Shear Alignment Research Articles

Two-species model for nonlinear flow of wormlike micelle solutions. Part I: Model

We develop a rheological model to approximate the nonlinear rheology of wormlike micelles using two constitutive models to represent a structural transition at high shear rates. The model is intended to describe the behavior of semidilute wormlike micellar solutions over a wide range of shear rates whose parameters can be determined mainly from small-amplitude equilibrium measurements. Length evolution equations are incorporated into reactive Rolie-Poly entangled-polymer rheology and dilute reactive-rod rheology, with a kinetic exchange between the two models. Although the micelle length is remarkably reduced during flow, surprisingly, we propose that they are not shortened by stress-enhanced breakage, which remains thermally driven. Instead, we hypothesize that stretching energy introduces a linear potential that decreases the rate of recombination and reduces the mean micelle length. This stress-hindered recombination approach accurately describes transient stress-growth upon start-up shear flow, and it predicts a transition of shear viscosity and alignment response observed at high shear rates. The proposed mechanism applies only when self-recombination occurs frequently. The effect of varying the relative rate of self-recombination on the rheology of wormlike micelles at high shear rates is yet to be explored.

Production of Nanofibrillar Patterned Collagen for Tissue Engineering.

Regenerative biomaterials are designed to facilitate cell-material interactions to guide the repair of damaged tissues and organs. These materials are designed to emulate the biophysical properties of native tissue, providing cellular phenotypic and morphological guidance that contributes to the restoration of the regenerative tissue niche. Collagen, a prevalent extracellular matrix protein, is a common component of these regenerative biomaterials due to its biocompatibility and other favorable properties. The current study describes a novel and straightforward method for the fabrication of engineered nanofibrillar collagen with directed fibril patterning. Through the manipulation of shear stress, temperature, and pH, collagen fibrillogenesis and alignment are precisely controlled without requiring specialized equipment. This approach allows for the creation of nanofibrillar collagen biomaterials that mimic the native structure of tissues exhibiting either anisotropic or isotropic characteristics. The flexibility in collagen nanofibril patterning not only facilitates the study of nanoscale patterning on cell behavior but also offers diverse possibilities for patterned tissue engineering applications.

Use microfluidics to study cell migration in response to fluid shear stress gradients

Cells respond not only to biological regulatory factors but also to physical stimuli such as electric fields, light, and shear stress in their environment. These stimuli can lead to cell migration and morphological changes. In the human body, cells encounter fluid shear stress induced by interstitial flow, lymphatic flow, blood flow, or organ-specific conditions within their micro-environments. Therefore, fluid shear stress, a classic mechanical force, has gained significant attention in wound healing and cancer metastasis. In this study, a microfluidic chip was developed to both culture cells and generate a shear stress gradient to direct cell migration. The design of this device’s geometry allows the generation of a shear stress gradient perpendicular to the direction of medium flow. This greatly eliminates the influence of flow-induced cell responses. Using mouse fibroblast cells (NIH3T3) and human lung cancer cells (CL1-5) as models, their migration directionality, migration rates, and alignment in response to the shear stress gradient were investigated. Within the stress range of 0.095–0.155 Pa and a gradient of 0.015 Pa/mm, NIH3T3 cells did not exhibit significant directional migration, differences in migration rates, or specific alignment patterns. In contrast, CL1-5 cells preferred higher shear stress environments and alignment parallel to the medium flow, suggesting that these conditions could induce higher mobility in these cancer cells.

Shear-aligned graphene oxide nanosheets incorporated PVDF composite membranes for selective dye rejection with high water flux.

