Cellulose constitutes the most abundant renewable polymer resource available today. As a chemical raw material, it is generally well-known that it has been used in the form of fibers or derivatives for nearly 150 years for a wide spectrum of products and materials in daily life. What has not been known until relatively recently is that when cellulose fibers are subjected to acid hydrolysis, the fibers yield defect-free, rod-like crystalline residues. Cellulose nanocrystals (CNs) have garnered in the materials community a tremendous level of attention that does not appear to be relenting. These biopolymeric assemblies warrant such attention not only because of their unsurpassed quintessential physical and chemical properties (as will become evident in the review) but also because of their inherent renewability and sustainability in addition to their abundance. They have been the subject of a wide array of research efforts as reinforcing agents in nanocomposites due to their low cost, availability, renewability, light weight, nanoscale dimension, and unique morphology. Indeed, CNs are the fundamental constitutive polymeric motifs of macroscopic cellulosic-based fibers whose sheer volume dwarfs any known natural or synthetic biomaterial. Biopolymers such as cellulose and lignin and † North Carolina State University. ‡ Helsinki University of Technology. Dr. Youssef Habibi is a research assistant professor at the Department of Forest Biomaterials at North Carolina State University. He received his Ph.D. in 2004 in organic chemistry from Joseph Fourier University (Grenoble, France) jointly with CERMAV (Centre de Recherche sur les Macromolecules Vegetales) and Cadi Ayyad University (Marrakesh, Morocco). During his Ph.D., he worked on the structural characterization of cell wall polysaccharides and also performed surface chemical modification, mainly TEMPO-mediated oxidation, of crystalline polysaccharides, as well as their nanocrystals. Prior to joining NCSU, he worked as assistant professor at the French Engineering School of Paper, Printing and Biomaterials (PAGORA, Grenoble Institute of Technology, France) on the development of biodegradable nanocomposites based on nanocrystalline polysaccharides. He also spent two years as postdoctoral fellow at the French Institute for Agricultural Research, INRA, where he developed new nanostructured thin films based on cellulose nanowiskers. Dr. Habibi’s research interests include the sustainable production of materials from biomass, development of high performance nanocomposites from lignocellulosic materials, biomass conversion technologies, and the application of novel analytical tools in biomass research. Chem. Rev. 2010, 110, 3479–3500 3479

Cellulose nanocrystals: chemistry, self-assembly, and applications.

Cellulose fibrils with widths in the nanometer range are nature-based materials with unique and potentially useful features. Most importantly, these novel nanocelluloses open up the strongly expanding fields of sustainable materials and nanocomposites, as well as medical and life-science devices, to the natural polymer cellulose. The nanodimensions of the structural elements result in a high surface area and hence the powerful interaction of these celluloses with surrounding species, such as water, organic and polymeric compounds, nanoparticles, and living cells. This Review assembles the current knowledge on the isolation of microfibrillated cellulose from wood and its application in nanocomposites; the preparation of nanocrystalline cellulose and its use as a reinforcing agent; and the biofabrication of bacterial nanocellulose, as well as its evaluation as a biomaterial for medical implants.

Nanocelluloses: A New Family of Nature-Based Materials

Review of Recent Research into Cellulosic Whiskers, Their Properties and Their Application in Nanocomposite Field

Microorganisms such as bacteria and many eukaryotic cells propel themselves with hair-like structures known as flagella, which can exhibit a variety of structures and movement patterns. For example, bacterial flagella are helically shaped and driven at their bases by a reversible rotary engine, which rotates the attached flagellum to give a motion similar to that of a corkscrew. In contrast, eukaryotic cells use flagella that resemble elastic rods and exhibit a beating motion: internally generated stresses give rise to a series of bends that propagate towards the tip. In contrast to this variety of swimming strategies encountered in nature, a controlled swimming motion of artificial micrometre-sized structures has not yet been realized. Here we show that a linear chain of colloidal magnetic particles linked by DNA and attached to a red blood cell can act as a flexible artificial flagellum. The filament aligns with an external uniform magnetic field and is readily actuated by oscillating a transverse field. We find that the actuation induces a beating pattern that propels the structure, and that the external fields can be adjusted to control the velocity and the direction of motion.

