| 1. |
- Almondo, Gino, et al.
(author)
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Intrinsic viscosity of a suspension of weakly Brownian ellipsoids in shear
- 2018
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In: Physical Review Fluids. - 2469-990X. ; 3:6
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Journal article (peer-reviewed)abstract
- We analyze the angular dynamics of triaxial ellipsoids in a shear flow subject to weak thermal noise. By numerically integrating an overdamped angular Langevin equation, we find the steady angular probability distribution for a range of triaxial particle shapes. From this distribution we compute the intrinsic viscosity of a dilute suspension of triaxial particles. We determine how the viscosity depends on particle shape in the limit of weak thermal noise. While the deterministic angular dynamics depends very sensitively on particle shape, we find that the shape dependence of the intrinsic viscosity is weaker, in general, and that suspensions of rodlike particles are the most sensitive to breaking of axisymmetry. The intrinsic viscosity of a dilute suspension of triaxial particles is smaller than that of a suspension of axisymmetric particles with the same volume and the same ratio of major to minor axis lengths.
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| 2. |
- Byron, M., et al.
(author)
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Shape-dependence of particle rotation in isotropic turbulence
- 2015
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In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 27:3
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Journal article (peer-reviewed)abstract
- We consider the rotation of neutrally buoyant axisymmetric particles suspended in isotropic turbulence. Using laboratory experiments as well as numerical and analytical calculations, we explore how particle rotation depends upon particle shape. We find that shape strongly affects orientational trajectories, but that it has negligible effect on the variance of the particle angular velocity. Previous work has shown that shape significantly affects the variance of the tumbling rate of axisymmetric particles. It follows that shape affects the spinning rate in away that is, on average, complementary to the shape-dependence of the tumbling rate. We confirm this relationship using direct numerical simulations, showing how tumbling rate and spinning rate variances show complementary trends for rod-shaped and disk-shaped particles. We also consider a random but non-turbulent flow. This allows us to explore which of the features observed for rotation in turbulent flow are due to the effects of particle alignment in vortex tubes. (C) 2015 AIP Publishing LLC.
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| 3. |
- Candelier, Fabien, et al.
(author)
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Angular Dynamics of a Small Particle in Turbulence
- 2016
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 117
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Journal article (peer-reviewed)abstract
- We compute the angular dynamics of a neutrally buoyant nearly spherical particle immersed in an unsteady fluid. We assume that the particle is small, that its translational slip velocity is negligible, and that unsteady and convective inertia are small perturbations. We derive an approximation for the torque on the particle that determines the first inertial corrections to Jeffery’s equation. These corrections arise as a consequence of local vortex stretching and can be substantial in turbulence, where local vortex stretching is strong and closely linked to the irreversibility of turbulence.
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| 4. |
- Candelier, F., et al.
(author)
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Role of inertia for the rotation of a nearly spherical particle in a general linear flow
- 2015
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 91:5
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Journal article (peer-reviewed)abstract
- We analyze the angular dynamics of a neutrally buoyant, nearly spherical particle immersed in a steady general linear flow. The hydrodynamic torque acting on the particle is obtained by means of a reciprocal theorem, a regular perturbation theory exploiting the small eccentricity of the nearly spherical particle, and by assuming that inertial effects are small but finite.
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| 5. |
- Einarsson, Emma, et al.
(author)
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Relating MR relaxation times of ex vivo meniscus to tissue degeneration through comparison with histopathology
- 2020
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In: Osteoarthritis and Cartilage Open. - : Elsevier BV. - 2665-9131. ; 2:2
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Journal article (peer-reviewed)abstract
- Background: Quantitative magnetic resonance imaging (MRI), e.g. relaxation parameter mapping, may be sensitive to structural and compositional tissue changes, and could potentially be used to non-invasively detect and monitor early meniscus degeneration related to knee osteoarthritis. Objective: To investigate MR relaxation times as potential biomarkers for meniscus degeneration through comparisons with histopathology. Methods: We measured MR relaxation parameters in the posterior horn of 40 menisci (medial and lateral) at a wide range of degenerative stages. T1, T2 and T2∗ were mapped using standard and ultrashort echo time sequences at 9.4 T and compared to gold standard histology using Pauli's histopathological scoring system, including assessment of surface integrity, collagen organization, cellularity and Safranin-O staining. Results: All three relaxation times increased with total Pauli score (mean difference per score (95% CI) for T2∗: 0.62 (0.37, 0.86), T2: 0.83 (0.53, 1.1) and T1: 24.7 (16.5, 32.8) ms/score). Clear associations were seen with scores of surface integrity (mean difference per score for T2∗: 3.0 (1.8, 4.2), T2: 4.0 (2.5, 5.5) and T1: 116 (75.6, 156) ms/score) and collagen organization (mean difference between highest and lowest score for T2∗: 5.3 (1.6, 8.9), T2: 6.1 (1.7, 11) and T1: 204 (75.9, 332) ms). The results were less clear for the remaining histopathological measures. Conclusions: MR relaxation times T1, T2 and T2∗ of ex vivo human menisci are associated with histologically verified degenerative processes, in particular related to surface integrity and collagen organization. If confirmed in vivo, MR relaxation times may thus be potential biomarkers for meniscus degeneration.