Membrane technology is crucial in addressing water pollution challenges, particularly in removing dyes from wastewater. This study presents a novel approach to fabricating shear-aligned graphene oxide (GO) nanosheets incorporated polyvinylidene fluoride (PVDF) membranes for achieving exceptional dye rejection efficiency while maintaining high water flux. The membranes were prepared by dispersing graphene oxide within a PVDF matrix and subsequent subjection to shear alignment techniques. Shear and flow-induced alignment were explored to achieve precise and controlled alignment of graphene oxide flakes within the PVDF matrix. The resulting membranes exhibited enhanced structural integrity and optimized molecular packing of PVDF and GO, enabling them to selectively reject dyes while allowing efficient water permeation. The fabricated membranes were extensively characterized using appropriate testing methods. The results demonstrated that the shear-aligned GO sheets infused PVDF composite membranes exhibited outstanding dye rejection (96-99%) performance, surpassing conventional membranes while maintaining high water flux. This innovative membrane fabrication approach holds significant promise for advanced water treatment applications, offering a sustainable solution for selective dye removal and efficient water purification.

Macroscopic Orientation and Local Mobility of PI during Shear Alignment of ISI and SIIS Triblock Copolymers Investigated by SAXS and Rheo-Dielectric Spectroscopy

Macroscopic Orientation and Local Mobility of PI during Shear Alignment of ISI and SIIS Triblock Copolymers Investigated by SAXS and Rheo-Dielectric Spectroscopy

Alignment of DNA Wrapped Single Walled Carbon Nanotubes Sensors

Single walled carbon nanotubes (SWNT) that are well aligned have a broad range of applications, including solar cells, sensors, transistors, and optoelectronic devices. Alignment of SWNT is typically performed either during SWNT fabrication or out of post-processed SWNT solutions through vacuum filtration, spin-coating, electromagnetic fields, surfactant-aided microfluidic alignment techniques or concentration-dependent shear alignment. Alignment of SWNT during fabrication requires a lot of time and money and precludes the ability to wrap the SWNT with DNA, which is what creates the SWNT’s sensing capabilities. Unfortunately, most of these techniques have not been widely adopted since it is labor-intensive, difficult to reproduce or hard to scale up. We developed a scalable method to align DNA wrapped SWNT through heat activation without altering the SWNT’s wrapping and sensing capabilities. This research will pave the way for the development of both research and industrial scale aligned SWNT from aqueous solutions for next-generation of SWNT based optical sensors.

Structure-induced Intelligence of Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are active soft matter-based materials with strong stimulus responsiveness and reversible, large-shape morphing capabilities. LCEs have demonstrated broad and growing applications in soft robotics, wearable devices, artificial muscles, and optical machines. The actuation intelligence and advanced functionality of LCEs depend on the smartness and properties of structures. In this review, we discuss recent advances in structure-induced intelligence of LCEs, specifically the integration of structural properties with the alignment and processing of LCEs. The structural design principles for three categories consisting of common structures (film, fiber, and tubule), smart structures (origami, kirigami, mechanical metamaterial, topology, and topography), and complex structures (monolithic and integrated) are presented. Various alignment controls of LCEs, including mechanical, surface, field-assisted, and shear alignment, are capable of inducing structural properties. The coupling and collaboration mechanisms of the LCE structures and the generated functions are discussed. The review concludes with perspectives on current challenges and emerging opportunities.

Shear Alignment Mechanisms of Close-Packed Spheres in a Bulk ABA Triblock Copolymer

Shear Alignment Mechanisms of Close-Packed Spheres in a Bulk ABA Triblock Copolymer

Exploring shear alignment of concentrated wormlike micelles using rheology coupled with small-angle neutron scattering

