Fiber Suspensions Research Articles

The influence of versatile thiol-norbornene modifications to cellulose nanofibers on rheology and film properties

Cellulose nanofibrils (CNF) can form impressive barrier layers but difficult rheological properties, brittleness, and sensitivity to moisture limit their use. To overcome these challenges, esterification reactions were performed in water without volatile organic solvents to create carbic-functionalized CNFs (cCNFs) that enabled versatile, thiol-norbornene secondary modifications. Chemical analysis determined that on average 5% anhydroglucose repeat units were functionalized with norbornene groups. Thiol-norbornene reactions added molecules with varying polar surface areas to the CNFs. Modifications did not change the film properties to a large extent. All CNF films were excellent grease barriers. The modifications significantly changed the rheology of CNF suspensions as the complex viscosity of the modified CNF was 27 times lower than unmodified CNFs. Modification also reduced the filtration rate by a factor of four. Surface modifications appeared to alter the colloidal forces between fibers in suspension that influence the flow and drainage properties.

Numerical simulation and modeling of the die swell for fiber suspension flows

A numerical model is developed to examine the die swell for fiber suspensions, which occurs in 3D printing extrusion processes. More specifically, it focuses on the fiber orientation distribution in a Newtonian suspending fluid and its effects on the shape of the free surface. In a first step, the flow in a 2D axisymmetric capillary die is explored in order to validate the numerical implementation for the fiber orientation model. It is found that the coupling effect flattens the velocity profile but has a small impact on the fiber orientation distribution in the suspension flow. In a second step, the modeling of die swell is considered and result from the literature for the final swelling height is obtained for an uncoupled flow. For fully coupled flows, different values for the coupling parameter and the fiber-fiber interaction coefficient have been tested in order to observe their effects on the free surface shape and on the final fiber orientation distribution. The increase of the fiber coupling parameter increases the die swell ratio when fibers are randomly distributed at the flow inlet, but the opposite behavior, i.e., a decrease of the swelling ratio, is noticed when a perfect fiber alignment condition is imposed at the entrance for low fiber interaction coefficients. The Tanner model, initially developed for viscoelastic fluids, is updated to estimate extrudate swell for fibers suspended in a Newtonian fluid in terms of the coupling parameter and the interaction coefficient. Although this analytical approach used a quadratic closure approximation, it gives qualitative results when compared with the numerical simulations.

Production and Characterization of Paper from Pineapple Leaves and Sugarcane Bagasse Pulp

Paper is the final product resulted from compression of fiber suspension of cellulose pulp, which is derived from wood or non-woody material. In this study, paper was made from mixture of pineapple leaves and sugarcane bagasse pulp (P:S, 100:0%, 80:20%, 60:40%, 40:60%, 20:80%, and 0:100%) at varying cooking liquor or NaOH concentration (1.5% and 3%) using semi-mechanical soda pulping method. Pulp characterisation (kappa number, tensile and tearing strength) was further carried out to ensure a stable pulp and paper quality. Results showed that the highest tensile strength of 6,709 kN was obtained at pineapple leaves and sugarcane bagasse pulp composition of 100:0% and NaOH 3%, whereas the highest tearing strength (1730,13 mN) was observed at the P:S of 40:60% with similar concentration of NaOH. The lowest kappa number (6,182) was also obtained at P:S of 0:100% and 3% NaOH. The pulp characterisation values have fullfil SNI No. 6519:2016 quality standard for plastic laminated paper.

