120 research outputs found
Droplet breakup driven by shear thinning solutions in a microfluidic T-Junction
Droplet-based microfluidics turned out to be an efficient and adjustable
platform for digital analysis, encapsulation of cells, drug formulation, and
polymerase chain reaction. Typically, for most biomedical applications, the
handling of complex, non-Newtonian fluids is involved, e.g. synovial and
salivary fluids, collagen, and gel scaffolds. In this study we investigate the
problem of droplet formation occurring in a microfluidic T-shaped junction,
when the continuous phase is made of shear thinning liquids. At first, we
review in detail the breakup process providing extensive, side-by-side
comparisons between Newtonian and non-Newtonian liquids over unexplored ranges
of flow conditions and viscous responses. The non-Newtonian liquid carrying the
droplets is made of Xanthan solutions, a stiff rod-like polysaccharide
displaying a marked shear thinning rheology. By defining an effective Capillary
number, a simple yet effective methodology is used to account for the
shear-dependent viscous response occurring at the breakup. The droplet size can
be predicted over a wide range of flow conditions simply by knowing the
rheology of the bulk continuous phase. Experimental results are complemented
with numerical simulations of purely shear thinning fluids using Lattice
Boltzmann models. The good agreement between the experimental and numerical
data confirm the validity of the proposed rescaling with the effective
Capillary number.Comment: Manuscript: 11 pages 5 figures, 65 References. Textual Supplemental
Material: 6 pages 3 figure. Video Supplemental Materials: 2 movie
Viscoplastic displacement flows in narrow channels
Les écoulements à déplacement se produisent fréquemment dans les applications naturelles et industrielles. Bien que les déplacements Newtoniens aient été pris en considération dans une grande variété d’études théoriques et expérimentales dans les dernières décennies, un nombre considérable de fluides pratiques présentent des caractéristiques viscoplastiques, rendant la prévision du comportement des écoulements plus difficile. Les écoulement de déplacement viscoplastiques sont généralement contrôlés par un équilibre entre diverses forces, y compris la force visqueuse, la force de flottabilité, la force d’inertie, contrainte d’écoulement, etc., en plus de caractéristiques miscibles et non miscibles. Une compétition entre ces forces peut conduire à des comportements imprévisibles et exotiques de déplacement. Permettant une compréhension approfondie de ces écoulements, dans cette thèse de doctorat nous avons étudié l’écoulement à déplacement d’un fluide viscoplastique par un fluide Newtonien dans une géométrie simple, c.-à-d. un canal étroit et confiné. Dans la première partie de cette thèse (chapitres 1 à 3), nous étudions expérimentalement les écoulements à déplacement non-miscibles d’un fluide viscoplastique par un fluide Newtonien. En particulier, nous analysons le mouvement d’air dans un gel de Carbopol, dans une cellule de Hele-Shaw de section rectangulaire. Cette géométrie est composée de deux plaques parallèles rigides. Nous étudions les résultats en termes d’efficacité de déplacement et de morphologie des modèles d’écoulement. Nous démontrons que les comportements complexes du gel Carbopol, c.-à-d. les fortes propriétés viscoplastiques et les faibles propriétés viscoélastiques, affectent les caractéristiques d’écoulement de déplacement. Ensuite, nous étendons cette étude au déplacement d’un gel de Carbopol par une huile de silicone afin de considérer les effets de la mouillabilité sur l’écoulement. Nous observons qu’une combinaison de comportements viscoplastiques et de mouillabilité exerce un impact significatif sur les modèles d’écoulement à déplacement, pour lesquels quatre régimes d’écoulement différents sont identifiés : un régime capillaire, un régime de contrainte d’écoulement, un régime visqueux et un régime élastoinertiel. Enfin, nous étudions les impacts du rapport d’aspect de la section transversale de la cellule sur les caractéristiques de déplacement viscoplastique. Dans la deuxième