Wormlike micelles (WLMs) are vital components of many consumer products and industrial fluids, adding a shear-dependent viscous texture through their entanglement in solutions. It is now well accepted from experiments such as coupling rheology and scattering that, similar to many polymer solutions and dispersions of highly anisotropic particles, WLM behavior during shear arises from the alignment of the “worms” with the shear field, resulting in ordering that is rapidly lost in the cessation of shear. Most studies of such systems have been limited to dilute systems that are far below concentrations used industrially and commercially, due to the complexity of analyzing shear-induced many-body effects in high volume fraction dispersions. Here, we explore the shear alignment of concentrated WLM solutions comprising sodium laureth sulfate and cocamidopropyl betaine in 0.38 M aqueous sodium chloride. By analyzing only scattering data at high values of the scattering vector (i.e., correlations at short length scales that are dominant in such concentrated systems), we explore whether useful information can be obtained by naïvely approximating the WLMs as an ensemble of unconnected short rods representing sections of the worms. By taking this reductionist approach to analyzing the obtained two-dimensional scattering patterns from these systems under shear, we find that in this regime, such concentrated worms can be approximated as cylinders that become more aligned with the direction of shear as volume fraction and shear rate increase.

Self-assembly of supramolecular complexes of charged conjugated polymers and imidazolium-based ionic liquid crystals

The use of supramolecular interactions to construct mesogens represents a versatile means of accessing new macromolecular architectures, and material properties in the resulting mesophases. Here, we demonstrate the formation of thermotropic liquid crystalline supramolecular conjugates of sulfonated poly(alkyl thiophenes) backbones coupled to biphenyl-containing 1-alkyl-3-methylimidazolium salts with bromide counter anions. Two types of liquid crystalline imidazolium salts are studied with chemical structures differing in the terminal group of the biphenyl mesogenic unit. Both ionic liquid crystals exhibit multiple phase transitions with lamellar structures, which correspond to crystal and smectic liquid crystalline transitions. Supramolecular complexes of sulfonated polythiophene and ionic liquid crystals display thermotropic liquid crystalline phases with lamellar-like nanostructures. Detailed analysis show that the lamellar periodicity increases in the supramolecular complexes, compared to those of the ionic liquid crystals. Aligned textures were produced in thin films of supramolecular complexes by shear and magnetic field alignment of the liquid crystalline mesophases. The incorporation of ionic liquid crystal units into a conjugated polymer opens opportunities for fabrication of aligned thin films of mixed electron-ion conducting polymers. Such molecular materials could be of interest in electrochemical devices.

Inline Rolling Shear Alignment: Deposition and Long-Range Order of Block Polymer Templates in a Fast, Single-Step Process

We report the first demonstration of a rapid, single-step technique to align block polymer, thin-film nanostructures, which is amenable to continuous speeds of ≥10 mm/s. This method, termed inline rolling shear alignment (IRSA), uses an elastomeric roller coupled with an electric hysteresis brake to shear films before the casting solvent fully evaporates, and it can enable high-throughput, large-area manufacturing of patterns such as nanowire templates. IRSA is adaptable to various polymer systems including diblock and triblock polymers, and a post-shear annealing step can yield high-quality nanostructures with orientation parameters >0.99 and defect densities as low as 5 defect-pairs/μm2. To demonstrate the effectiveness of IRSA, platinum nanowires were produced from this rapid deposition and alignment approach using poly(styrene-b-2-vinylpyridine), followed by open-air, atmospheric-pressure plasma etching. This integrated concept, with 50× faster operation than existing processes, could be leveraged for cost-effective manufacturing of macroscopically aligned nanostructures in roll-to-roll systems.

Confined Shear Alignment of Ultrathin Films of Cellulose Nanocrystals.

Cellulose nanocrystals (CNCs) are a naturally abundant nanomaterial derived from cellulose which exhibit many exciting mechanical, chemical, and rheological properties, making CNCs attractive for use in coatings. Furthermore, the alignment of CNCs is important to exploit their anisotropic mechanical and piezoelectric properties. Here, we demonstrate and study the fabrication of submonolayer to 25 nm thick films of CNCs via solution-based shear alignment. CNC solution is forced through a sub-millimeter tall channel at high volumetric flow rates generating shear. The half-width at half-maximum of the spread in CNC alignment significantly improves from 78 to 17° by increasing the shear rate from 19 to 19,000 s-1. We demonstrate that the film thickness is increased by increasing the volume of CNC solution flowed over the substrate and/or increasing the CNC solution concentration, with a degradation in film uniformity at higher (≥7 wt %) concentrations, likely due to CNC aggregates in the solution. Deposition of ultrathin aligned CNC films occurs within seconds and the technique is inherently scalable, demonstrating the promise of solution-based shear for the fabrication of ultrathin aligned CNC films, thereby enabling the future study of their inherent material properties or use in high-performance coatings and applications.