Effects of sucrose addition on the rheological properties of citrus peel fiber suspensions before and after drying

In this study we investigated the impact of sucrose on the rheological properties of high pressure homogenized citrus peel fiber suspensions before and after drying. Drying of such cellulose-rich microfibrillated systems often goes paired with structural collapse which leads to a product with sub-optimal functional properties (e.g. swelling, water holding …) upon reconstitution. The purpose was to better understand the role of sucrose on preventing structural collapse during drying and consequently identify the optimum fiber/sucrose mixing ratio allowing optimal recovery of the functional properties upon reconstitution. High pressure homogenized samples were prepared by fixing the fiber concentration at 1% w/w while varying sucrose content from 0% to ~60% w/w. Through these investigations, we demonstrated that the presence of sucrose (at the given concentrations) seems to have a negative impact on the functionalized fiber properties prior drying, whereas a positive effect could be observed for the dried functionalized fibers when they were co-dried in presence of 30% w/w sucrose. This sucrose concentration allowed recovery of the functionalized fiber properties prior drying. The initial fiber properties recovery suggests that sucrose can act as a hydrogen bond blocker or a spacer that limits the irreversible aggregation effect usually observed for plant cell wall-derived fibers upon drying.

Pipe rheology of microfibrillated cellulose suspensions

The shear rheology of two mechanically manufactured microfibrillated cellulose (MFC) suspensions was studied in a consistency range of 0.2–2.0% with a pipe rheometer combined with ultrasound velocity profiling. The MFC suspensions behaved at all consistencies as shear thinning power law fluids. Despite their significantly different particle size, the viscous behavior of the suspensions was quantitatively similar. For both suspensions, the dependence of yield stress and the consistency index on consistency was a power law with an exponent of 2.4, similar to some pulp suspensions. The dependence of flow index on consistency was also a power law, with an exponent of − 0.36. The slip flow was very strong for both MFCs and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress, slip flow caused drag reduction with consistencies higher than 0.8%. When inspecting the slip velocities of both suspensions as a function of wall shear stress scaled with the yield stress, a good data collapse was obtained. The observed similarities in the shear rheology of both the MFC suspensions and the similar behavior of some pulp fiber suspensions suggests that the shear rheology of MFC suspensions might be more universal than has previously been realized.

Fibre kinematics in dilute non-Newtonian fibre suspensions during confined and lubricated squeeze flow: Direct numerical simulation and analytical modelling

The properties of short fibre-reinforced polymer composites depend on the distribution and the orientation of fibres which drastically changes during the forming of composites. During this stage, these materials behave as fibre suspensions and usually flow in confined geometries. To analyse their flow-induced fibrous microstructures, we previously conducted 3D real-time in situ observations of the compression of non-Newtonian dilute fibre suspensions using fast X-ray microtomography [T. Laurencin et al., Compos Sci Technol134 (2016)]. Here, we successfully simulated these experiments with a multi-domain Finite Element code and compared them with the predictions of Jeffery's model. Often, the Jeffery's equations agree with the experimental and numerical data. However, for fibres closed to compression platens, important deviations were observed with faster simulated and experimental fibre rotation. Adopting the dumbbell approach and revisiting the recent work of Perez et al. [J non-Newtonian Fluid Mech233 (2016)], an extension of the Jeffery's model is proposed to account both for the non-Newtonian rheology of the suspending fluid and confinement effects. Despite its simplicity, the new model allows a good description of simulation and experimental results.

Flow characteristics of drag-reducing natural bamboo fiber suspensions with minimal environmental load

The reduction of pipe friction loss by adding drag-reducing agents has attracted attention as an aid to energy conservation. Drag-reducing agents induce drag reduction (DR) effects and should have a minimal environmental load, with natural resource-saving potential. This study demonstrates bamboo fiber as a drag-reducing agent that saves natural resources and has a low environmental load. Using pressure drop measurements, we report DR with suspensions of bamboo fibers with the average diameter of 13.3 μm and aspect ratio of 98.7. The maximum DR obtained in this experiment is 43% at the concentration of 4000 ppm and pipe diameter of 30 mm; DR is affected by the Reynolds number, suspension concentration, and pipe diameter. In addition, the bamboo fibers can be easily removed from the suspensions by filtration. We found that low-environmental-load bamboo fiber has DR effects like those of other fibers; its effects are greater than those of conventional synthetic fibers and wood pulp. Furthermore, it is resistant to mechanical degradation, recoverable, and recyclable. Therefore, DR effects can be selectively obtained by adding the fibers only when DR is needed; the fibers can then be collected when DR is no longer necessary. This method might greatly expand the application range of DR agents. The results demonstrate the usefulness of bamboo fibers as DR additives.

