| 1. |
- Bagge, Joar, 1991-, et al.
(författare)
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Parabolic velocity profile causes shape-selective drift of inertial ellipsoids
- 2021
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 926
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Tidskriftsartikel (refereegranskat)abstract
- Understanding particle drift in suspension flows is of the highest importance in numerous engineering applications where particles need to be separated and filtered out from the suspending fluid. Commonly known drift mechanisms such as the Magnus force, Saffman force and Segre-Silberberg effect all arise only due to inertia of the fluid, with similar effects on all non-spherical particle shapes. In this work, we present a new shape-selective lateral drift mechanism, arising from particle inertia rather than fluid inertia, for ellipsoidal particles in a parabolic velocity profile. We show that the new drift is caused by an intermittent tumbling rotational motion in the local shear flow together with translational inertia of the particle, while rotational inertia is negligible. We find that the drift is maximal when particle inertial forces are of approximately the same order of magnitude as viscous forces, and that both extremely light and extremely heavy particles have negligible drift. Furthermore, since tumbling motion is not a stable rotational state for inertial oblate spheroids (nor for spheres), this new drift only applies to prolate spheroids or tri-axial ellipsoids. Finally, the drift is compared with the effect of gravity acting in the directions parallel and normal to the flow. The new drift mechanism is stronger than gravitational effects as long as gravity is less than a critical value. The critical gravity is highest (i.e. the new drift mechanism dominates over gravitationally induced drift mechanisms) when gravity acts parallel to the flow and the particles are small.
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| 2. |
- Brouzet, Christophe, et al.
(författare)
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Effect of Electric Field on the Hydrodynamic Assembly of Polydisperse and Entangled Fibrillar Suspensions
- 2021
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 37:27, s. 8339-8347
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Tidskriftsartikel (refereegranskat)abstract
- Dynamics of colloidal particles can be controlled by the application of electric fields at micrometer-nanometer length scales. Here, an electric field-coupled microfluidic flow-focusing device is designed for investigating the effect of an externally applied alternating current (AC) electric field on the hydrodynamic assembly of cellulose nanofibrils (CNFs). We first discuss how the nanofibrils align parallel to the direction of the applied field without flow. Then, we apply an electric field during hydrodynamic assembly in the microfluidic channel and observe the effects on the mechanical properties of the assembled nanostructures. We further discuss the nanoscale orientational dynamics of the polydisperse and entangled fibrillar suspension of CNFs in the channel. It is shown that electric fields induced with the electrodes locally increase the degree of orientation. However, hydrodynamic alignment is demonstrated to be much more efficient than the electric field for aligning CNFs. The results are useful for understanding the development of the nanostructure when designing high-performance materials with microfluidics in the presence of external stimuli.
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| 3. |
- Rosén, Tomas, 1985-, et al.
(författare)
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Elucidating the Opportunities and Challenges for Nanocellulose Spinning
- 2021
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 33:28, s. 2001238-
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Tidskriftsartikel (refereegranskat)abstract
- Man-made continuous fibers play an essential role in society today. With the increase in global sustainability challenges, there is a broad spectrum of societal needs where the development of advanced biobased fibers could provide means to address the challenges. Biobased regenerated fibers, produced from dissolved cellulose are widely used today for clothes, upholstery, and linens. With new developments in the area of advanced biobased fibers, it would be possible to compete with high-performance synthetic fibers such as glass fibers and carbon fibers as well as to provide unique functionalities. One possible development is to fabricate fibers by spinning filaments from nanocellulose, Nature's nanoscale high-performance building block, which will require detailed insights into nanoscale assembly mechanisms during spinning, as well as knowledge regarding possible functionalization. If successful, this could result in a new class of man-made biobased fibers. This work aims to identify the progress made in the field of spinning of nanocellulose filaments, as well as outline necessary steps for efficient fabrication of such nanocellulose-based filaments with controlled and predictable properties.
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| 4. |
- Rosén, Tomas, 1985-, et al.
(författare)
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Shear-free mixing to achieve accurate temporospatial nanoscale kinetics through scanning-SAXS : ion-induced phase transition of dispersed cellulose nanocrystals
- 2021
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Ingår i: Lab on a Chip. - : Royal Society of Chemistry (RSC). - 1473-0197 .- 1473-0189. ; 21:6, s. 1084-1095
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved in situ characterization of well-defined mixing processes using small-angle X-ray scattering (SAXS) is usually challenging, especially if the process involves changes of material viscoelasticity. In specific, it can be difficult to create a continuous mixing experiment without shearing the material of interest; a desirable situation since shear flow both affects nanoscale structures and flow stability as well as resulting in unreliable time-resolved data. Here, we demonstrate a flow-focusing mixing device for in situ nanostructural characterization using scanning-SAXS. Given the interfacial tension and viscosity ratio between core and sheath fluids, the core material confined by sheath flows is completely detached from the walls and forms a zero-shear plug flow at the channel center, allowing for a trivial conversion of spatial coordinates to mixing times. With this technique, the time-resolved gel formation of dispersed cellulose nanocrystals (CNCs) was studied by mixing with a sodium chloride solution. It is observed how locally ordered regions, so called tactoids, are disrupted when the added monovalent ions affect the electrostatic interactions, which in turn leads to a loss of CNC alignment through enhanced rotary diffusion. The demonstrated flow-focusing scanning-SAXS technique can be used to unveil important kinetics during structural formation of nanocellulosic materials. However, the same technique is also applicable in many soft matter systems to provide new insights into the nanoscale dynamics during mixing.
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| 5. |
- Rosén, Tomas, 1985-, et al.
(författare)
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Understanding ion-induced assembly of cellulose nanofibrillar gels through shear-free mixing and in situ scanning-SAXS
- 2021
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Ingår i: Nanoscale Advances. - : Royal Society of Chemistry (RSC). - 2516-0230. ; 3:17, s. 4940-4951
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Tidskriftsartikel (refereegranskat)abstract
- During the past decade, cellulose nanofibrils (CNFs) have shown tremendous potential as a building block to fabricate new advanced materials that are both biocompatible and biodegradable. The excellent mechanical properties of the individual CNF can be transferred to macroscale fibers through careful control in hydrodynamic alignment and assembly processes. The optimization of such processes relies on the understanding of nanofibril dynamics during the process, which in turn requires in situ characterization. Here, we use a shear-free mixing experiment combined with scanning small-angle X-ray scattering (scanning-SAXS) to provide time-resolved nanoscale kinetics during the in situ assembly of dispersed cellulose nanofibrils (CNFs) upon mixing with a sodium chloride solution. The addition of monovalent ions led to the transition to a volume-spanning arrested (gel) state. The transition of CNFs is associated with segmental aggregation of the particles, leading to a connected network and reduced Brownian motion, whereby an aligned structure can be preserved. Furthermore, we find that the extensional flow seems to enhance the formation of these segmental aggregates, which in turn provides a comprehensible explanation for the superior material properties obtained in shear-free processes used for spinning filaments from CNFs. This observation clearly highlights the need for different assembly strategies depending on morphology and interactions of the dispersed nanoparticles, where this work can be used as a guide for improved nanomaterial processes.
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