
Persistence diagrams, which summarize the birth and death of homological
features extracted from data, are employed as stable signatures for
applications in image analysis and other areas. Besides simply considering the
multiset of intervals included in a persistence diagram, some applications need
to find representative cycles for the intervals. In this paper, we address the
problem of computing these representative cycles, termed as persistent
1-cycles, for -persistent homology with
coefficients. The definition of persistent cycles is based on the interval
module decomposition of persistence modules, which reveals the structure of
persistent homology. After showing that the computation of the optimal
persistent 1-cycles is NP-hard, we propose an alternative set of meaningful
persistent 1-cycles that can be computed with an efficient polynomial time
algorithm. We also inspect the stability issues of the optimal persistent
1-cycles and the persistent 1-cycles computed by our algorithm with the
observation that the perturbations of both cannot be properly bounded. We
design a software which applies our algorithm to various datasets. Experiments
on 3D point clouds, mineral structures, and images show the effectiveness of
our algorithm in practice.Comment: Correct the algorithm numbering issu
Magnetism in the Early Universe
Blazar observations point toward the possible presence of magnetic fields
over intergalactic scales of the order of up to Mpc, with strengths of
at least G. Understanding the origin of these large-scale
magnetic fields is a challenge for modern astrophysics. Here we discuss the
cosmological scenario, focussing on the following questions: (i) How and when
was this magnetic field generated? (ii) How does it evolve during the expansion
of the universe? (iii) Are the amplitude and statistical properties of this
field such that they can explain the strengths and correlation lengths of
observed magnetic fields? We also discuss the possibility of observing
primordial turbulence through direct detection of stochastic gravitational
waves in the mHz range accessible to LISA.Comment: Invited talk given at FM8 "New Insights in Extragalactic Magnetic
Fields", XXX IAU GA, Vienna, Austria, August 29, 2018; 4 pages; 2 figures;
accepted for publicatio
Statistical Properties of Scale-Invariant Helical Magnetic Fields and Applications to Cosmology
We investigate the statistical properties of isotropic, stochastic, Gaussian
distributed, helical magnetic fields characterized by different shapes of the
energy spectra at large length scales and study the associated realizability
condition. We discuss smoothed magnetic fields that are commonly used when the
primordial magnetic field is constrained by observational data. We are
particularly interested in scale-invariant magnetic fields that can be
generated during the inflationary stage by quantum fluctuations. We determine
the correlation length of such magnetic fields and relate it to the infrared
cutoff of perturbations produced during inflation. We show that this scale
determines the observational signatures of the inflationary magnetic fields on
the cosmic microwave background. At smaller scales, the scale-invariant
spectrum changes with time. It becomes a steeper weak-turbulence spectrum at
progressively larger scales. We show numerically that the critical length scale
where this happens is the turbulent-diffusive scale, which increases with the
square root of time.Comment: 24 pages, 9 figures. Discussion expanded, references added, results
unchanged. Accepted for publication in JCA
Cross-stream migration characteristics of a deformable droplet in a non-isothermal Poiseuille Flow through Microfluidic Channel
The migration characteristics of a suspended deformable droplet in a parallel
plate microchannel is studied, both analytically and numerically, under the
combined influence of a constant temperature gradient in the transverse
direction and an imposed pressure driven flow. Any predefined transverse
position in the micro channel can be attained by the droplet depending on the
applied temperature gradient in the cross-stream direction or how small the
droplet is with respect to the channel width. For the analytical solution, an
asymptotic approach is used, where we neglect any effect of inertia or thermal
convection of the fluid in either of the phases. To obtain a numerical
solution, we use the conservative level set method. Variation of temperature in
the flow field causes a jump in the tangential component of stress at the
droplet interface. This jump in stress component, which is the thermal
Marangoni stress, is an important factor that controls the trajectory of the
droplet. The direction of cross-stream migration of the droplet is decided by