Theoretical prediction of atomization performance of fibre suspensions and the effect of feed temperature and air velocity

Atomization performances of juice-fibre suspensions, which are non-Newtonian extensional liquids, inside a two-fluid nozzle were predicted theoretically by using two different relationships, in which one of the relationships does not incorporate the extensional rheological properties in its prediction. A good correlation was obtained between the theoretical and experimental droplet sizes of fibre suspensions when the theoretical droplet sizes were predicted by considering the extensional rheological property which is the relaxation time of the fibre suspensions. This finding confirms that the atomization of fibre suspensions, which the fibre suspensions exhibit significant extensional properties, are dominantly controlled by extensional resistance with minor effects of shear resistance and surface tension. It also implies that the relatively bigger droplet sizes of the fibre suspensions, when compared to Newtonian droplets, were due to the extensional resistance of the suspensions, with little effect by shear viscosity and surface tension. The atomization performance was observed at elevated temperatures ranging between 20 and 70 °C. By using a two-fluid nozzle at a low atomizing air velocity of 150 m/s, the D(v,90) significantly reduced from >1000 μm to 650 μm when the temperature was increased from 20 to 70 °C. Meanwhile, at a high atomizing air velocity of 240 m/s, the D(v,90) can be said to remain constant between 350 and 400 μm.

Shear reversal experiments on concentrated rigid fiber suspensions

Shear reversal is a smart rheological protocol allowing inspection of the contribution of the nonhydrodynamic contact forces between particles on the rheology of concentrated suspensions. In this paper, this protocol is applied to concentrated suspensions of rigid fibers dispersed in a Newtonian liquid at three aspect ratios ranging from 10 to 31 and various concentrations. When the shear direction is reversed, a low viscosity pseudoplateau is observed and we show that the value of the viscosity at pseudoplateau is close to the steady-state viscosity value expected for semidilute suspensions where the fibers interact mainly via hydrodynamic forces, while the contact interactions are released for a short time just after shear reversal. The low viscosity pseudoplateau is followed by a viscosity overshoot that is interpreted in terms of the interplay between contact interparticle interactions (that reappear at the end of the low viscosity plateau) and fiber reorientation. In particular, we show that the accumulated strain at which the overshoot occurs is controlled by a strain scale associated with the reconstruction of the contact network. After the viscosity overshoot, the viscosity decreases and reaches the steady value that it had before shear reversal.

Effect of fiber suspension jet stability on alignment quality of discontinuous carbon fiber tapes

A hydrodynamic alignment process has been developed for converting discontinuous random carbon fibers into tapes with a highly aligned orientation distribution to greatly improve the applicability of recovered fibers to composite parts. In hydrodynamic alignment processes short fibers are aligned by the velocity gradient along the flow direction in a convergent nozzle. Thereafter the jet of fiber suspension is deposited on a nylon mesh and the now redundant dispersion medium is drained away to leave an aligned fiber tape. The fundamental physical principles at work in the process have not been widely studied and are shown in the present work to greatly influence the properties of the resulting materials. In this work, the influence of suspension jet stability on the fiber orientation distribution was examined and the liquid jet break-up regime was determined. To explore the factors which can affect the suspension jet stability, different nozzle geometries, viscosities of dispersion media, fiber lengths and Reynolds numbers were applied in experimental work. The shear rate profiles inside different nozzles were simulated by Computational Fluid Dynamics methods and the results described in this paper.

