Shear-induced Structural Changes Research Articles

Effect of Europium and Gadolinium Alloying Elements on the Tribological Response of Low Hydrogen Content Amorphous Carbon.

Dopants and alloying elements are commonly introduced in amorphous carbon (a-C) materials to tailor their mechanical and tribological properties. While most published studies have focused on doping and alloying a-C coatings with metals or metalloids, doping a-C films with rare-earth elements has only recently been explored. Notably, our understanding of the shear-induced structural changes occurring in rare-earth-element-containing a-C films is still elusive, even in the absence of any liquid lubricants. Here, the friction response of Eu- and Gd-containing a-C films with low hydrogen content deposited by HiPIMS on silicon was evaluated in open air and at room temperature. The load-dependent friction measurements indicated that the introduction of Gd ((2.3 ± 0.1) at.%) and Eu ((2.4 ± 0.1) at.%) into the a-C matrix results in a significant reduction of the shear strength of the sliding interfaces ((41 ± 2) MPa for a-C, (16 ± 1) MPa for a-C:Gd2.3at.%, and (11 ± 2) MPa for a-C:Eu2.4at.%). NEXAFS spectromicroscopy experiments provided evidence that no stress-assisted sp3-to-sp2 rehybridization of carbon atoms was induced by the sliding process in the near-surface region of undoped a-C, while the amount of sp2-bonded carbon progressively increased in a-C:Gd2.3at.% and a-C:Eu2.4at.% upon increasing the applied normal load in tribological tests. The formation of an sp2-bonded carbon-rich surface layer in a-C:Gd2.3at.% and a-C:Eu2.4at.% films was not only proposed to be the origin for the reduced duration of the running-in period in tribological test, but was also postulated to induce shear localization within the sp2-carbon-rich layer and transfer film formation on the countersurface, thus decreasing the interfacial shear strength. These findings open the path for the use of Gd- and Eu-containing a-C even under critical conditions for nearly hydrogen-free a-C films (i.e., humid air).

Hyperreflective photonic crystals created by shearing colloidal dispersions at ultrahigh volume fraction

Colloidal crystallization serves as one of the most economic and scalable production methods for photonic crystals. However, insufficient optical performance, nonuniformity and low reproducibility remain challenges for advanced high-value applications. In this study, we optimally formulate a photocurable dispersion of silica particles and apply shear flow to unify the orientation of the colloidal crystals, ensuring high optical performance and uniformity. The silica particles experience strong repulsion at ultrahigh volume fractions of 50% but demonstrate low mobility, leading to polycrystalline structures. Applying shear flow to the dispersions allows the silica particles to rearrange into larger crystalline domains with a unidirectional orientation along the flow. This shear-induced structural change produces absolute reflectivity at the stopband as high as 90% and a high transparency of 90% at off-resonant wavelengths with minimal diffusive scattering. Furthermore, the strong interparticle repulsion ensures a uniform volume fraction of particles throughout the dispersion, reducing deviations in the optical properties. We intricately micropattern the photocurable dispersions using photolithography. Additionally, the photonic films and patterns can be stacked to form multiple layers, displaying mixed structural colors and multiple reflectance peaks without sacrificing reflectivity. These superior photonic materials hold promise for various optical applications, including optical components and anticounterfeiting patches.

Shear-induced structural and viscosity changes of amphiphilic patchy nanocubes in suspension

Our study demonstrates the relationship between the design of patchy nanocubes, their self-assembled structures including shear-induced structural changes and the rheological properties of suspensions through coarse-grained molecular simulations.

Shear-induced nano structural changes in micelles formed by sugar-based surfactants with varied anomeric configuration

Shear-induced nano structural changes in micelles formed by sugar-based surfactants with varied anomeric configuration

Shear-induced structural changes and denaturation of bovine immunoglobulin G and serum albumin at different temperatures

