Shear-induced Structure Research Articles

Assembly process of locust bean gum and xanthan gum for synergistic gelling revealed by atomic force microscopy

Synergistic interaction gels (SIGs) have become attractive in food industry due to their ability to modify rheology features, realize safe swallowing and apply as fat simulants. Locust bean gum (LBG) and xanthan give rise to SIG, however, their assembly process remain elusive. This work demonstrated that, in xanthan-rich system, xanthan chains transformed into helical structure and aggregated into thick bundles upon cooling. Subsequently they bound with LBG clusters, constructing a xanthan-like 3D gel network. However, in LBG-rich system, LBG chains gradually dissociated from clusters and associated into helical structure during cooling (“dissociation-association” process), forming an LBG-like “sea-island” structure. With further cooling, LBG continued to undergo “dissociation-association” process, along with specifically binding by xanthan chains (helical structure or thick bundles), generating thick grid contour lines and larger sparse gel network. The enthalpy change values indicated that with the decrease of xanthan content, synergistic binding extent was increased and then decreased, reaching the maximum at LBG: xanthan ratio of 7:3. This was responsible for the increment of G′, gel hardness, and capacity to accumulate energy for the weak shear-induced structure for LBG/xanthan SIGs. This study serves to facilitate the design and production of SIGs with the requisite performance characteristics.

Influence of shear deformation on the formation of dynamic structure in protic ionic liquids. Representation of the temperature dependence of viscosity within the framework of Angell’s fragility concept

The rheological properties of a liquid containing two types of basic centers and hydroxyl groups (BGE-Im), as well as protic ionic liquids (ILs) derived from it, were studied in a wide range of shear rates (γ̇) (10−6–103 s−1) at various temperatures from 10 to 50 °C. Viscosity (η) of the BGE-Im does not depend on the scanning direction of shear stress (σ), i.e. this compound behaves like a Newtonian fluid over the entire range of σ. The dependence log η (1/T) of BGE-Im in Arrhenius coordinates is linear. Protonation of basic centers of BGE-Im leads to a strong increase in the viscosity of ILs obtained on its basis, and the temperature dependence of the viscosity of these ILs is described by the Vogel–Fulcher–Tammann (VFT) equation. It has been established that in the region of low shear rates, the viscosity of the ILs increases with decreasing shear rate, which leads to the appearance of a yield stress (σY). This indicates the formation of a fluctuation rheopectic structure in the ILs. In this sense, ILs are heterogeneous systems and their behavior is similar to that of highly concentrated suspensions and emulsions. For the first time, a quantitative comparison between the parameters of the VFT equation, the fragility (according to Angell’s model) of ILs, the relative coordination number of ILs molecules and the rheological yield stress characterizing the shear-induced fluctuation rheopectic structure of ILs has been made and the relationship between these characteristics has been shown.

Structure Strengthening Phenomena of Gluten Matrices under Different Stress Types.

To predict the achievable product volume with respect to the gas retention capacity of the gluten matrix in wheat flour doughs, strain hardening evaluation is crucial. But assessing these structure hardening phenomena in wheat flour dough systems is still a challenging task. In this work, a simple shear method applied to kneaded dough samples was tested and compared to biaxial extension tests performed with a lubricated squeezing flow method. The comparability of shear-induced structure hardening with biaxial extension tests was shown. Structure hardening and breakdown after overload were visualized using shear flow and a comparison of the obtained shear flow over Hencky strain curve peaks. To predict the behavior of the analyzed flours according to their composition, a correlation analysis of the flour and dough properties was performed. The influence of the HMW glutenin subunits on the sensitivity of the dough matrix according to the applied shear speed (0.1 and 1.0 mm/s) could be shown with a correlation coefficient of 0.94. The LMW glutenin subunits, on the other hand, showed a high correlation coefficient of 0.89 with the achievable network connectivity parameter z [-] gained from frequency sweep testing.