the magnitude of the critical Marangoni stress, corresponding to which the
droplet remains stationary. In order to analyze practical microfluidic setup,
we do numerical simulations where we consider wall effects as well as the
effect of thermal convection and finite shape deformation on the cross-stream
migration of the droplet
End-to-End Bengali Speech Recognition
Bengali is a prominent language of the Indian subcontinent. However, while
many state-of-the-art acoustic models exist for prominent languages spoken in
the region, research and resources for Bengali are few and far between. In this
work, we apply CTC based CNN-RNN networks, a prominent deep learning based
end-to-end automatic speech recognition technique, to the Bengali ASR task. We
also propose and evaluate the applicability and efficacy of small 7x3 and 3x3
convolution kernels which are prominently used in the computer vision domain
primarily because of their FLOPs and parameter efficient nature. We propose two
CNN blocks, 2-layer Block A and 4-layer Block B, with the first layer
comprising of 7x3 kernel and the subsequent layers comprising solely of 3x3
kernels. Using the publicly available Large Bengali ASR Training data set, we
benchmark and evaluate the performance of seven deep neural network
configurations of varying complexities and depth on the Bengali ASR task. Our
best model, with Block B, has a WER of 13.67, having an absolute reduction of
1.39% over comparable model with larger convolution kernels of size 41x11 and
21x11.Comment: 4 pages, 2 figures, 3 table
Primordial magnetic helicity evolution with a homogeneous magnetic field from inflation
Motivated by a scenario of magnetogenesis in which a homogeneous magnetic
field is generated during inflation, we study the magnetohydrodynamic evolution
of the primordial plasma motions for two kinds of initial conditions -- (i) a
spatially homogeneous field with an unlimited correlation length, and (ii) a
zero flux scale-invariant statistically homogeneous magnetic field. In both
cases, we apply, for a short initial time interval, monochromatic forcing at a
certain wave number so that the correlation length is finite, but much smaller
than the typical length scale of turbulence. In particular, we investigate the
decay of nonhelical and helical hydromagnetic turbulence. We show that, in the
presence of a homogeneous magnetic field, the decay of helical and nonhelical
small-scale fields can occur rapidly. This is a special property of a system
with a perfectly homogeneous magnetic field, which is sometimes considered as a
local approximation to a slowly varying background field. It can never change
and acts as an imposed magnetic field. This is in a sharp contrast to the case
of a statistically homogeneous magnetic field, where we recover familiar decay
properties: a much slower decay of magnetic energy and a faster growth of the
correlation length, especially in the case with magnetic felicity. The result
suggests that a homogeneous magnetic field, if generated during inflation,
should persist under the influence of small-scale fields and could be the
origin of the large-scale magnetic field in the Universe.Comment: 16 pages, 7 figures, 1 table. Journal versio
Computing Minimal Persistent Cycles: Polynomial and Hard Cases
Persistent cycles, especially the minimal ones, are useful geometric features
functioning as augmentations for the intervals in a purely topological
persistence diagram (also termed as barcode). In our earlier work, we showed
that computing minimal 1-dimensional persistent cycles (persistent 1-cycles)
for finite intervals is NP-hard while the same for infinite intervals is
polynomially tractable. In this paper, we address this problem for general
dimensions with coefficients. In addition to proving that it is
NP-hard to compute minimal persistent d-cycles (d>1) for both types of
intervals given arbitrary simplicial complexes, we identify two interesting
cases which are polynomially tractable. These two cases assume the complex to
be a certain generalization of manifolds which we term as weak pseudomanifolds.
For finite intervals from the d-th persistence diagram of a weak
(d+1)-pseudomanifold, we utilize the fact that persistent cycles of such
intervals are null-homologous and reduce the problem to a minimal cut problem.
Since the same problem for infinite intervals is NP-hard, we further assume the
weak (d+1)-pseudomanifold to be embedded in so that the
complex has a natural dual graph structure and the problem reduces to a minimal
cut problem. Experiments with both algorithms on scientific data indicate that
the minimal persistent cycles capture various significant features of the data.Comment: Content same as appeared in the proceeding of SODA20
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