Shearing can modify native properties of proteins alone or in combination with heating. Effects of shearing on secondary structure, extent of denaturation and interactions of immunoglobulin G (IgG) and bovine serum albumin (BSA) were studied at temperatures where these changes start to take place. Pure IgG (0.8 mL mL−1) or BSA (0.4 mg mL−1) and their binary mixture were subjected to selected temperature and time combinations (70, 72, 75 or 85 °C for 0–2 min or 20 °C for 20 min) without or with a constant shear at 1000 s−1. Shearing of pure IgG or BSA for 20 min at 20 °C resulted in protein denaturation (≥4%) and loss of secondary structure, mainly β-sheets in IgG and α-helices and β-turns in BSA. The shearing appeared to preserve native proteins to a varying degree when pure IgG and BSA were sheared at 72 °C and 75 °C for 0–2 min, respectively, in comparison to their non-sheared counterparts, possibly through a countereffect generated by shear against thermal effect. The shearing mostly induced denaturation of IgG and BSA under other conditions tested. The changes in the secondary structure largely coincided with protein denaturation or preservation results. The shearing had no apparent impact on aggregation of protein molecules due to covalent interactions in both, pure and binary, dispersions at 75 or 85 °C for 0–2 min. Heating at 85 °C for 2 min resulted in formation of covalently bound intra- and intermolecular aggregates in the pure and binary mixture, respectively, regardless of shearing.

Backbone resonance assignments of the A2 domain of mouse von Willebrand factor.

von Willebrand factor (vWF) is an adhesive plasma protein that is important for platelet adhesion in normal hemostasis in response to vascular injury. Although large vWF multimers are released from storage granules of platelets and (sub-)endothelial cells in response to hemostatic stimuli, for normal physiological function, vWF multimers are required to be cleaved into smaller multimeric forms. The plasma metalloproteinase ADAMTS13 specifically cleaves the peptide bond located in the middle of the A2 domain of vWF (vWF-A2), but the cleavage site is buried inside the structure of vWF and is difficult to access in the absence of elevated flow shear stress. On the other hand, in the presence of high vascular shear stress, the structure of vWF-A2 is supposed to be unfolded, thereby becoming accessible for proteolysis by ADAMTS13. However, the atomic-level mechanism underlying shear-induced structural changes of vWF-A2 remains unclear and to date no solution NMR information is available. In this study, we present the backbone 1H, 13C, and 15N resonance assignments of mouse vWF-A2; side chain assignments of 13Cβ are also provided. Secondary structure propensity analysis based on the assigned chemical shifts showed that mouse vWF-A2 forms similar secondary structures in solution to the previously determined crystal structure of human vWF-A2. The obtained NMR assignment data will contribute to an atomic-level characterization of shear-induced unfolding of vWF-A2 in solution.

Multiple-State Monitoring of SOD1 Amyloid Formation at Single-Residue Resolution by Rheo-NMR Spectroscopy.

Formation of protein aggregates or fibrils entails the conversion of soluble native protein monomers via multiple molecular states. No spectroscopic techniques have succeeded in capturing the transient molecular-scale events of fibrillation in situ. Here we report residue- and state-specific real-time monitoring of the fibrillation of amyotrophic lateral sclerosis-related SOD1 by rheology NMR (Rheo-NMR) spectroscopy. Under moderately denaturing conditions, where NMR signals of folded and unfolded monomeric SOD1 are simultaneously observable, the cross-peak intensities of folded monomeric SOD1 decreased faster than those of the unfolded species, and a 310-helix in folded SOD1 was deformed prior to global unfolding. Furthermore, real-time protein dynamics analysis identified residues involved in the core structure formation of SOD1 oligomers. Our findings provide insight into local and global unfolding events in SOD1 and fibril formation. This Rheo-NMR analysis will be applicable not only to atomic-level monitoring of other amyloidogenic proteins but also to quantification of shear-induced structural changes of non-amyloidogenic proteins and elucidation of shear-enhanced chemical phenomena such as viscosity increase and crystallization of various solution-state compounds.

A rheological and microstructural study of two-step yielding in mud samples from a port area

Natural fine-grained suspensions usually exhibit a complex rheological fingerprint – in particular a two-step yielding phenomenon – due to the presence of mineral clay particles and organic matter (often found in a flocculated state). These rheological properties may vary considerably from one location to another due to the differences in mud composition (specifically in organic matter content). In this study, the origin of this two-step yielding behaviour for natural suspensions is discussed with the help of different experimental techniques including rheology, particle sizing, rheo-optics, and video microscopy. The samples were collected from different locations in the Port of Hamburg, Germany. A rheological analysis of the samples was performed with amplitude sweep, frequency sweep, stress ramp-up and structural recovery tests. The shear-induced structural changes of mud samples was studied by using a parallel plate shearing device with a microscope. Mineral clay-organic matter flocs were studied using video microscopy to obtain the floc size, floc density and settling velocity of flocs. Higher values of rheological properties such as cross-over stress, yield stress, and moduli were observed for samples having higher organic matter content. These samples also produced the largest floc sizes. The rheo-optical analysis showed the formation of cylinder-like structures in fine-grained suspensions upon shearing action, which reflect the origin of two-step yielding behaviour in mud samples, observed in stress ramp-up and amplitude sweep tests. • Mud samples with different TOC content were analysed using rheology. • Two-step yielding was observed in mud from amplitude and stress sweep experiments. • Rheo-optical analysis showed the formation of cylinder-like structure during shearing. • Larger and porous flocs were observed for samples with higher TOC content. • This study provides a useful understanding of mud flocs during shearing.

