Shear driven vorticity aligned flocs in a suspension of attractive rigid rods

Surfactant induced catastrophic collapse of carbon black suspensions used in flow battery application

HYPOTHESIS

Carbon black particles act as electronically conductive additives in the slurry electrodes used in electrochemical redox flow batteries. Modifying the carbon black slurry formulation with the addition of a nonionic surfactant could impart improved particle dispersion, gravitational stability, and flowability leading to better battery performance.

EXPERIMENTS

Carbon black particles were dispersed in 1 M H₂SO₄ with volume fractions Φ = 0.01 to 0.06 and a nonionic surfactant (Triton X-100) concentration of c_surf. = 0, 0.05, and 0.1 M. Particle size was characterized using microscopy and surfactant adsorption using UV-vis spectroscopy. Sedimentation kinetics was measured using a custom camera set-up that tracks the height of the settling particle bed. Rheology experiments were conducted to measure linear viscoelasticity and shear flow behavior.

FINDINGS

The sedimentation dynamics of the slurry resembled that of a gel collapse. At short times we observed fast sedimentation associated with structural gel collapse and at long times very slow sedimentation associated with compaction of the sediment. Rheological investigations revealed that the slurry indeed behaved like colloidal gels. Addition of nonionic surfactant at α (= (c_surf./c_CB)) < 0.75 improved particle dispersion and increased gel elasticity. However, α> 0.75 led to a weaker gel that exhibits a fast 'catastrophic collapse' under gravity.

Shear induced tuning and memory effects in colloidal gels of rods and spheres

Shear history plays an important role in determining the linear and nonlinear rheological response of colloidal gels and can be used for tuning their structure and flow properties. Increasing the colloidal particle aspect ratio lowers the critical volume fraction for gelation due to an increase in the particle excluded volume. Using a combination of rheology and confocal microscopy, we investigate the effect of steady and oscillatory preshear history on the structure and rheology of colloidal gels formed by silica spheres and rods of length L and diameter D (L/D = 10) dispersed in 11 M CsCl solution. We use a non-dimensional Mason number, Mn (=F_visc./F_attr.), to compare the effect of steady and oscillatory preshear on gel viscoelasticity. We show that after preshearing at intermediate Mn, attractive sphere gel exhibits strengthening, whereas attractive rod gel exhibits weakening. Rheo-imaging of gels of attractive rods shows that at intermediate Mn, oscillatory preshear induces large compact rod clusters in the gel microstructure, compared to steady preshear. Our study highlights the impact of particle shape on gel structuring under flow and viscoelasticity after shear cessation.

Stress-controlled shear flow alignment of collagen type I hydrogel systems

Disease research and drug screening platforms require in vitro model systems with cellular cues resembling those of natural tissues. Fibrillar alignment, occurring naturally in extracellular matrices, is one of the crucial attributes in tissue development. Obtaining fiber alignment in 3D, in vitro remains an important challenge due to non-linear material characteristics. Here, we report a cell-compatible, shear stress-based method allowing to obtain 3D homogeneously aligned fibrillar collagen hydrogels. Controlling the shear-stress during gelation results in low strain rates, with negligible effects on the viability of embedded SH-SY5Y cells. Our approach offers reproducibility and tunability through a paradigm shift: The shear-stress initiation moment, being the critical optimization parameter in the process, is related to the modulus of the developing gel, whereas state of the art methods often rely on a predefined time to initiate the alignment procedure. After curing, the induced 3D alignment is maintained after the release of stress, with a linear relation between the total acquired strain and the fiber alignment. This method is generally applicable to 3D fibrillar materials and stress/pressure-controlled setups, making it a valuable addition to the fast-growing field of tissue engineering. STATEMENT OF SIGNIFICANCE: Controlling fiber alignment in vitro 3D hydrogels is crucial for developing physiologically relevant model systems. However, it remains challenging due to the non-linear material characteristics of fibrillar hydrogels, limiting the scalability and repeatability. Our approach tackles these challenges by utilizing a stress-controlled rheometer allowing us to monitor structural changes in situ and determine the optimal moment for applying a shear-stress inducing alignment. By careful parameter control, we infer the relationship between time, induced strain, alignment and biocompatibility. This tunable and reproducible method is both scalable and generally applicable to any fibrillar hydrogel, therefore, we believe it is useful for research investigating the link between matrix anisotropy and cell behavior in 3D systems, organ-on-chip technologies and drug research.

Nanochitin: Chemistry, Structure, Assembly, and Applications

Chitin, a fascinating biopolymer found in living organisms, fulfills current demands of availability, sustainability, biocompatibility, biodegradability, functionality, and renewability. A feature of chitin is its ability to structure into hierarchical assemblies, spanning the nano- and macroscales, imparting toughness and resistance (chemical, biological, among others) to multicomponent materials as well as adding adaptability, tunability, and versatility. Retaining the inherent structural characteristics of chitin and its colloidal features in dispersed media has been central to its use, considering it as a building block for the construction of emerging materials. Top-down chitin designs have been reported and differentiate from the traditional molecular-level, bottom-up synthesis and assembly for material development. Such topics are the focus of this Review, which also covers the origins and biological characteristics of chitin and their influence on the morphological and physical-chemical properties. We discuss recent achievements in the isolation, deconstruction, and fractionation of chitin nanostructures of varying axial aspects (nanofibrils and nanorods) along with methods for their modification and assembly into functional materials. We highlight the role of nanochitin in its native architecture and as a component of materials subjected to multiscale interactions, leading to highly dynamic and functional structures. We introduce the most recent advances in the applications of nanochitin-derived materials and industrialization efforts, following green manufacturing principles. Finally, we offer a critical perspective about the adoption of nanochitin in the context of advanced, sustainable materials.

Microstructure and elasticity of dilute gels of colloidal discoids

The linear elasticity of dilute colloidal gels formed from discoidal latex particles is quantified as a function of aspect ratio and modeled by confocal microscopy characterization of their fractal cluster microstructure. Colloidal gels are of fundamental interest because of their widespread use to stabilize complex fluids in industry. Technological interest in producing gels of desired moduli using the least number of particles drives formulators to produce gels at dilute concentrations. However, dilute gels self-assembled from isotropic spheres offer limited scope for rheological tunability due to the universal characteristics of their fractal microstructure. Our results show that changing the building block shape from sphere to discoid yields very large shifts in gel elasticity relative to the universal behavior reported for spheres. This shift - tunable through aspect ratio - yields up to a 100-fold increase in elastic modulus at a fixed volume fraction. From modeling the results using the theory for fractal cluster gel rheology, which is applicable at the dilute conditions of this study, we reveal that the efficient generation of elasticity by the colloidal discoids is the consequence of the combined effects of shape anisotropy on the fractal microstructure of the gel network, the anisotropy of the attractive interparticle pair potentials, and the volumetric compactness of the fractal cluster. These results extend prior characterizations of the rheology of non-spherical particulate gels by providing quantitative estimates of how the specific mechanisms of fractality, pair potential, and clustering mediate the profound effects of particle shape anisotropy on the elastic rheology of colloidal gels.