Preparation of Al(OH)3-based layered structural material by shear alignment from aqueous dispersion of colloidal gibbsite platelets

Development of the technological and industrial capabilities to obtain hexagonal gibbsite nanoplatelets with a regularly well-defined configuration is an ongoing consideration that has not yet covered a required level of feasible architecture. Here we present a convenient method to synthesize the colloidal gibbsite particles based on a simple hydrothermal process from aluminum alkoxides in weak acid media. These colloids are near-regularly shaped and fairly monodisperse hexagons with an average diameter of 185 nm and a thickness of ~5.3 nm. The aqueous dispersion of gibbsite platelets can be shear-aligned to form highly ordered, continuous films on a substrate by an industrially adaptable method for producing large-area membranes. Overall, the modified synthesis method exhibits an excellent potential for scalable-up production with high quality in various practical applications. The aligned films also have outstanding properties due to the self-assemblies and interfacial physical interactions between adjacent gibbsite platelets and the possibility of industrial fabrication from those layered structural materials. Moreover, we prepare this shear-aligned membrane by the in-plane shearing method to introduce the great promise of disk-like-derived gibbsite as new gas-barrier material or surface-protective materials.

Shear-Rolling Process for Unidirectionally and Perpendicularly Oriented Sub-10-nm Block Copolymer Patterns on the 4 in Scale.

Shear alignment of the block copolymer (BCP) thin film is one of the promising directed self-assembly (DSA) methodologies for the unidirectional alignment of sub-10 nm microdomains of BCPs for next-generation nanolithography and nanowire-grid polarizers. However, because of the differences in the surface/interfacial energies at the top surface/bottom interface, the shear-induced ordering of BCP nanopatterns has been restricted to BCPs with spherical and cylindrical nanopatterns and cannot be realized for high-aspect-ratio perpendicular lamellar structures, which is essential for practical application to semiconductor pattern processes. It is still a difficult challenge to fabricate the unidirectional alignment in a short time over a large area. In this study, we propose an approach for combining the shear-rolling process with the filtered plasma treatment of BCP films for the fabrication of unidirectionally aligned and perpendicularly oriented lamellar nanostructures. This approach enables fabrication within 1 min on a 4 in scale. We treated filtered plasma on the BCP film for perpendicular orientation and executed the hot-rolling process with different roller and stage speeds. Large-scale shear was generated only at the location where the BCP film was in contact with both the roller and stage, effectively applying shear stress to a large area of the BCP film within a short time. The repeated application of this shear-rolling process can achieve a higher level of unidirectional alignment. Our aligned BCP vertical lamellae were used to fabricate a high-aspect-ratio sub-10-nm-wide metallic nanowire array via dry/wet processes. In addition, shear-rolling with chemoepitaxy patterns can achieve higher orientational order and lower defectivity.

Self-assembly of aramid amphiphiles into ultra-stable nanoribbons and aligned nanoribbon threads.