partie de cette thèse (chapitres 4 à 5), nous étudions numériquement les écoulements à déplacement miscibles d’un fluide viscoplastique par un fluide Newtonien dans un long canal plan 2D. Pour un déplacement «heavy-light», l’analyse des modèles d’écoulement en fonction de divers paramètres sans dimension nous permet d’identifier trois régimes d’écoulement distincts : déplacements «center-type»/«slump- type», «back flow»/«no-back flow» et déplacement «stable/instable». Nous décrivons les effets du rapport de viscosité des fluides, de la flottabilité, de la contrainte d’écoulement et de l’inclinaison du canal sur les régimes d’écoulement susmentionnés.Displacement flows frequently occur in natural and industrial applications. Although Newtonian displacements have been considered in a wide range of theoretical and experimental studies in the recent decades, a considerable number of practical fluids exhibit viscoplastic features, making it hard to predict the flow behaviors. Viscoplastic displacement flows are generally controlled by a balance between a variety of forces, including viscous, buoyant, inertial, yield stress, etc., in addition to miscible and immiscible features. A competition between these forces may lead to exotic, unpredictable displacement flow behaviors. To provide a deep understanding of these flows, in this Ph.D. thesis we investigate the displacement flow of a viscoplastic fluid by a Newtonian fluid in a simple flow geometry, i.e., a narrow confined channel. In the first part of this thesis (Chapters 1-3), we experimentally study immiscible displacement flows of a viscoplastic fluid by a Newtonian fluid. In particular, we analyze the invasion of air into a Carbopol gel in a rectangular cross-section Hele-Shaw cell. This flow geometry is composed of two rigid parallel plates with a small gap. We study the results in terms of the displacement efficiency and morphology of the flow patterns. We demonstrate that the complex behaviors of the Carbopol gel, i.e., strong viscoplastic properties and weak viscoelastic properties, affect the displacement flow features. We then extend this study to the displacement of a Carbopol gel by silicon oil in order to consider the effects of wettability on the flow. We observe that a combination of viscoplastic behaviors and wettability exerts a significant impact on the displacement flow patterns, for which four different flow regimes are identified a capillary regime, a yield stress regime, a viscous regime and an elasto-inertial regime. Finally, we investigate the impacts of the cell cross-section aspect ratio on viscoplastic displacement flow features. In the second part of this thesis (Chapters 4-5), we numerically study miscible displacement flows of a viscoplastic fluid by a Newtonian fluid in a long 2D plane channel. For a heavy-light displacement, analyzing the displacement flow patterns as a function of various dimensionless parameters allows us to identify three distinct flow regimes center/slump-type, back/no-backflow and stable/unstable displacements. We describe the effects of the viscosity ratio of fluids, buoyancy, yield stress and channel inclination on the aforementioned flow regimes
Water Flooding and Viscous Fingering in Fracture and Porous Media by Lattice Boltzmann Method
The study of fluid front in porous media in enhanced oil recovery is important. The purpose of this study is to simulate water flooding, and investigate the factors affecting the fluid front across a microfracture and simple porous media using Shan-Chen type of the Lattice Boltzmann Method (LBM). Various factors, including velocity and dynamic viscosity that define the capillary number and wettability are considered. Independently, the increase in velocity and dynamic viscosity ratio results in viscous fingering and its narrowness. Increasing the wettability of the displacing fluid decreases viscous fingering, and as a result, it makes the fluid move in piston form. The lowest sweep efficiency occurs when the displacing fluid has a neutral wettability. Simulation results show the strength and accuracy of Shan-Chen type of LBM in fluid front tracking in porous media in pore scale.
