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

Decoupling the effects of shear and extensional flows on the alignment of colloidal rods

Cellulose nanocrystals (CNC) can be considered as model colloidal rods and have practical applications in the formation of soft materials with tailored anisotropy. Here, we employ two contrasting microfluidic devices to quantitatively elucidate the role of shearing and extensional flows on the alignment of a dilute CNC dispersion. Characterization of the flow field by micro-particle image velocimetry is coupled to flow-induced birefringence analysis to quantify the deformation rate--alignment relationship. The deformation rate required for CNC alignment is 4$\times$ smaller in extension than in shear. Alignment in extension is independent of the deformation rate magnitude, but is either 0$^\circ$ or 90$^\circ$ to the flow, depending on its sign. In shear flow the colloidal rods orientate progressively towards 0$^\circ$ as the deformation rate magnitude increases. Our results decouple the effects of shearing and extensional kinematics at aligning colloidal rods, establishing coherent guidelines for the manufacture of structured soft materials

Cellulose Nanocrystal Liquid Crystal Phases: Progress and Challenges in Characterization Using Rheology Coupled to Optics, Scattering, and Spectroscopy

Cellulose nanocrystals (CNCs) self-assemble and can be flow-assembled to liquid crystalline orders in a water suspension. The orders range from nano- to macroscale with the contributions of individual crystals, their micron clusters, and macroscopic assemblies. The resulting hierarchies are optically active materials that exhibit iridescence, reflectance, and light transmission. Although these assemblies have the potential for future renewable materials, details about structures on different hierarchical levels that span from the nano- to the macroscale are still not unraveled. Rheological characterization is essential for investigating flow properties; however, bulk material properties make it difficult to capture the various length-scales during assembly of the suspensions, for example, in simple shear flow. Rheometry is combined with other characterization methods to allow direct analysis of the structure development in the individual hierarchical levels. While optical techniques, scattering, and spectroscopy are often used to complement rheological observations, coupling them in situ to allow simultaneous observation is paramount to fully understand the details of CNC assembly from liquid to solid. This Review provides an overview of achievements in the coupled analytics, as well as our current opinion about opportunities to unravel the structural distinctiveness of cellulose nanomaterials

Thixotropy of cellulose nanocrystal suspensions

The thixotropy of cellulose nanocrystal (CNC) water suspensions is intrinsically dependent on the hierarchical structure of the suspension. The diverse hierarchies that comprise individual CNC nanoparticles and mesophase liquid crystalline domains, chiral nematic and nematic structures, contribute selectively to the rheological material response. Here, we combine rheology with polarized light imaging (PLI) to elucidate the thixotropic behavior of CNCs suspended in water. The simultaneous monitoring of PLI and rheological tests enables the observation of mesogens and their orientation dynamics. Creep, dynamic time sweep, ramped hysteresis loop, and thixotropic recovery tests combined with PLI aim to differentiate the contribution of the different hierarchical levels of CNC suspensions to their thixotropy. The range of concentrations investigated comprised biphasic (4 and 5 wt. %) and liquid crystalline phase suspensions (6, 7, and 8 wt. %). The CNC suspensions exhibited complex thixotropy behavior, such as viscosity bifurcations in creep tests and overshoot in ramped hysteresis loop tests. The restructuring and destructuring appeared to correspond to different levels of their hierarchical structure, depending mainly on the phase, in agreement with previous studies. Restructuring was attributed to re-organizations of an individual CNC, e.g., in the isotropic fraction of biphasic suspensions and at the mesogen interfaces in liquid crystalline phase suspensions. However, by increasing liquid crystalline fraction in the biphasic concentrations, restructuring could also involve mesogens, as indicated in the creep tests. For flow conditions above the yield stress, as evidenced by the ramped hysteresis and thixotropy recovery tests, destructuring was dominated by orientation in the flow direction, a process that is readily observable in the form of PLI “Maltese-cross” patterns. Finally, we show that a simple thixotropy model, while unable to capture the finer details of the suspension’s thixotropic behavior, could be employed to predict general features thereof