Artificial neural network and partial least square regressions for rapid estimation of cellulose pulp dryness based on near infrared spectroscopic data

The content of water in fiber suspension and affects pulp refining, bleaching and draining operations. Cellulose pulp dryness estimate through near infrared (NIR) spectroscopy coupled with multivariate regressions or artificial neural network (ANN) techniques are not well explored yet. In this study models were developed to estimate cellulose pulp dryness in pads based on the NIR spectra. Thus, the cellulose pulp pads (4 mm thick) were weighed and their NIR spectra were obtained in several stages during desorption from 13.1 to 98.3% of content of solids. Partial least square regression (PLS-R) was developed from whole NIR spectra (1300 Absorbance values) and six spectral variables (from 1300) were selected for developing the PLS-R (6) and the ANN model. Both trained neural network and regression can predict pulp dryness of unknown cellulose pulp pads from their NIR data with an error of 2.5%. PLS-R models based on whole NIR spectra showed accurate predictions (the R² of lab-determined and estimated values plot was 0.99) while the ANN showed the same predictive performance from only six NIR variables. Predictive models developed from full NIR spectra and those based on only 6 variables were compared. Our findings indicate that NIR spectroscopy coupled with multivariate analysis and Artificial neural networks are a promising tool for monitoring the weight variation due to dewatering of the cellulose pulps in real time.

Transparent and antimicrobial cellulose film from ginger nanofiber

As demand on the safety and quality of the product in food packaging is increased, nanocellulose based transparent paper with resistance against microbial activity becomes more important. In this study, transparent cellulose film was prepared from ginger nanofiber with chemicals and ultrasonication. After purification using acid hydrolysis, ginger fiber had high cellulose content (88%). Ultrasonication of the suspension of the chemically treated fibers for 1 h decreased their diameter to 54.3 nm. Drying of the nano-sized fiber suspension resulted in a transparent film with 5 μm thickness. This film transmitted 83.3% of the light at 650 nm and showed antimicrobial activity. It had high thermal stability with the maximum decomposition temperature peak at 353 °C. This was 17% higher than that of raw ginger fiber (302 °C). The transparent film had a crystallinity index of 48%, and lower moisture resistance than film from chemically treated cellulose fiber. This study promoted transparent nanocellulose film with good antimicrobial properties from abundant and available ginger fibers.

Control of Direct Written Ink Droplets Using Electrowetting.

Here, we investigate the feasibility and effectiveness of electrowetting in the motion control of droplets of different liquids, which are widely used as inks in direct writing (DW)-based three-dimensional (3D) printing processes for various applications. To control the movement of DW ink droplets on dielectric substrates, the electrodes were embedded in the substrate. It is demonstrated that droplets of pure liquid inks, aqueous polymer solution inks, and carbon fiber suspension inks can be moved on multi-angled surfaces. Also, experimental results reveal that droplets of a commercial hydrogel, agar-agar, alginate, xanthan gum, and gum arabic can be moved by electrowetting. Droplets of sizes 200 μm-3 mm were manipulated and moved by the electric field on different dielectric substrates accurately and repeatedly. Effective electrowetting-based control and movement of droplets were observed on horizontal, vertical, and even inverted substrates. These findings imply the feasibility and potential application of electrowetting as a flexible, rapid, and new method for ink droplet control in 3D printing processes.

NANOFIBRILLATED CELLULOSE, THE SMALL PROMISING FIBER: CHARACTERISTICS AND POTENTIALITIES

Cellulose: an abundant, biodegradable and renewable plant material that is available all around the globe, from which nanofibrillated cellulose (a homogeneous suspension of fibers with nanometer-scale diameters) can be extracted. The nanofibrillated cellulose, in what concerns its range of application, may completely or partially replace other materials due to its particular physical properties. Knowing its characteristics and properties is important in understanding its behavior in various applications. In light of this remarks, the objective of this research was to extract the nanocellulose from bleached eucalyptus pulp through mechanical milling to characterize the resulting nanofibrillated cellulose and compare both. The nanofibrillated cellulose presented a very distinct appearance in relation to the original cellulose pulp when they were analysed in the same concentrations, demonstrating greater water absorption and fibrillar interweaving. The nanofibrillated material showed different responses in comparison to the original in terms of dimensions, thermal stability, crystallinity, viscosity, and degree of polymerization. The nanofibrillated cellulose displays the same chemical composition through infrared analysis and has diameters of around 21.7 nm, smaller than the cellulose diameter of 11.4 μm. The nanofibrillated cellulose also exhibits lower thermal resistance: its decomposition temperature was about 20 ºC lower than that of the original cellulose pulp, and its crystallinity index has decreased. As for viscosity and degree of polymerization, there was reduction of 25% compared to the original material. The results presented hereby indicate that the nanofibrillated cellulose has unique features in relation to the original cellulose pulp, which supports its application as an alternative to existing materials.