Shear-Induced Adhesion of Alternating Peptides Prepared by Ugi Four-Center Three-Component Reaction.

The development of new peptide-based glues has been strongly urged from the viewpoints of industrial applications and biomedical engineering. However, the large-scale synthesis of polypeptides with an ordered sequence is highly challenging, which strictly restricts materials resources for the research and development of polypeptides. In this work, the framework of adhesive alternating peptides has been designed to be glycine (Gly)-N-substituted valine (Val) as the dipeptide repeating sequence, considering the peptapeptide repeating sequence of viscoelastic natural elastin as a motif. The alternating peptides are prepared via three-component polymerization exploiting Ugi four-center three-component reaction as the elemental polymerization reaction. The adhesive strength (SAdh ) values of the polymers are evaluated by a shear adhesive test method using two glass plates. Alternating peptides with Gly-N-benzylated Val dipeptide repeating units exhibit the optimal adhesive properties such as much higher SAdh than that of conventional fibrin glue and a unique readhesion capability. It is indicated that the remarkably high SAdh would be attributed to the shear-induced structural change of single polymer chain, the slow relaxation of extended structure, and the weak interchain interactions. Due to the favorable adhesive properties of alternating peptides, these adhesives may be highly suitable for real-world applications.

Structural Change of an α-Gel (α-Form Hydrated Crystal) Induced by Temperature and Shear Flow in an Oleic Acid Based Gemini Surfactant System.

We studied the effects of temperature and shear flow on the structures of α-gel bilayers and domains. The α-gel samples were prepared by a carboxylate-type gemini surfactant synthesized from oleic acid and a long-chain alcohol (1-tetradecanol) with water. The structural change as a function of temperature was investigated using small- and wide-angle X-ray scattering (SWAXS) measurements, spin-spin relaxation time (T2) measurements, and optical microscopy observations. SWAXS measurements suggested that the decreased temperature yielded the α-gel phase from a lamellar liquid-crystal phase. We also found that the lamellar d-spacing drastically decreased at the phase transition temperature. The T2 measurements suggested that two kinds of protons with different mobilities coexisted in amphiphiles consisting of lamellar bilayers. The abundance of the protons with low mobility increased with decreasing temperature. Optical microscopy results indicated that the size of the α-gel domains increased with decreasing temperature. We assumed that the increased abundance of the low-mobility protons, indicating low flexibility of lamellar bilayers, led to a decreased lamellar d-spacing and increased size of the α-gel domains. Shear-induced structural changes in the α-gel were also studied using simultaneous small-angle neutron scattering and rheological measurements. The α-gel can maintain bilayer structures even at high shear rates. We also found that the lamellar d-spacing was independent of the shear rate.

Neutral hydrocolloids promote shear-induced elasticity and gel strength of gelatinized waxy potato starch

Shear forces are important during food processing, food consumption, and food digestion, and often are estimated to evaluate their effects on product quality. However, few studies have been done on shear-induced structure changes of starch/hydrocolloid systems. Our previous work showed that gelatinized waxy potato starch forms a strong viscoelastic gel under the application of long-term low shear at low temperatures, a phenomenon that is not noted in other waxy starches such as rice and corn. In the present study, the rheological properties of gelatinized waxy potato starch in the presence of neutral (guar gum and konjac glucomannan) and anionic hydrocolloids (gum arabic, xanthan gum, sodium alginate, and pectin) were investigated. Results showed that gelatinized starch dispersions containing hydrocolloids with flexible polymer chains (guar gum, konjac glucomannan, sodium alginate, and pectin) or a highly-branched structure (gum arabic) were able to produce gels with a more liquid-like behaviour when compared to gels produced by dispersions containing hydrocolloids with rigid chains (i.e., xanthan gum). Phase separation in starch/xanthan gum system and formation of a 3D-network in the xanthan gum phase may be responsible for the strengthened gel structure resulting in high elasticity. When shearing the gelatinized waxy potato starch/hydrocolloid dispersions at 5 °C for 24 h at 20 s−1, the presence of neutral and highly viscous hydrocolloids drastically increased the gel strength, while anionic and low viscous hydrocolloids decreased it. Such decrease in gel strength was attributed to the electrostatic repulsion between anionic hydrocolloids and the naturally negatively charged phosphorylated chains of waxy potato starch.