Bendable transparent films from cellulose nanocrystals–Study of surface and microstructure-property relationship

The presented work focuses on preparing transparent bendable films from nanocellulose. In comparison to cellulose nanofibrils and bacterial cellulose, nanocrystalline cellulose are shorter and have higher crystallinity (CI<95 %). Sulfated CNC (CNCIH-OSO3H) were prepared, and by changing their counter ions from H+ to Na+ and Et4N+ (Tetraethyl ammonium) flexible films were prepared with a strength of 70.5 MPa and 2.6 % elongation at break. The CNC suspensions showed excellent dispersibility in DI water with Zeta-potential (ζ) values > -35 mV. In the preparation of films, pre-sonication was key in improving the tensile strength and improved elongation (>30 % increase compared to films prepared without sonication) and hydrophobicity. The change of counter ion, H+ to Na+ or Et4N+, improved the thermal and mechanical properties of CNC films. The films were investigated with UV–Vis spectroscopy and optical polarized spectroscopy to explain the arrangement of nanocellulose crystals in correlation with the mechanical properties. The wettability of CNC samples was also studied and explained in detail. CNC from CelluForce was also studied as commercial reference samples. The modified CNC films have adequate properties for application in flexible electronics, energy storage, and biodegradable smart packaging

Exploring the nonlinear rheological behavior and optical properties of cellulose nanocrystal suspensions

Cellulose nanocrystals (CNCs), with their versatile properties, offer immense potential in a range of applications, whether used independently or as sustainable reinforcements in polymers. They also find utility as renewable rheology modifiers in industries such as cosmetics, paints, and foods, where precise control over rheological characteristics is crucial for factors like product stability, prevention of splattering, and efficient processing and transportation. To enhance their properties and unlock new applications, surface modification of CNCs is essential. However, studying flow-induced structuring requires the use of accurate and reliable analysis methods, particularly when dealing with fast and large deformations in suspensions, multiphase systems, and composites.This thesis presents a novel approach for studying the interactions between flow fields and CNCs by investigating nonlinear rheological parameters using a combination of Fourier-Transform rheology (i) and Polarized Light Imaging (PLI) techniques (ii). The utilization of (i) allows for the capture of nonlinear parameters that cannot be obtained through conventional rheological characterization. Concurrently, (ii) provides visual insights into flow-induced CNC structuring and optical properties.By employing these two distinct techniques, it becomes possible to discern alterations in the microstructure of CNCs, enabling the determination of critical concentrations for phase transitions, percolation, and gelation. To validate the proposed methodology, several different CNC systems were examined, categorized as either (1) self-assembling or (2) non-self-assembling CNC suspensions. These systems varied in terms of surface charge, concentrations, surface modification with azetidinium salts or monovalent counterions, and aspect ratio.This comprehensive investigation expands our understanding of CNC behavior under flow conditions and offers valuable insights into the rheological properties of CNC suspensions, potentially paving the way for the development of improved materials and applications in various industries

Unexpected microphase transitions in flow towards nematic order of cellulose nanocrystals

Organization of nanoparticles is essential in order to control their light-matter interactions. We present cellulose nanocrystal suspension organization in flow towards a unidirectional state. Visualization of evolving polarization patterns of the cellulose nanocrystal suspensions is combined with steady and oscillatory shear rheology. Elucidation of the chiral nematic mesophase in a continuous process towards unidirectional order enables control of alignment in a suspension precursor for structural films and reveals thus far in situ unrevealed transition states that were not detectable by rheology alone. The coupled analytics enabled the suspensions of interest to be divided into rheological gels and rheological liquid crystal fluids with detailed information on the microtransition phases. Both populations experienced submicron organization and reached macro-scale homogeneity with unidirectional ordering in continued shear. We quantify the time, shear rate, and recovery time after shear to design an optimizing formation process for controlled wet structures as precursors for dry products

