24 research outputs found
Dynamic characterization of cellulose nanofibrils in sheared and extended semi-dilute dispersions
New materials made through controlled assembly of dispersed cellulose
nanofibrils (CNF) has the potential to develop into biobased competitors to
some of the highest performing materials today. The performance of these new
cellulose materials depends on how easily CNF alignment can be controlled with
hydrodynamic forces, which are always in competition with a different process
driving the system towards isotropy, called rotary diffusion. In this work, we
present a flow-stop experiment using polarized optical microscopy (POM) to
study the rotary diffusion of CNF dispersions in process relevant flows and
concentrations. This is combined with small angle X-ray scattering (SAXS)
experiments to analyze the true orientation distribution function (ODF) of the
flowing fibrils. It is found that the rotary diffusion process of CNF occurs at
multiple time scales, where the fastest scale seems to be dependent on the
deformation history of the dispersion before the stop. At the same time, the
hypothesis that rotary diffusion is dependent on the initial ODF does not hold
as the same distribution can result in different diffusion time scales. The
rotary diffusion is found to be faster in flows dominated by shear compared to
pure extensional flows. Furthermore, the experimental setup can be used to
quickly characterize the dynamic properties of flowing CNF and thus aid in
determining the quality of the dispersion and its usability in material
processes.Comment: 45 pages, 13 figure
Fluidtransport i gasdiffusionsskikt i polymerelektrolytsbrÀnsleceller berÀknade med Lattice Boltzmann metoden
Angular dynamics of non-spherical particles in linear flows related to production of biobased materials
Dispersed particle flows are encountered in many biological, geophysical but also in industrial situations, e.g. during processing of materials. In these flows, the particles usually are non-spherical and their angular dynamics play a crucial role for the final material properties. Generally, the angular dynamics of a particle is dependent on the local flow in the frame-of-reference of this particle. In this frame, the surrounding flow can be linearized and the linear velocity gradient will determine how the particle rotates. In this thesis, the main objective is to improve the fundamental knowledge of the angular dynamics of non-spherical particles related to two specific biobased material processes. Firstly, the flow of suspended cellulose fibers in a papermaking process is used as a motivation. In this process, strong shear rates close to walls and the size of the fibers motivates the study of inertial effects on a single particle in a simple shear flow. Through direct numerical simulations combined with a global stability analysis, this flow problem is approached and all stable rotational states are found for spheroidal particles with aspect ratios ranging from moderately slender fibers to thin disc-shaped particles. The second material process of interest is the production of strong cellulose filaments produced through hydrodynamic alignment and assembly of cellulose nanofibrils (CNF). The flow in the preparation process and the small size of the particles motivates the study of alignment and rotary diffusion of CNF in a strain flow. However, since the particles are smaller than the wavelength of visible light, the dynamics of CNF is not easily captured with standard optical techniques. With a new flow-stop experiment, rotary diffusion of CNF is measured using Polarized optical microscopy. This process is found to be quite complicated, where short-range interactions between fibrils seem to play an important role. New time-resolved X-ray characterization techniques were used to target the underlying mechanisms, but are found to be limited by the strong degradation of CNF due to the radiation. Although the results in this thesis have limited direct applicability, they provide important fundamental stepping stones towards the possibility to control fiber orientation in flows and can potentially lead to new tailor-made materials assembled from a nano-scale.QC 20160929</p
Angular dynamics of non-spherical particles in linear flows related to production of biobased materials
Dispersed particle flows are encountered in many biological, geophysical but also in industrial situations, e.g. during processing of materials. In these flows, the particles usually are non-spherical and their angular dynamics play a crucial role for the final material properties. Generally, the angular dynamics of a particle is dependent on the local flow in the frame-of-reference of this particle. In this frame, the surrounding flow can be linearized and the linear velocity gradient will determine how the particle rotates. In this thesis, the main objective is to improve the fundamental knowledge of the angular dynamics of non-spherical particles related to two specific biobased material processes. Firstly, the flow of suspended cellulose fibers in a papermaking process is used as a motivation. In this process, strong shear rates close to walls and the size of the fibers motivates the study of inertial effects on a single particle in a simple shear flow. Through direct numerical simulations combined with a global stability analysis, this flow problem is approached and all stable rotational states are found for spheroidal particles with aspect ratios ranging from moderately slender fibers to thin disc-shaped particles. The second material process of interest is the production of strong cellulose filaments produced through hydrodynamic alignment and assembly