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Passive and Active Particles in Viscoelastic Fluids.

Record Type:	Language materials, manuscript : Monograph/item
Title/Author:	Passive and Active Particles in Viscoelastic Fluids./
Author:	Neo, Boon Siong.
Description:	1 online resource (154 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 85-11, Section: A.
Contained By:	Dissertations Abstracts International85-11A.
Subject:	Polymers. -
Online resource:	click for full text (PQDT)
ISBN:	9798382636474

Passive and Active Particles in Viscoelastic Fluids.
Neo, Boon Siong.

Passive and Active Particles in Viscoelastic Fluids. - 1 online resource (154 pages)

Source: Dissertations Abstracts International, Volume: 85-11, Section: A.

Thesis (Ph.D.)--Stanford University, 2023.

Includes bibliographical references

Particles suspended in viscoelastic fluids appear in a variety of settings, both industrial and biological; examples include particulate suspensions with polymer additives in consumer goods or food products, or cells suspended in biological fluids containing high molecular weight macromolecules. These particles can be rigid or deformable, and passive (e.g. particulate filler) or active (e.g. swimming microbes). Understanding particle-fluid interactions is a key building block in predicting the behavior of these systems, as the material properties of the particle and fluid affect stress distributions in a suspension under flow, or the motion of a swimming microorganism.Due to the length and time scales involved, many of these systems can be analyzed at the limit of vanishing inertia. In this limit, the motion of rigid particles, or particles with simple interfaces, has been extensively studied in Newtonian fluids. However, with viscoelasticity in the suspending fluid, the local stresses are no longer linear in the local rate-of-strain, leading to qualitatively different rheological behavior. Further, flow with deformable particles is made even more complex as the particle's shape evolves over time. To tackle these problems, we use constitutive continuum viscoelastic fluid models and employ a mix of analytic theory and direct numerical simulations to investigate the underlying physics.We first examine passive particles suspended in viscoelastic fluids under flow; specifically, capsules, where the combined effects of fluid viscoelasticity and solid elasticity can lead to interesting rheological behavior. We study the effect of suspending fluid elasticity at the macro scale (the bulk suspension rheology) as well as the micro scale (particle deformation and tensions), for two different flows of rheological significance that appear in many applications.For dilute spherical capsules in shear flows, we find that for constant particle stiffness, increasing the suspending fluid elasticity leads to a non-monotonic trend in the suspension shear viscosity, with an initial decrease and a subsequent increase. Decomposing the stress contributions to the suspension shear viscosity, we find the stress contribution from the capsule domain monotonically decreases iv with suspending fluid elasticity; on the other hand, the stress contribution from the suspending fluid domain monotonically increases. The decreasing stress contribution in the capsule domain reflects a decrease in the average shear stress in that domain, hence increasing suspending fluid elasticity also decreases the particle deformation for constant capsule stiffness, leading to decreasing tensions in the capsule membrane. The trend of decreasing shear stress in the particle domain was also reported for dilute rigid spherical particles in viscoelastic shear; to provide a mechanistic explanation, we study the surface tractions on a rigid sphere analytically and find that the surface tractions can be related to the oscillatory shear response of the suspending viscoelastic fluid. We derive a scaling for these tractions, and the resultant rigid particle contribution to the suspension shear stress - the "stresslet" - in the limit of an undisturbed flow field, and see good agreement with previously reported results. Replacing the spherical capsules with a red blood cell (RBC), we find similar trends in the suspension rheology and particle deformation, but weaker trends in the membrane tension distributions due to the reduced volume of the red blood cells.For pressure-driven flow of dilute spherical capsules and RBCs through ducts at high confinement (i.e. the particles occupy a significant fraction of the cross-sectional area), we find that for constant particle stiffness, increasing the suspending fluid elasticity leads to a monotonic increase in the particle deformation, while the suspension extrusional viscosity exhibits non-monotonic behavior, for both types of particles studied. We run complementary simulations of rigid spheres with sharplydefined particle interfaces, examine how viscoelasticity affects the surface tractions for this flow, and relate these to the particle deformation and suspension rheology.Finally, we study a specific type of active particle suspended in a viscoelastic fluid. At low Reynolds numbers, "swirlers" - swimmers with an axisymmetric "head" and "tail" counterrotating about the axis of symmetry - generate no net propulsion in a Newtonian fluid as a consequence of the "scallop theorem". Viscoelasticity in the suspending fluid breaks the time-reversibility of the governing equations and allows swirlers to propel themselves, with the swim speed being a function of swimmer geometry, fluid elasticity, and swimming gait. We study the unsteady motion of a freelysuspended self-propelled swirler though viscoelastic fluids described by the Giesekus model, allowing for general axisymmetric geometry and time-dependent tail rotation rate. We show how the steady swim speed under weakly elastic conditions can be calculated for general arbitrary axisymmetric geometries, via the reciprocal theorem and the solution of two Newtonian flow problems. In this "weak flow" limit, we analytically determine the swim speed and its dependence on the parameters of the Giesekus fluid which in turn are related to the primary and secondary normal stress coefficients v - rheological parameters of interest for characterizing viscoelastic fluids. Furthermore, for weak fluid elasticity, we derive the unsteady swim speed as an analytic function of a specified unsteady tail rotation rate and the material properties of the suspending fluid. We show that for a particular tail rotation rate, the unsteady swim speed can be analyzed to recover the spectrum of fluid relaxation times, analogous to small-amplitude oscillatory shear measurements on a benchtop rheometer. This study expands upon the design space for a "swimming rheometer" by increasing its functionality to make and interpret rheological measurements.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2024