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| 6. |
- Einarsson, Emma, et al.
(author)
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The role of cartilage glycosaminoglycan structure in gagCEST
- 2020
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In: NMR in Biomedicine. - : Wiley. - 0952-3480 .- 1099-1492. ; 33:5
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Journal article (peer-reviewed)abstract
- Glycosaminoglycan (GAG) chemical exchange saturation transfer (gagCEST) is a potential method for cartilage quality assessment. The aim of this study was to investigate how the gagCEST effect depends on the types and molecular organization of GAG typically found in articular cartilage. gagCEST was performed on different concentrations of GAG in various forms: free chains of chondroitin sulfate (CS) of different types (-A and -C) and GAG bound to protein in aggregated and nonaggregated aggrecan extracted from calf articular cartilage. The measured magnetization transfer ratio asymmetry (MTRasym ) was compared with known GAG concentrations or GAG concentrations determined through biochemical analysis. The gagCEST effect was assessed through the linear regression coefficient with 95% confidence interval of MTRasym per GAG concentration. We observed a lower gagCEST effect in phantoms containing a mixture of CS-A and CS-C compared with phantoms containing mainly CS-A. The difference in response corresponds well to the difference in CS-A concentration. GAG bound in aggrecan from calf articular cartilage, where CS-A is assumed to be the major type of GAG, produed a similar gagCEST effect as that observed for free CS-A. The effect was also similar for aggregated (ie, bound to hyaluronic acid) and nonaggregated aggrecan. In conclusion, our results indicate that the aggrecan structure in itself does not impact the gagCEST effect, but that the effect is strongly dependent on GAG type. In phantoms, the current implementation of gagCEST is sensitive to CS-A while for CS-C, the main GAG component in mature human articular cartilage, the sensitivity is limited. This difference in gagCEST sensitivity between GAG types detected in phantoms is a strong motivation to also explore the possibility of a similar effect in vivo.
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| 7. |
- Einarsson, Jonas
(author)
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Angular dynamics of small particles in fluids
- 2015
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Doctoral thesis (other academic/artistic)abstract
- This thesis concerns the angular motion of small particles suspended in fluid flows. A small particle experiences a hydrodynamic torque due to the local fluid velocity, and this torque leads to rotational motion. When inertial effects are negligible the torque on an ellipsoidal particle is given by Jeffery's theory [Jeffery, G. B. Proc. R. Soc. Lond. A 102, 161–179 (1922)]. In this thesis and the appended papers I describe three studies that all relate to this well-known result. First, we derive an effective equation of motion for the orientation of a spheroid in a simple shear flow, valid for small values of the shear Reynolds number $\textrm{Re}_s=sa^2/\nu$, where $s$ is the shear rate, $a$ the particle size and $\nu$ the kinematic viscosity of the suspending fluid. In absence of inertia the equation of motion has infinitely many periodic solutions, the 'Jeffery orbits'. We show how this degeneracy is lifted by the effects of inertia. Second, we describe experimental observations of the orientational dynamics of asymmetric particles advected in a microchannel. We record several trajectories with each particle by resetting the initial condition with an optical trap. We find that the dynamics depend sensitively on both particle shape and initial conditions. This confirms earlier theoretical results, which are also described in this thesis. Third, we discuss the angular dynamics of axisymmetric particles in turbulent and random flow. In these flows the statistical averages of the angular dynamical quantities depend crucially on the intricate correlations between the particle orientation, angular velocity, and the flow vorticity relative to the principal straining directions of the fluid flow. We illustrate this by direct numerical simulation, experimental measurements and statistical model calculations. Finally, this thesis contains an introduction to the field aimed at new students, as well as an accessible popular science introduction to low Reynolds particle dynamics.
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| 8. |
- Einarsson, Jonas, et al.
(author)
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Effect of weak fluid inertia upon Jeffery orbits
- 2015
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 91
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Journal article (peer-reviewed)abstract
- We consider the rotation of small neutrally buoyant axisymmetric particles in a viscous steady shear flow. When inertial effects are negligible the problem exhibits infinitely many periodic solutions, the “Jeffery orbits.” We compute how inertial effects lift their degeneracy by perturbatively solving the coupled particle-flow equations. We obtain an equation of motion valid at small shear Reynolds numbers, for spheroidal particles with arbitrary aspect ratios. We analyze how the linear stability of the “log-rolling” orbit depends on particle shape and find it to be unstable for prolate spheroids. This resolves a puzzle in the interpretation of direct numerical simulations of the problem. In general, both unsteady and nonlinear terms in the Navier-Stokes equations are important.
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| 9. |
- Einarsson, Jonas
(author)
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New biological device not faster than regular computer
- 2016
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In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 113:23
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Journal article (peer-reviewed)abstract
- Nicolau et al. (1) describe a proof-of-concept biological computing device that performs parallel computations by letting protein filaments simultaneously explore all branches of a problem-specific structured network. This innovative device represents a new direction in alternative means of computing, and its construction is an impressive technical achievement. However, I find it necessary to comment on the authors’ statements on solving large combinatorial problems. In particular, the new device does not circumvent the superpolynomial time requirement to solve the nondeterministic-polynomial-time (NP) complete subset sum problem (SSP), and it does not move forward the limit of the size of combinatorial problems that can …
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| 10. |
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