of cellulose nanofibrils (CNF). The flow in the preparation process and the small size of the particles motivates the study of alignment and rotary diffusion of CNF in a strain flow. However, since the particles are smaller than the wavelength of visible light, the dynamics of CNF is not easily captured with standard optical techniques. With a new flow-stop experiment, rotary diffusion of CNF is measured using Polarized optical microscopy. This process is found to be quite complicated, where short-range interactions between fibrils seem to play an important role. New time-resolved X-ray characterization techniques were used to target the underlying mechanisms, but are found to be limited by the strong degradation of CNF due to the radiation. Although the results in this thesis have limited direct applicability, they provide important fundamental stepping stones towards the possibility to control fiber orientation in flows and can potentially lead to new tailor-made materials assembled from a nano-scale.QC 20160929</p
The influence of inertia on the rotational dynamics of spheroidal particles suspended in shear flow
Dispersed particle flows occur in many industrial, biological and geophysical applications. The knowledge of how these flow behave can for example lead to improved material processes, better predictions of vascular diseases or more accurate climate models. These particle flows have certain properties that depend on single particle motion in fluid flows and especially how they are distributed both in terms of spatial position and, if they are non-spherical, in terms of orientation. Much is already known about the motion of perfectly spherical particles. For non-spherical particles, apart from their translation, it is important to know the the rotational motion due to local velocity gradients. Such studies have usually been restricted by the assumption that particles are extremely small compared to fluid length scales. In this limit, both inertia of the particle and inertia of the fluid can be neglected for the particle motion. This thesis gives a complete picture of how a spheroidal particle (a particle described by a rotation of an ellipse around one of its principal axes) behave in a linear shear flow when including both fluid and particle inertia, using numerical simulations. It is observed that this very simple problem possess very interesting dynamical behavior with different stable rotational states appearing as a competition between the two types of inertia. The effect of particle inertia leads to a rotation where the mass of the particle is concentrated as far away from the rotational axis as possible, i.e.\ a rotation around the minor axis. Typically, the effect of fluid inertia is instead that it tries to force the particle in a rotation where the streamlines of the flow remain as straight as possible. The first effect of fluid inertia is thus the opposite of particle inertia and instead leads to a particle rotation around the major axis. Depending on rotational state, the particles also affect the apparent viscosity of the particle dispersion. The different transitions and bifurcations between rotational states are characterized in terms of non-linear dynamics, which reveal that the particle motion probably can be described by some reduced model. The results in this theses provides fundamental knowledge and is necessary to understand flows containing non-spherical particles.Flöden med dispergerade partiklar pÄtrÀffas i mÄnga industriella, biologiska och geofysiska tillÀmpningar. Kunskap om hur dessa flöden beter sig kan bl.a. leda till förbÀttrade materialprocesser, bÀttre förutsÀgelser om hjÀrt- och kÀrlsjukdomar eller mer noggranna vÀderprognoser. Dessa flödens egenskaper beror pÄ hur enskilda partiklar rör sig i en fluid och speciellt hur de Àr fördelade bÄde i termer av position och, om de Àr icke-sfÀriska, i termer av orientering. Mycket Àr redan kÀnt om rörelsen av perfekt sfÀriska partiklar. För icke-sfÀriska partiklar Àr det inte bara translationen som Àr av intresse utan det Àr Àven viktigt att veta hur partiklarna roterar till följd av lokala hastighetsgradienter. SÄdana studier har tidigare varit begrÀnsade av antagandet att partiklarna Àr extremt smÄ jÀmfört med fluidens typiska lÀngdskalor. I denna grÀns kan bÄde partikelns och fluidens tröghet antas försumbar. Den hÀr avhandlingen ger en komplett bild av hur en sfÀroidisk partikel (en partikel som beskrivs av en rotation av en ellips runt en av dess huvudaxlar) beter sig i ett linjÀrt skjuvflöde nÀr tröghetseffekter inkluderas. Resultaten har erhÄllits genom numeriska simuleringar. Det visar sig att detta enkla problem Àr vÀldigt rikt pÄ olika dynamiska beteenden med flera stabila rotationstillstÄnd som uppstÄr tilll följd av bÄde partikel- och fluidtröghet. Inverkan av partikeltröghet leder till en rotation dÀr massan av partikeln Àr koncentrerad sÄ lÄngt ifrÄn rotationsaxeln som möjligt, d.v.s. en rotation runt lillaxeln. Den typiska inverkan av fluidtröghet Àr istÀllet att fluiden försöker pÄtvinga partikeln en rotation dÀr strömlinjer förblir sÄ raka som möjligt. PrimÀrt leder detta till att partikeln istÀllet roterar runt storaxeln. Beroende pÄ rotationstillstÄnd, sÄ har partikeln Àven olika inverkan pÄ den mÀrkbara viskositeten av partikeldispersionen. De olika övergÄngarna och bifurkationerna mellan rotationstillstÄnd Àr karaktÀriserade i termer av icke-linjÀr dynamik, vilket visar pÄ att partikelrörelserna förmodligen kan beskrivas med en reducerad modell. Resultaten i denna avhandling Àr dÀrför fundamental kunskap och ett nödvÀndigt steg mot att förstÄ beteendet av flöden med dispergerade, icke-sfÀriska partiklar.QC 20140328</p