Mode of access: World Wide Web

ISBN: 9798382636474Subjects--Topical Terms:

561233
Polymers.
Index Terms--Genre/Form:

554714
Electronic books.

Passive and Active Particles in Viscoelastic Fluids.
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502 $a Thesis (Ph.D.)--Stanford University, 2023.
504 $a Includes bibliographical references
520 $a Particles suspended in viscoelastic fluids appear in a variety of settings, both industrial and biological; examples include particulate suspensions with polymer additives in consumer goods or food products, or cells suspended in biological fluids containing high molecular weight macromolecules. These particles can be rigid or deformable, and passive (e.g. particulate filler) or active (e.g. swimming microbes). Understanding particle-fluid interactions is a key building block in predicting the behavior of these systems, as the material properties of the particle and fluid affect stress distributions in a suspension under flow, or the motion of a swimming microorganism.Due to the length and time scales involved, many of these systems can be analyzed at the limit of vanishing inertia. In this limit, the motion of rigid particles, or particles with simple interfaces, has been extensively studied in Newtonian fluids. However, with viscoelasticity in the suspending fluid, the local stresses are no longer linear in the local rate-of-strain, leading to qualitatively different rheological behavior. Further, flow with deformable particles is made even more complex as the particle's shape evolves over time. To tackle these problems, we use constitutive continuum viscoelastic fluid models and employ a mix of analytic theory and direct numerical simulations to investigate the underlying physics.We first examine passive particles suspended in viscoelastic fluids under flow; specifically, capsules, where the combined effects of fluid viscoelasticity and solid elasticity can lead to interesting rheological behavior. We study the effect of suspending fluid elasticity at the macro scale (the bulk suspension rheology) as well as the micro scale (particle deformation and tensions), for two different flows of rheological significance that appear in many applications.For dilute spherical capsules in shear flows, we find that for constant particle stiffness, increasing the suspending fluid elasticity leads to a non-monotonic trend in the suspension shear viscosity, with an initial decrease and a subsequent increase. Decomposing the stress contributions to the suspension shear viscosity, we find the stress contribution from the capsule domain monotonically decreases iv with suspending fluid elasticity; on the other hand, the stress contribution from the suspending fluid domain monotonically increases. The decreasing stress contribution in the capsule domain reflects a decrease in the average shear stress in that domain, hence increasing suspending fluid elasticity also decreases the particle deformation for constant capsule stiffness, leading to decreasing tensions in the capsule membrane. The trend of decreasing shear stress in the particle domain was also reported for dilute rigid spherical particles in viscoelastic shear; to provide a mechanistic explanation, we study the surface tractions on a rigid sphere analytically and find that the surface tractions can be related to the oscillatory shear response of the suspending viscoelastic fluid. We derive a scaling for these tractions, and the resultant rigid particle contribution to the suspension shear stress - the "stresslet" - in the limit of an undisturbed flow field, and see good agreement with previously reported results. Replacing the spherical capsules with a red blood cell (RBC), we find similar trends in the suspension rheology and particle deformation, but weaker trends in the membrane tension distributions due to the reduced volume of the red blood cells.For pressure-driven flow of dilute spherical capsules and RBCs through ducts at high confinement (i.e. the particles occupy a significant fraction of the cross-sectional area), we find that for constant particle stiffness, increasing the suspending fluid elasticity leads to a monotonic increase in the particle deformation, while the suspension extrusional viscosity exhibits non-monotonic behavior, for both types of particles studied. We run complementary simulations of rigid spheres with sharplydefined particle interfaces, examine how viscoelasticity affects the surface tractions for this flow, and relate these to the particle deformation and suspension rheology.Finally, we study a specific type of active particle suspended in a viscoelastic fluid. At low Reynolds numbers, "swirlers" - swimmers with an axisymmetric "head" and "tail" counterrotating about the axis of symmetry - generate no net propulsion in a Newtonian fluid as a consequence of the "scallop theorem". Viscoelasticity in the suspending fluid breaks the time-reversibility of the governing equations and allows swirlers to propel themselves, with the swim speed being a function of swimmer geometry, fluid elasticity, and swimming gait. We study the unsteady motion of a freelysuspended self-propelled swirler though viscoelastic fluids described by the Giesekus model, allowing for general axisymmetric geometry and time-dependent tail rotation rate. We show how the steady swim speed under weakly elastic conditions can be calculated for general arbitrary axisymmetric geometries, via the reciprocal theorem and the solution of two Newtonian flow problems. In this "weak flow" limit, we analytically determine the swim speed and its dependence on the parameters of the Giesekus fluid which in turn are related to the primary and secondary normal stress coefficients v - rheological parameters of interest for characterizing viscoelastic fluids. Furthermore, for weak fluid elasticity, we derive the unsteady swim speed as an analytic function of a specified unsteady tail rotation rate and the material properties of the suspending fluid. We show that for a particular tail rotation rate, the unsteady swim speed can be analyzed to recover the spectrum of fluid relaxation times, analogous to small-amplitude oscillatory shear measurements on a benchtop rheometer. This study expands upon the design space for a "swimming rheometer" by increasing its functionality to make and interpret rheological measurements.
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650 4 $a Mechanics. $3 527684
650 4 $a Physics. $3 564049
650 4 $a Polymer chemistry. $3 1182163
650 4 $a Recreation. $3 559433
650 4 $a Chemistry. $3 593913
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