Shear-induced liquid-crystalline phase transition behaviour of colloidal solutions of hydroxyapatite nanorod composites.

Liquid-crystalline (LC) bio-inspired materials based on colloidal nanoparticles with anisotropic morphologies such as sheets, plates, rods and fibers were used as functional materials. They show stimuli-responsive behaviour under mechanical force and in electric and magnetic fields. Understanding the effects of external stimuli on the structures of anisotropic colloidal particles is important for the development of highly ordered structures. Recently, we have developed stimuli-responsive hydroxyapatite (HAP)-based colloidal LC nanorods that are environmentally-friendly functional materials. In the present study, the ordering behaviour of HAP nanorod dispersions, which show LC states, has been examined using in situ small-angle neutron scattering and rheological measurements (Rheo-SANS) under shearing force. The structural analyses and dynamic viscosity observations provided detailed information about the effects of shear force on the structural changes of HAP nanorods in D2O dispersion. The present Rheo-SANS measurements unraveled three kinds of main effects of the shear force: the enhancement of interactions between the HAP nanorods, the alignment of HAP nanorods to the shear flow direction, and the formation and disruption of HAP nanorod assemblies. Simultaneous analyses of dynamic viscosity and structural changes revealed that the HAP nanorod dispersions exhibited distinctive rheological properties accompanied by their ordered structural changes.

Shear-Induced Structural Changes and Origin of Ultralow Friction of Hydrogenated Diamond-like Carbon (DLC) in Dry Environment.

The origins of run-in and ultralow friction states of a sliding contact of hydrogenated diamond-like carbon (H-DLC) and sapphire were studied with an in situ Raman tribometer as well as ex situ analyses of transmission electron microscopy (TEM), Raman spectroscopy, and nanoindentation. Prior to ultralow friction behavior, H-DLC exhibits a run-in period. During the run-in period in dry nitrogen atmosphere, the transfer film was formed and its uniformity and thickness as well as structure were varied. The duration and friction behaviors during the run-in depended on the initial surface state of the H-DLC coatings. A comparative study of pristine and thermally oxidized H-DLC revealed the role of surface oxide layer on run-in friction and transfer film formation. Attainment of the ultralow friction state appeared to correlate with the uniformity and structure of the transfer film evolved during the run-in, rather than its final thickness. TEM cross-section imaging of the wear track and the counter surfaces showed a trace of nanocrystalline graphite and a thin modified surface layer on both rubbing bodies. The comparison of hardness and reduced modulus of the wear tracks and the unworn surfaces as well as the ex situ Raman spectra suggested the densification of the wear track surfaces. Combining the in situ and ex situ analysis results, a comprehensive model was proposed for the formation and structure of the ultralow friction sliding contact of H-DLC.

Challenges in Rheological Characterization of Highly Concentrated Suspensions — A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching. The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

Challenges in Rheological Characterization of Highly Concentrated Suspensions - A Case Study for Screen-printing Silver Pastes.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

Structure-rheology relationship for a homogeneous colloidal gel under shear startup

The structural anisotropy and shear rheology of colloidal gels under startup of shear flow are calculated by Brownian dynamics simulations modified to include particle surface-mediated attractions, which is a model for thermoreversible colloidal gels. Shear-induced structural changes are analyzed in both real and reciprocal space through computation of the pair distribution function and structure factor. A distinct structural anisotropy is evident as alignment along the compressional and vorticity axes. A spherical harmonics expansion of pair distribution function is calculated to analyze structural anisotropy. In reciprocal space, structural anisotropy is quantified through an alignment factor, which shows overshoot behavior similar to the stresses. Based on the microstructure analysis, the evolution of the structural anisotropy is explained by the anisotropic rupture of the colloidal gel microstructure. This result provides evidence for how shearing creates structural anisotropy. The structural anisotro...