Shear-Induced Structure Formation in MAH-g-PP Compatibilized Polypropylenes

Shear-Induced Structure Formation in MAH-g-PP Compatibilized Polypropylenes

Probing emulsion-gel transition in aqueous two-phase systems stabilized by charged nanoparticles: A simple pathway to fabricate water-in-water emulsion-filled gels

Nanoparticle-stabilized water-in-water (w/w) emulsions, an example of aqueous two-phase systems (ATPS), can produce low-fat food colloids, edible gels, bio-polymer-based bijels, scaffolds for tissue-engineering, and porous materials. This w/w emulsion is made from two thermodynamically incompatible aqueous-polymer solutions, such as polyethylene oxide (PEO) and dextran. Changing the molecular weight of PEO in one aqueous phase stabilizes w/w emulsion. The state diagram we generated using electron and light microscopy provides a novel approach to producing stable emulsion-filled gels and emulsion droplets. The visual examination, brightfield, and fluorescent microscopy studies highlight the role of molecular weight and storage duration. The production of an emulsion-filled gel is ascribed to the “active-filler-particles” arrangement, typically seen when the affinity between droplets stabilized by particles and polymer is substantial. In contrast, we expect the “inactive-filler-particles” arrangement for the samples that undergo phase inversion. The temporal evolution of shear-induced structures recorded using a rheometer demonstrates that these emulsion-filled gels’ viscoelastic properties correspond directly with time, molecular weight, and polymer composition. The emulsion-filled gels generated displayed 90-day storage stability. Our work would help us understand the complex dynamics of w/w emulsion-based formulations that need suitable size, shape, appearance, and shelf life management.

Gas channeling control with an in-situ smart surfactant gel during water-alternating-CO2 enhanced oil recovery

Undesirable gas channeling always occurs along the high-permeability layers in heterogeneous oil reservoirs during water-alternating-CO2 (WAG) flooding, and conventional polymer gels used for blocking the “channeling” paths usually suffer from either low injectivity or poor gelation control. Herein, we for the first time developed an in-situ high-pressure CO2-triggered gel system based on a smart surfactant, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM), which was introduced into the aqueous slugs to control gas channeling in WAG processes. The water-like, low-viscosity UC22AMPM brine solution can be thickened by high-pressure CO2 owing to the formation of wormlike micelles (WLMs), as well as their growth and shear-induced structure buildup under shear flow. The thickening power can be further potentiated by the generation of denser WLMs resulting from either surfactant concentration augmentation or a certain range of heating, and can be impaired via pressurization above the critical pressure of CO2 because of its soaring solvent power. Core flooding tests using heterogeneous cores demonstrated that gas channeling was alleviated by plugging of high-capacity channels due to the in-situ gelation of UC22AMPM slugs upon their reaction with the pre- or post-injected CO2 slugs under shear flow, thereupon driving chase fluids into unrecovered low-permeability areas and producing an 8.0% higher oil recovery factor than the conventional WAG mode. This smart surfactant enabled high injectivity and satisfactory gelation control, attributable to low initial viscosity and the combined properties of one component and CO2-triggered gelation, respectively. This work could provide a guide towards designing gels for reducing CO2 spillover and reinforcing the CO2 sequestration effect during CO2 enhanced oil recovery processes.

Development of Synthesis Method for Gel Fibers with Shear-Induced Multi-Core Structure

Development of Synthesis Method for Gel Fibers with Shear-Induced Multi-Core Structure

Large amplitude oscillatory shear behavior of thermoresponsive hydrogels: Single versus double network

Double network (DN) hydrogels have been recognized as new tough materials for several industries due to their precise structural platforms and significant properties. However, a comprehensive understanding of microstructural changes of DN hydrogels under large deformations is required to extend their applications. In this work, we use the large amplitude oscillatory shear (LAOS) technique to study the nonlinear response of a thermoresponsive κ-carrageenan/polyacrylamide DN system and its nanocomposite containing graphene oxide (GO) in comparison to its single network components. The results show a combination of strain stiffening and shear thickening nonlinear responses. The elastic intracycle strain stiffening was mainly attributed to the shear-induced increase in the elasticity of network chains and non-Gaussian stretching of individual chains. In addition, the orientation of the κ-carrageenan double helix segments and their enhancing effect on molecular orientation could be proposed as another possible mechanism of strain stiffening. The viscous intracycle shear thickening is also interpreted by two mechanisms of shear-induced temporary structure formation and reformation of dissociated physical interactions. It is also found that the GO nanosheets could contribute to the viscoelastic response by increasing the molecular interactions and, thus, amplification of energy dissipation. Furthermore, temperature dependency of the DN hydrogel owing to the conformational changes of the κ-carrageenan network at sufficiently high temperatures is used to investigate the effect of temperature on nonlinear behaviors. Increasing the temperature is found to have a significant decreasing effect on viscous nonlinearity, while its effect on the elastic nonlinearity was strongly dependent on the strain amplitude. This study provides a better understanding of the correlation between the microstructure and viscoelastic properties for designing tough hydrogels.