3D bioprinting of nanocellulose based inks

3D bioprinting is an emerging technology in the tissue engineering field to treat or replace damaged body tissues and organs. Bioink is the printable material that is required for 3D bioprinting of structures that may contain cells and biological components. Designing a bioink remains a challenge as it needs to be viscous enough for printing, however it needs to be compatible with cells and biological components. The aim of this study is to develop a conducting bioink for 3D bioprinting applications. Thus, I need to develop a viscous mixture based on not only cell growth media to support the cells but also conductive components to provide conductivity. I designed the bioink based on nanocellulose, due to its excellent properties such as its highly elongated structure and biocompatibility which can provide a desirable structure for 3D bioprinting. Moreover, nanocellulose is obtained from renewable sources. In the first chapter of this thesis, I explored if it is possible to stabilize the bioink via photopolymerization considering the fact that it needs to contain cell growth media to keep the cells alive. This investigation showed that photopolymerization of bioinks is not completely controllable in the presence of cell culture media as the media components can interfere with photopolymerization processes through different pathways such as radical chain transfer and radical scavenging effects. Therefore, in the second chapter, I explored a combination of photocrosslinking and ionic crosslinking to obtain suitable viscous bioinks for 3D printing. A bioink based on nanocellulose and poly (ethylene glycol) was developed and optimized for 3D bioprinting by adjusting the physical crosslinking of nanocellulose with CaCl2 and chemical crosslinking of poly (ethylene glycol) under visible light irradiation. The study of physical, mechanical, rheological, and cellular interactions of the designed bioink revealed that the developed bioink has a high potential for various regenerative medicine applications. In the last chapter, I investigated if it is possible to blend conducting material into the bioink and achieve alignment in the material to improve its directional conductivity. This feature would have the potential for future applications in anisotropic tissue engineering such as neural and cardiac regeneration

Phase transitions of cellulose nanocrystal suspensions from nonlinear oscillatory shear

Cellulose nanocrystals (CNCs) self- assemble in water suspensions into liquid crystalline assemblies. Here, we elucidate the microstructural changes associated with nonlinear deformations in (2–9 wt%) CNC suspensions through nonlinear rheological analysis, that was performed in paral- lel with coupled rheology—polarized light imaging. We show that nonlinear material parameters from Fourier-transform rheology and stress decomposition are sensitive to all CNC phases investigated, i.e. iso- tropic, biphasic and liquid crystalline. This is in con- trast to steady shear and linear viscoelastic dynamic moduli where the three-region behavior and weak strain overshoot cannot distinguish between biphasic and liquid crystalline phases. Thus, the inter-cycle and intra-cycle nonlinear parameters investigated are a more sensitive approach to relate rheological meas- urements to CNC phase behavior

Exploring the role of nanocellulose as potential sustainable material for enhanced oil recovery:New paradigm for a circular economy

Presently, due to growing global energy demand and depletion of existing oil reservoirs, oil industry is focussing on development of novel and effective ways to enhance crude oil recovery and exploration of new oil reserves, which are typically found in challenging environment and require deep drilling in high temperature and high-pressure regime. The nanocelluloses with numerous advantages such as high temperature and pressure stability, ecofriendly nature, excellent rheology modifying ability, interfacial tension reduction capability, etc., have shown a huge potential in oil recovery over conventional chemicals and macro/micro sized biopolymers-based approach. In present review, an attempt has been made to thoroughly investigate the potential of nanocellulose (cellulose nanocrystals/nanofibers) in development of drilling fluid and in enhancement of oil recovery. The impact of various factors such as nanocellulose shape, charge density, inter-particle or inter-fibers interactions after surface functionalization, rheometer geometries, additives, post processing techniques, etc., which provides insight into the attributes of nanocellulose suspension and exemplify their behaviour during oil recovery have also been reviewed and discussed. Finally, the conclusion and challenges in utility of nanocellulose for oilfield applications are addressed. Knowing how to adjust/quantify nanocrystals/nanofibers shape and size; and monitor their interactions might promote their utility in oilfield industry.</p