Coarse graining the dynamics of immersed and driven fiber assemblies

An important class of fluid-structure problems involve the dynamics of ordered arrays of immersed, flexible fibers. While specialized numerical methods have been developed to study fluid-fiber systems, they become infeasible when there are many, rather than a few, fibers present, nor do these methods lend themselves to analytical calculation. Here, we introduce a coarse-grained continuum model, based on local-slender body theory, for elastic fibers immersed in a viscous Newtonian fluid. It takes the form of an anisotropic Brinkman equation whose skeletal drag is coupled to elastic forces. This model has two significant benefits: (1) the density effects of the fibers in a suspension become analytically manifest, and (2) it allows for the rapid simulation of dense suspensions of fibers in regimes inaccessible to standard methods. As a first validation, without fitting parameters, we achieve very reasonable agreement with 3D Immersed Boundary simulations of a bed of anchored fibers bent by a shear flow. Secondly, we characterize the effect of density on the relaxation time of fiber beds under oscillatory shear, and find close agreement to results from full numerical simulations. We then study buckling instabilities in beds of fibers, using our model both numerically and analytically to understand the role of fiber density and the structure of buckling transitions. We next apply our model to study the flow-induced bending of inclined fibers in a channel, as has been recently studied as a flow rectifier, examining the nature of the internal flows within the bed, and the emergence of inhomogeneous permeability. Finally, we extend the method to study a simple model of metachronal waves on beds of actuated fibers, as a model for ciliary beds. Our simulations reproduce qualitatively the pumping action of coordinated waves of compression through the bed.

Effect of temperature on the rheology of concentrated fiber suspensions

The effect of temperature on the apparent rheological properties of concentrated synthetic fiber suspensions was investigated experimentally. Aqueous suspensions of viscose rayon, acrylic, and nylon 6,6 fibers of various fiber concentrations, sizes, and shapes were used. At a fixed shear rate, the apparent viscosity of all the suspensions decreased reversibly with increasing temperature. The steady-state flow behavior is well described by the Bingham fluid model where the yield stress is a decreasing function of temperature and follows an Arrhenius dependence with an activation energy in the range of 2–80 kJ/mol, which is the same order of magnitude as that reported for 20 wt. % fibrous biomass suspensions below 55 °C. The fiber suspensions exhibited a negative plastic viscosity at low temperatures, and as the temperature was increased, the plastic viscosity became less negative. This temperature-dependent rheological behavior is qualitatively similar to that observed for concentrated fibrous biomass suspensions. The fiber suspensions formed heterogeneous networks where the state of aggregation depended on the experimental conditions and thus affected the macroscopic rheology.

Fabrication of Designable and Suspended Microfibers via Low-Voltage 3D Micropatterning.

Building two-dimensional (2D) and three-dimensional (3D) fibrous structures in the micro- and nanoscale will offer exciting prospects for numerous applications spanning from sensors to energy storage and tissue engineering scaffolds. Electrospinning is a well-suited technique for drawing micro- to nanoscale fibers, but current methods of building electrospun fibers in 3D are restrictive in terms of printed height, design of macroscopic fiber networks, and choice of polymer. Here, we combine low-voltage electrospinning and additive manufacturing as a method to pattern layers of suspended mesofibers. Layers of fibers are suspended between 3D-printed supports in situ in multiple fiber layers and designable orientations. We examine the key working parameters to attain a threshold for fiber suspension, use those behavioral observations to establish a “fiber suspension indicator”, and demonstrate its utility through design of intricate suspended fiber architectures. Individual fibers produced by this method approach the micrometer/submicrometer scale, while the overall suspended 3D fiber architecture can span over a centimeter in height. We demonstrate an application of suspended fiber architectures in 3D cell culture, utilizing patterned fiber topography to guide the assembly of suspended high-cellular-density structures. The solution-based fiber suspension patterning process we report offers a unique competence in patterning soft polymers, including extracellular matrix-like materials, in a high resolution and aspect ratio. The platform could thus offer new design and manufacturing capabilities of devices and functional products by incorporating functional fibrous elements.