This work is licensed under a Creative Commons Attribution 4.0 International License
Single-Phase Flow of Non-Newtonian Fluids in Porous Media
The study of flow of non-Newtonian fluids in porous media is very important
and serves a wide variety of practical applications in processes such as
enhanced oil recovery from underground reservoirs, filtration of polymer
solutions and soil remediation through the removal of liquid pollutants. These
fluids occur in diverse natural and synthetic forms and can be regarded as the
rule rather than the exception. They show very complex strain and time
dependent behavior and may have initial yield-stress. Their common feature is
that they do not obey the simple Newtonian relation of proportionality between
stress and rate of deformation. Non-Newtonian fluids are generally classified
into three main categories: time-independent whose strain rate solely depends
on the instantaneous stress, time-dependent whose strain rate is a function of
both magnitude and duration of the applied stress and viscoelastic which shows
partial elastic recovery on removal of the deforming stress and usually
demonstrates both time and strain dependency. In this article the key aspects
of these fluids are reviewed with particular emphasis on single-phase flow
through porous media. The four main approaches for describing the flow in
porous media are examined and assessed. These are: continuum models, bundle of
tubes models, numerical methods and pore-scale network modeling.Comment: 94 pages, 12 figures, 1 tabl
Local Fluidization of Concentrated Emulsion in Microfluidic Channels Textured at the Droplet Scale
The rheology of soft-flowing systems, such as concentrated emulsions, foams, gels, slurries, colloidal glasses and related complex fluids, has a larger and larger impact in modern science and engineering. Much of the fascination of these systems stems from the fact that they do not fall within any of three basic states of matter, gas-liquid-solid, but live rather on a moving border between them. To understand the flow mechanism, it is necessary to have a look at the micro-scale dynamics of its constituents (i.e, droplets for emulsions, bubbles for foams, blobs for gels, etc.). In fact, in these fluids, the flow occurs via successive elastic deformations and plastic rearrangements, which create fragile regions enhancing the “fluidization” of the material. Despite the fluidization of Soft Glassy Materials (SGMs) is strongly affected by the surface roughness, the role played by the density, the orientation and the periodicity of rough elements has not been quantitatively addressed so far. In fact, predict and control the flow of SGMs is particularly important for an ample variety of technological applications from food to pharmaceutical industries.
In this work, we study the flow of concentrated emulsions in microfluidic channels, one wall of which is patterned with micron-size grooves with different patterns. Using equally spaced grooves, we find a scaling law describing the roughness-induced fluidization as a function of the density of the grooves, thus fluidization can be predicted and quantitatively regulated. Furthermore, we quantitatively report the existence of two physically different scenarios. When the gap is large, compared to the droplets in the emulsion, the droplets hit the solid obstacles and easily escape scrambling with their neighbors. Conversely, as the gap spacing is reduced, droplets get trapped inside, creating a “soft roughness” layer, i.e., a complementary series of deformable posts. Introducing an asymmetrical micro-roughness (herringbone pattern), the flow presents, in turn an asymmetric behavior. The emulsion flows faster in the same direction of the herringbone groove respect when it flows in the opposite direction.
Our experimental observations are suitably complemented and confirmed by lattice Boltzmann simulations. These numerical simulations are key to highlight the change in the spatial distribution of the plastic rearrangements caused by surface roughness and to elucidate the micro-mechanics of the roughness induced fluidization
Mesoscale fluid simulation with the Lattice Boltzmann method
PhDThis thesis describes investigations of several complex fluid effects., including
hydrodynamic spinodal decomposition, viscous instability. and self-assembly of a
cubic surfactant phase, by simulating them with a lattice Boltzmann computational
model.
The introduction describes what is meant by the term "complex fluid", and why
such fluids are both important and difficult to understand. A key feature of complex
fluids is that their behaviour spans length and time scales. The lattice Boltzmann
method is presented as a modelling technique which sits at a "mesoscale" level
intermediate between coarse-grained and fine-grained detail, and which is therefore
ideal for modelling certain classes of complex fluids.
The following chapters describe simulations which have been performed using
this technique, in two and three dimensions. Chapter 2 presents an investigation
into the separation of a mixture of two fluids. This process is found to involve several
physical mechanisms at different stages. The simulated behaviour is found to be in
good agreement with existing theory, and a curious effect, due to multiple competing
mechanisms, is observed, in agreement with experiments and other simulations.
Chapter 3 describes an improvement to lattice Boltzmann models of Hele-Shaw
flow, along with simulations which quantitatively demonstrate improvements in both
accuracy and numerical stability. The Saffman-Taylor hydrodynamic instability is
demonstrated using this model.
Chapter 4 contains the details and results of the TeraGyroid experiment, which
involved extremely large-scale simulations to investigate the dynamical behaviour
of a self-assembling structure. The first finite- size-effect- free dynamical simulations
of such a system are presented. It is found that several different mechanisms are
responsible for the assembly; the existence of chiral domains is demonstrated, along
with an examination of domain growth during self-assembly.
Appendix A describes some aspects of the implementation of the lattice Boltzmann
codes used in this thesis; appendix B describes some of the Grid computing
techniques which were necessary for the simulations of chapter 4.
Chapter 5 summarises the work, and makes suggestions for further research and
improvement.Huntsman Corporation Queen Mary University Schlumberger Cambridge Researc
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