The Connection between Biaxial Orientation and Shear Thinning for Quasi-Ideal Rods

Rheology and Microstructural Behavior of Semidilute Suspensions of Semiflexible Rods across Five Decades of Shear Rate

Rod-like particles are efficient rheology modifiers and are commonly found in a variety of biological and industrially relevant suspensions, from biofilaments to worm-like surfactant micelles. These suspensions display strong shear-thinning behavior, and ongoing efforts aim to understand the microstructural changes of these fluids: how they depend upon the properties of the suspended particles and how these changes manifest in the resulting rheology. With suspensions of fd bacteriophage as a model monodisperse rod system, we use capillary microrheometry and flow birefringence to determine rheological behavior across nearly six decades of shear rate up to 6 × 105 s-1, at various semidilute concentrations. A single, and surprisingly large, primary fitting parameter accounting for the characteristic distance of hydrodynamic interactions is consistent with viscosity data up to very high shear rates. These results may prompt other work to understand the mechanics of these interactions.

Alignment-Rheology Relationship of Biosourced Rod-Like Colloids and Polymers under Flow

Fluids composed of biosourced rod-like colloids (RC) and rod-like polymers (RP) have been extensively studied due to various promising applications relying on their flow-induced orientation (e.g., fiber spinning). However, the relationship between RC and RP alignment and the resulting rheological properties is unclear due to experimental challenges. We investigate the alignment-rheology relationship for a variety of biosourced RC and RP, including cellulose-based particles, filamentous viruses, and xanthan gum, by simultaneous measurements of the shear viscosity and fluid anisotropy under rheometric shear flows. For each system, the RC and RP contribution to the fluid viscosity, captured by the specific viscosity η_sp, follows a universal trend with the extent of the RC and RP alignment independent of concentration. We further exploit this unique rheological-structural link to retrieve a dimensionless parameter (β) directly proportional to η_sp at zero shear rate (η_0,sp), a parameter often difficult to access from experimental rheometry for RC and RP with relatively long contour lengths. Our results highlight the unique link between the flow-induced structural and rheological changes occurring in RC and RP fluids. We envision that our findings will be relevant in building and testing microstructural constitutive models to predict the flow-induced structural and rheological evolution of fluids containing RC and RP.

Wet spinning imogolite nanotube fibres: an in situ process study

Imogolite nanotubes (INTs) form transparent aqueous liquid-crystalline solutions, with strong birefringence and X-ray scattering power. They provide an ideal model system for studying the assembly of one-dimensional nanomaterials into fibres, as well as offering interesting properties in their own right. Here, in situ polarised optical microscopy is used to study the wet spinning of pure INTs into fibres, illustrating the influence of process variables during extrusion, coagulation, washing and drying on both structure and mechanical properties. Tapered spinnerets were shown to be significantly more effective than thin cylindrical channels for forming homogeneous fibres; a result related to simple capillary rheology by fitting a shear thinning flow model. The washing step has a strong influence of structure and properties, combining the removal of residual counter-ions and structural relaxation to produce a less aligned, denser and more networked structure; the timescales and scaling behavior of the processes are compared quantitatively. Both strength and stiffness are higher for INT fibres with a higher packing fraction and lower degree of alignment, indicating the importance of forming a rigid jammed network to transfer stress through these porous, rigid rod assemblies. The electrostatically-stabilised, rigid rod INT solutions were successfully cross-linked using multivalent anions, providing robust gels, potentially useful in other contexts.

Naturally derived colloidal rods in microfluidic flows

Naturally derived colloidal rods (CR) are promising building blocks for developing sustainable soft materials. Engineering new materials based on naturally derived CR requires an in-depth understanding of the structural dynamics and self-assembly of CR in dispersion under processing conditions. With the advancement of microfabrication techniques, many microfluidic platforms have been employed to study the structural dynamics of CR under flow. However, each microfluidic design has its pros and cons which need careful evaluation in order to fully meet the experimental goal and correctly interpret the data. We analyze recent results obtained from naturally derived CR and relevant rod-like macromolecules under microfluidic flows, with emphasis on the dynamical behavior in shear- and extensional-dominated flows. We highlight the key concepts required in order to assess and evaluate the results obtained from different CR and microfluidic platforms as a whole and to aid interconnections with neighboring fields. Finally, we identify and discuss areas of interest for future research directions.

Biopolymer Filament Entanglement Softens Then Hardens with Shear

It is unsatisfactory that regarding the problem of entangled macromolecules driven out of equilibrium, experimentally based understanding is usually inferred from the ensemble average of polydisperse samples. Here, confronting with single-molecule imaging this common but poorly understood situation, over a wide range of shear rate we use single-molecule fluorescence imaging to track alignment and stretching of entangled aqueous filamentous actin filaments in a homebuilt rheo-microscope. With increasing shear rate, tube "softening" is followed by "hardening." Physically, this means that dynamical localization first weakens from molecular alignment, then strengthens from filament stretching, even for semiflexible biopolymers shorter than their persistence length.