Thermodynamic restrictions on constitutive functions for fiber suspensions

The constitutive properties of fiber suspensions are investigated with the methods of thermodynamics. Fiber orientation, orientation change velocity and gradient of orientation are considered as relevant variables for the constitutive functions, and the Second Law of Thermodynamics in the form of the dissipation inequality is exploited by a method introduced by I-S. Liu [Arch. Rat. Mech. Analysis 46 , 131 (1972)]. The restrictions on the constitutive functions show, that the fiber suspension is a micropolar continuum with non-vanishing couple stresses. In addition it is shown that the classical relation of entropy flux being heat flux over temperature holds only in the special case that the free energy density does not depend on the orientation change velocity, otherwise there exists an extra entropy flux.

Fiber suspension in 2D nonhomogeneous flow: The effects of flow/fiber coupling for Newtonian and power-law suspending fluids

A numerical study is presented for fiber suspension flows through a parallel plate channel and a planar 4:1 contraction. Besides examining a Newtonian suspending fluid, a non-Newtonian matrix exhibiting a pseudoplastic behavior and describing a power-law model is also investigated. Furthermore, instead of using orientation tensors for the macroscopic constitutive modeling, the proposed approach addresses the macroscopic scale by describing the fiber orientation state with the probability distribution function (PDF). It enables us to eliminate the error introduced due to the closure approximation when using orientation tensor description as our numerical scheme solves the PDF in both the spatial and configurational spaces. This allows us to correctly implement expressions for both the fiber extra stress, especially for the suspending matrix displaying a pseudoplastic behavior, and the fiber orientation state, describing the configuration. Hence, these two constitutive relations for suspensions are used to perform simulations in which flow and fiber orientation are fully coupled. Results are presented in planar geometries involving channel and 4:1 contraction flows. It is found that the coupling effect flattens the velocity profile for both suspending fluids but has a small impact on the fiber orientation distributions at the geometry outlets. However, in the corner region where a vortex is observed, its magnitude increases with the coupling and this enhancement is more pronounced for the Newtonian suspending fluid. The Newtonian viscosity model is replaced with the Carreau model and results are compared to a bi-viscosity model. It gives qualitatively correct results if no rapid fiber orientation change occurs along the streamlines.

Atomization behaviour of juice-fibre suspensions in a two-fluid nozzle

Atomization of non-Newtonian viscoelastic liquids was carried out using a two-fluid nozzle. Juice-fibre suspension is a shear thinning liquid that exhibits significant extensional resistance when forced through an extensional flow-field. Two types of fibres were used with aspect ratios of 2–3 and 8–10. The atomization behaviour of the suspensions discharging in the air was photographed using a backlit setup to visualise the atomization pattern of viscoelastic fibre suspensions. The effect of the extensional resistance is to delay the droplet formation by forming filamentary structures that link successive droplets together. As a consequence, droplets of viscoelastic fibre suspensions are usually bigger than Newtonian droplets. Using a two-fluid nozzle, at an atomizing air velocity of 150 m/s, the average droplet size of fibre suspension with aspect ratio 2–3 was 450 μm while the average droplet size of fibre suspension with aspect ratio 8–10 was 530 μm. Increasing the atomizing air velocity in the two-fluid nozzle yielded better atomization by forming smaller droplets as proved by the droplet size analysis. At the highest tested atomizing air velocity of 240 m/s, both average droplet sizes were reduced to approximately 200–250 μm.