Small-molecule self-assembly is an established route for producing high-surface-area nanostructures with readily customizable chemistries and precise molecular organization. However, these structures are fragile, exhibiting molecular exchange, migration and rearrangement-among other dynamic instabilities-and are prone to dissociation upon drying. Here we show a small-molecule platform, the aramid amphiphile, that overcomes these dynamic instabilities by incorporating a Kevlar-inspired domain into the molecular structure. Strong, anisotropic interactions between aramid amphiphiles suppress molecular exchange and elicit spontaneous self-assembly in water to form nanoribbons with lengths of up to 20 micrometres. Individual nanoribbons have a Young's modulus of 1.7 GPa and tensile strength of 1.9 GPa. We exploit this stability to extend small-molecule self-assembly to hierarchically ordered macroscopic materials outside of solvated environments. Through an aqueous shear alignment process, we organize aramid amphiphile nanoribbons into arbitrarily long, flexible threads that support 200 times their weight when dried. Tensile tests of the dry threads provide a benchmark for Young's moduli (between ~400 and 600 MPa) and extensibilities (between ~0.6 and 1.1%) that depend on the counterion chemistry. This bottom-up approach to macroscopic materials could benefit solid-state applications historically inaccessible by self-assembled nanomaterials.

Revealing filler morphology in 3D-printed thermoset nanocomposites by scanning microbeam X-ray scattering

Abstract Room temperature direct-ink-write printing of epoxy-nanoclay-carbon fiber composites produces parts with high stiffness and strength. Establishing clear relationships between print parameters, filler orientation, and properties is difficult, in part owing to challenges in characterization. Here, we perform scanning microbeam X-ray scattering with 5 micrometer spatial resolution on cross-sections of printed parts with (a) epoxy-nanoclay composite and (b) epoxy-nanoclay-carbon fiber reinforced composite. The nanoclay morphology is directly visualized, illuminating the road geometry with far greater clarity than other techniques. Near the boundary of each road, the nanoclay platelets are preferentially oriented coplanar with the road boundary. Shear alignment within the nozzle during extrusion, and road-to-road shear upon deposition are two proposed factors leading to this orientation. In the sample containing carbon fiber, wide angle X-ray diffraction enables the mapping and visualization of the fibers directly onto the road geometry. The carbon fiber does not significantly affect the nanoclay morphology. Finally, from the small angle X-ray scattering map, we qualitatively reproduce a polarized optical microscope image, revealing that optical microscopy is capable of visualizing the large-scale road structure in these epoxy-nanoclay systems.

Hierarchical self-assembly of supramolecular polymer complexes mediated by various generations of bent-core mesogenic dendrimers hydrogen-bonded with triblock copolymer

A series of side-chain supramolecular polymer complexes containing proton acceptor triblock copolymer, i.e., poly(4-vinylpyridine)-block-polystyrene-block-poly(4-vinylpyridine) (VSV, i.e., P4VP-PS-P4VP), hydrogen-bonded (H-bonded) with two generations of proton donor bent-core mesogenic dendritic pendants (G1 and G2) were prepared and investigated. These two supramolecular polymer complexes (i.e., VSV/G1 and VSV/G2) were characterized by wide- and small-angle X-ray scattering (WAXS and SAXS), and the formation of a microphase separated smectic A (SmA) structure corresponding to a head-to-head layered arrangement was confirmed. The hierarchical structures within the morphologies of supramolecular polymer complexes were proved not only by the SAXS patterns but also by the transmission electron microscopy (TEM). The novel hierarchical lamellar domains of corresponding tetragonal and hexagonal arrangements in different generations of dendritic proton donors (G1 and G2) were self-assembled with triblock copolymer proton acceptor VSV to induce respective BCC (body-centered cubic) and FCC (face-centered cubic) structures in supramolecular polymer complexes VSV/G1 and VSV/G2. Therefore, the shear alignments of characteristic cylindrical column phase micro-domains were developed for the first time to control the unique hierarchical constructions of functionalized self-assembled bent-core structures by utilization of various generations of dendritic proton donors to be H-bonded with triblock copolymer proton acceptor.