Surface and friction forces between grafted polysaccharide layers in the absence and presence of surfactant

We analyzed the interaction between chemically grafted polysaccharide layers in aqueous solutions. To fabricate such layers, an end-terminated dextran silane coupling agent was synthesized and the polydextran was grafted to oxidized silicon wafers and to silica particles. This resulted in the formation of a 28nm thick layer (in air) and a grafted amount of 40mg/m2 as determined by ellipsometry. The physical properties of the grafted layer were investigated in aqueous solutions by atomic force microscope imaging and colloidal probe force measurements. Surface and friction forces were measured between one bare and one polydextran coated silica surface. A notable feature was a bridging attraction due to affinity between dextran and the silica surface. Surface interactions and friction forces were also investigated between two surfaces coated with grafted polydextran. Repulsive forces were predominant, but nevertheless a high friction force was observed. The repulsive forces were enhanced by addition of sodium dodecyl sulfate (SDS) that associates with the tethered polydextran layers. SDS also decreased the friction force. Our data suggests that energy dissipation due to shear-induced structural changes within the grafted layer is of prime importance for the high friction forces observed, in particular deformation of protrusions in the surface layer.

Shear-induced floc structure changes for enhanced dewatering of coal preparation plant tailings

The handling and disposal of industrial waste slurries containing colloidal particulate matter, with particular reference to coal and mineral processing plant tailings with a high proportion of clay materials present significant challenge. To develop effective disposal methods requires an improved understanding of floc structure that promote better water recovery and higher sediment density during settling and consolidation of flocculated tailings. In this paper, the effect of floc structure on coal tailings dewatering and the impact of shear-induced modification of floc network structure on additional sediment consolidation and densification have been examined. Four real tailings and a high molecular weight anionic polymeric flocculant were used. Flocculation tests were conducted under reproducible hydrodynamic conditions in batch mode. Flocculated aggregates have complex structures with different sizes, shapes and voidages. The structures of the flocs were characterised by fractal analysis using light scattering technique and the dewatering characteristics of the flocs were investigated in terms of settling rates and sediment consolidation. The fractal analysis by laser scattering identified optimum mixing speed and flocculant dosage required for the production of flocs with compact structure of fractal dimensions greater than 2. It was determined that hydrodynamic conditions during flocculation have a profound impact on floc settling rates and to a lesser extent sediment consolidation. Low mixing speeds produced large flocs with high settling rates, while very high mixing speed during flocculation resulted in irreversible floc degradation leading to much reduced settling rates with some additional sediment consolidation. Application of shear to pre-sedimented flocs resulted in significant additional sediment consolidation for the tailings samples and flocculants studied. This approach would allow flocculation to be optimised for fast settling with subsequent application of shear to the settled sediment to promote the removal of inter and intra-floc liquor to improve consolidation and sediment density.

Investigating liquid-solid interfacial phenomena in a Couette flow at nanoscale

This study investigates shear-induced liquid structure changes in nanoscale Couette flows and their corresponding flow boundary conditions. Molecular dynamics simulations are used to model a liquid argon slab confined between two smooth rigid copper walls with an applied velocity at the upper wall to generate a planar Couette flow. Depending on the applied wall velocity, different liquid structures or the orderings of the liquid at liquid-solid interfaces are identified when reaching steady states. We define three regimes based on the ordering of the liquid structure: Newtonian, layer, and oversheared. Each regime is characterized by the spatial probability distribution and structure factor. These liquid structures are strongly correlated with the liquid velocity and density profiles in the flow. Ultimately, the liquid structures also determine the boundary conditions from pure slip to multilayer locking at liquid-solid interfaces. Our results show that temperature and liquid-solid interaction parameter are also important factors in influencing the liquid structures formed near interfaces.

Rheodielectric study on shear-induced structural change in the smectic-Aphase of4−n-octyl-4′-cyanobiphenyl(8CB)

Simultaneous measurements of rheological and dielectric properties are made to investigate shear-induced structural change in the smectic-A phase of 8CB (4-n-octyl-4'-cyanobiphenyl). With increasing the shear rate, the fluidity changes from non-Newtonian to Newtonian flow via an unstable flow region, accompanied by a characteristic change in the dielectric permittivity. In the non-Newtonian flow region, a dielectric dispersion, which can be ascribed to an undulation motion of smectic layer, is recognized. On the basis of these results, it is suggested that with increasing the shear rate the undulation changes to a chaotic structure, which is followed by a more simple structure with the layer normal along the neutral axis, and that these structural changes are responsible for the fluidity change.