Alignment of Colloidal Rods in Crowded Environments

Understanding the hydrodynamic alignment of colloidal rods in polymer solutions is pivotal for manufacturing structurally ordered materials. How polymer crowding influences the flow-induced alignment of suspended colloidal rods remains unclear when rods and polymers share similar length scales. We tackle this problem by analyzing the alignment of colloidal rods suspended in crowded polymer solutions and comparing that to the case where crowding is provided by additional colloidal rods in a pure solvent. We find that the polymer dynamics govern the onset of shear-induced alignment of colloidal rods suspended in polymer solutions, and the control parameter for the alignment of rods is the Weissenberg number, quantifying the elastic response of the polymer to an imposed flow. Moreover, we show that the increasing colloidal alignment with the shear rate follows a universal trend that is independent of the surrounding crowding environment. Our results indicate that colloidal rod alignment in polymer solutions can be predicted on the basis of the critical shear rate at which polymer coils are deformed by the flow, aiding the synthesis and design of anisotropic materials.

On the Mechanism of Shear-Thinning in Viscous Oppositely Charged Polyelectrolyte Surfactant Complexes (PESCs)

Semidilute mixtures of the cationically modified cellulose-based polyelectrolyte JR 400 and the anionic surfactant sodium dodecyl sulfate (SDS) form highly viscous solutions if a slight excess of charges from the polyelectrolyte is present. The reason for this is the formation of mixed rodlike aggregates in which the surfactant cross-links several polyelectrolyte chains. The same solutions also show shear-thinning behavior. In this paper, we use rheoSANS to investigate the structural evolution of the rodlike aggregates under steady shear and thereby elucidate the mechanism of shear-thinning in these viscous oppositely charged polyelectrolyte surfactant complexes.

Effects of particle stiffness on the extensional rheology of model rod-like nanoparticle suspensions

The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semiflexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is compared in detail with theoretical predictions for worm-like chains. The extensional properties are compared to Batchelor's theory, generalized for the shear thinning nature of the suspensions. Theoretical predictions agree well with the measured complex moduli at low concentrations as well as the nonlinear shear and elongational viscosities at high flow rates. The results in this work provide guidelines for enhancing the elongational viscosity based on purely frictional effects in the absence of strong normal forces which are characteristic for high molecular weight polymers.

Near wall dynamics of a spherical particle in crowded suspensions of colloidal rods - dynamic information from TIRM revisited

We performed total internal reflection microscopy (TIRM) experiments to determine the depletion potentials between probe spheres and a flat glass wall which are induced by long and thin, rod-shaped colloids (fd-virus), and probe the spatially resolved dynamics of the probe spheres. The dynamic information from the same raw TIRM intensity time traces is extracted in three different ways: by determining the spatially averaged diffusion constant of the probe sphere normal to the wall, by measuring the position dependence of the diffusion coefficient, and by measuring the particle's local drift velocity. Up to a concentration of about 6 times the overlap concentration of the rod-like colloids, the spatially averaged diffusion coefficient and the amplitude of the depletion potential are in surprisingly good agreement with theoretical predictions in which mutual interactions between the rods are neglected, that is, where the concentration is less than the overlap concentration. On increasing the depletant content even further, however, both the static and the averaged dynamic quantities begin to deviate from such theoretical predictions. In particular we find large deviations from the prediction by Mao, Cates, and Lekkerkerker [J. Chem. Phys., 1997, 106, 3721] based on the third order virial expansion for the rod concentration. It is shown that there are significant inaccuracies in TIRM measurements of diffusion coefficients due to the limited time range in which the mean squared displacements vary linearly in time, whereas mean displacements give much more accurate information concerning the probe sphere dynamics.

Direct observation of orientation distributions of actin filaments in a solution undergoing shear banding

Shear banding is frequently observed in complex fluids. However, the configuration of macromolecules in solutions undergoing shear banding has not yet been directly observed. In this study, by using the fact that F-actin solutions exhibit shear banding and actin filaments are visualized by fluorescent labels, we directly observed the intrinsic states of an actin solution undergoing shear banding. By combining the 3D imaging of labeled actin filaments and particle image velocimetry (PIV), we obtained orientation distributions of actin filaments in both high and low shear rate regions, whose quantitative differences are indicated. In addition, by using the orientation distributions and applying stress expression for rod-like polymers, we estimated stress tensors in both high and low shear rate regions. This evaluation indicates that different orientation distributions of filamentous macromolecules can exhibit a common shear stress.