Decoding pyroclastic density current flow direction and shear conditions in the flow boundary zone via particle-fabric analysis

One of the greatest challenges in volcanology is extracting information from volcanic deposits that inform the conditions at the time of emplacement. Here we explore particle-shape fabric within samples from the column-collapse pyroclastic density current (PDC) deposits generated by the 18 May 1980 eruption of Mt. Saint Helens (MSH). Particle shape-fabric is the mutual alignment of particles within a sample, which is dependent on shearing conditions at the time of deposition. Here, particle shape-fabric is used to address the following hypotheses: (1) particle shape-fabric will align with bulk flow direction estimates made by previous studies, while resolving fine scale flow direction variations; (2) samples extracted from PDC deposits immediately above a basal contact will show higher extents of fabric development (i.e. clast alignment) relative to samples extracted well above the contact; and (3) samples extracted from cross-stratified deposits will have higher extents of fabric development relative to samples extracted from the massive facies. We find that fabric orientations approximate previous flow direction interpretations of general flow, from south to north, and channelization due to paleo-topography. Furthermore, shape-fabric records flow interaction with the substrate, created by debris avalanche deposits, leading to fine scale variations in flow paths. These variations are recorded in samples extracted near the substrate-deposit contact. Moreover, these samples do not show a higher measure of clast alignment relative to samples taken well above flow contacts but do exhibit bimodal clast alignment. This suggests some degree of traction transport and thus higher shearing conditions relative to when the currents deposited massive lapilli tuffs. Samples extracted from the cross-stratified deposits do not show higher extents of fabric development relative to the massive deposits, supporting previous interpretations that these deposits formed from a rapidly sedimenting, concentrated flow boundary zone that was thin enough to be frequently interrupted or influenced by the overriding turbulent portion of the flow. Finally, observations of fine scale variability in measured fabric within a single outcrop indicates unsteady flow conditions and step-wise aggradation during deposition. This work demonstrates that particle shape-fabric analyses can aid in decoding PDC flow conditions. Future complementary experimental work may allow for the extraction of quantitative information regarding the magnitudes of shear and subsequent particle alignments within the flow boundary zone, providing further insight into PDC dynamics.

Chiral Structure Formation during Casting of Cellulose Nanocrystalline Films.

A Landau-de Gennes formulation coupled with a mass-transfer equation was used to track the evaporation front and the development of chiral microstructures during the casting of sulfuric acid-hydrolyzed cellulose nanocrystal (CNC) films. These simulations are compared to thin-film casting experiments that used analogous processing parameters and environments. The results show that the initial concentration, chiral strength, surface anchoring, speed of drying, and the influence of initial shear alignment all affect the uniformity of the microstructure and the orientation of the chiral director. In this report, we aim to show that under optimal casting conditions, the lateral size of planar microstructural domains that exhibit uniform selective reflection can be achieved on the order of millimeters.

Cellulose nanofibrils and nanocrystals in confined flow: Single-particle dynamics to collective alignment revealed through scanning small-angle x-ray scattering and numerical simulations.

Nanostructured materials made through flow-assisted assembly of proteinaceous or polymeric nanosized fibrillar building blocks are promising contenders for a family of high-performance biocompatible materials in a wide variety of applications. Optimization of these processes relies on improving our knowledge of the physical mechanisms from nano- to macroscale and especially understanding the alignment of elongated nanoparticles in flows. Here, we study the full projected orientation distributions of cellulose nanocrystals (CNCs) and nanofibrils (CNFs) in confined flow using scanning microbeam SAXS. For CNCs, we further compare with a simulated system of dilute Brownian ellipsoids, which agrees well at dilute concentrations. However, increasing CNC concentration to a semidilute regime results in locally arranged domains called tactoids, which aid in aligning the CNC at low shear rates, but limit alignment at higher rates. Similarly, shear alignment of CNF at semidilute conditions is also limited owing to probable bundle or flock formation of the highly entangled nanofibrils. This work provides a quantitative comparison of full projected orientation distributions of elongated nanoparticles in confined flow and provides an important stepping stone towards predicting and controlling processes to create nanostructured materials on an industrial scale.