Microscopic artificial swimmers

Microfibrillated cellulose - its barrier properties and applications in cellulosic materials: a review.

Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20–40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

The specific magnetic-field-induced birefringence is measured in aqueous suspensions composed of the charged rodlike particle tobacco mosaic virus (TMV) as a function of temperature, TMV concentration, ionic strength, and TMV polydispersity over the entire isotropic range. This quantity is proportional to the magnitude of the interparticle angular correlations at zero field. Theoretical expressions for the field-induced birefringence for both the mono- and polydisperse samples are derived based on extensions of the Onsager model [L. Onsager, Ann. N.Y. Acad. Sci. 51, 627 (1949)] and compare well with experiment. In addition, the isotropic-nematic phase coexistence concentrations are measured as a function of ionic strength and temperature. The agreement between experiment and theory indicates that the TMV particles interact primarily through electrostatic repulsion and that attractive forces are negligible.

Angular correlations and the isotropic-nematic phase transition in suspensions of tobacco mosaic virus.

Measurements are presented of the long‐time self‐diffusion coefficient and of the low shear limiting viscosity of dispersions of charge stabilized colloidal silica spheres. Long‐time self‐diffusion coefficients were measured using fluorescence recovery after photobleaching (FRAP), the theory of which is presented and generalized to Gaussian laser beams. The particles, suspended in solutions of LiCl in dimethylformamide, interacted via a screened Coulomb potential, the range of which was varied through the ionic strength. Measurements were made up to volume fractions beyond freezing where a coexistence occurred between a colloidal crystal and a colloidal fluid. It is often speculated that the long‐time self‐diffusion coefficient and the low shear viscosity of a dispersion are related through a simple Stokes–Einstein‐like relation, but this expectation is not confirmed by the experiments. A slightly modified relation, however, does seem to provide a reasonable empirical description of the data.

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A comparison between the long-time self-diffusion and low shear viscosity of concentrated dispersions of charged colloidal silica spheres

We report the first observation of Faraday rotation of light in multiple-scattering media. Speckle patterns generated by diffusive transport of laser light through turbid granular samples are strongly modified by high magnetic fields (B). The speckle intensity correlation function, measured at various sample thicknesses and Verdet constants, decays with increasing B in agreement with the simple model discussed. In addition the coherent backscattering cone is partially suppressed by the field, again consistent with the model. This effect provides a unique way to destroy interferences between time-reversed scattering paths and may be expected to affect light localization.

Multiple Light Scattering in Magneto-optically Active Media

We report dynamic light scattering experiments on turbid colloidal suspension under stationary and laminar flow, as well as in the regime of flow instabilities. It is shown that the time autocorrelation function C 1 ( t ) = E (0) E ∗ ( t )>/ E (0)¦ 2 > of the scattered light field E ( t ) is not sensitive to the mean velocity flow but rather to the root mean square of velocity gradient. C 1 ( t ) is characterised on the level of each scattering event by the correlation time required by a pair of scatterers initially separated by a transport mean free path to move a relative distance of optical wavelength due to the velocity gradient. We verified this theoretical analysis using planar Couette flow as an example for homogenous velocity gradients, and planar Poiseuille flow for inhomogeneous velocity gradients. Agreement between expirement and theory is excellent. Finally, this technique is applied to spatially varying velocity gradient fields for measuring the threshold and wave number of the Taylor-Coutte instability. This illustrates the possibility of studying hydrodynamic instabilities and quasi-local velocity gradients even under conditions of strong multiple scattering.

G. Maret

Papers

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Angular correlations and the isotropic-nematic phase transition in suspensions of tobacco mosaic virus.

A comparison between the long-time self-diffusion and low shear viscosity of concentrated dispersions of charged colloidal silica spheres

Multiple Light Scattering in Magneto-optically Active Media

Multiple light scattering in Taylor-Couette flow