High temperature tribology behavior of silicon and nitrogen doped hydrogenated diamond-like carbon (DLC) coatings

To improve the thermal stability and mechanical performances of the DLC coating at high temperature, thermodynamically stable Si-N and C-N bonds were introduced into the DLC coating by co-doping Si and N elements. The friction coefficient and wear rate changed from 0.055 and 1.93 × 10−7 mm3/Nm (25 °C) to 0.033 and 7.3 × 10−6 mm3/Nm (500 °C). The superior tribological performances of Si, N-DLC coating at 500 °C was attributed to the formation of compacted layers with high hardness (9.2 GPa) and elastic recovery (~89%) on the wear track during running-in period. The compacted layer possessed a high compliance and large deformation capacity due to the fullerene-like nanostructure within amorphous carbon network and the shear-induced stacked structure, thereby reducing the friction.

Solution to the two-body Smoluchowski equation with shear flow for charge-stabilized colloids at low to moderate Péclet numbers

We developed an analytical theoretical method to determine the microscopical structure of weakly to moderately sheared colloidal suspensions in dilute conditions. The microstructure is described by the static structure factor, obtained by solving the stationary two-body Smoluchowski advection-diffusion equation. The singularly perturbed PDE problem is solved by performing an angular averaging over the extensional and compressing sectors and by the rigorous application of boundary-layer theory (intermediate asymptotics). This allows us to expand the solution to a higher order in P\'eclet with respect to previous methods. The scheme is independent of the type of interaction potential. We apply it to the example of charge-stabilized colloidal particles interacting via the repulsive Yukawa potential and study the distortion of the structure factor. It is predicted that the distortion is larger at small wavevectors $k$ and its dependence on $Pe$ is a simple power law. At increasing $Pe$, the main peak of the structure factor displays a broadening and shift towards lower $k$ in the extensional sectors, which indicates shear-induced spreading out of particle correlations and neighbor particles locally being dragged away from the reference one. In the compressing sectors, instead, a narrowing and shift towards high $k$ is predicted, reflecting shear-induced ordering near contact and concomitant depletion in the medium-range. An overall narrowing of the peak is also predicted for the structure factor averaged over the whole solid angle. Calculations are also performed for hard spheres, showing good overall agreement with experimental data. It is also shown that the shear-induced structure factor distortion is orders of magnitude larger for the Yukawa repulsion than for the hard spheres.

Characteristic Time for Disappearance of Shear-Induced Structure in Wormlike Micelle Solutions

Characteristic Time for Disappearance of Shear-Induced Structure in Wormlike Micelle Solutions

A Study on the Condition of No Shear-Induced Structure Generation in Wormlike Micelle Solutions

A Study on the Condition of No Shear-Induced Structure Generation in Wormlike Micelle Solutions

Formation of Gel-Like Shear-Induced Structure with Dosing of Dilute Surfactant Solution and Its Effect on Turbulent Channel Flow

A shear-induced structure (SIS) is formed under appropriate concentration and shear conditions in a surfactant micellar solution. In this study, we performed experiments of surfactant solution dosing in a fully developed two-dimensional turbulent channel flow from a sintered metallic wire mesh plate attached to a side wall. We investigated the behavior of the solution under the elongation during its passing through the wire mesh and under the strong shear due to the channel flow. It was confirmed that the dosed solution containing a laser dye was visualized by a laser sheet, and the accumulated gel from the wire mesh formed a layer and developed with time. Consequently, on dosing the dilute surfactant solution from the wire mesh, a gel-like SIS layer was formed, which majorly covered the wire mesh plate. The gel-like SIS layer on the wire mesh plate acted as a sticky solid and restricted the flow in the channel. This layer continued to grow while dosing, owing to which the pressure drop of the channel flow significantly increased. The gel-like SIS layer grew rapidly even in the turbulent flow and reached the equilibrium thickness. After the termination of the dosing, the gel layer collapsed gradually. In addition, the thickness of the gel-like SIS layer (indicating the strength indirectly) strongly depended on the surfactant concentration and the elongation rate in the wire mesh.

Shear-banding fluid(s) under time-dependent shear flows. Part II: A test of the Moorcroft–Fielding criteria

Complex systems often exhibit shear banding—the coexistence of two different states characterized by their internal structuring and local shear rates. For some of them, the heterogeneous flow corresponds to the final steady-state response, while for others, shear banding can only be transient, the banding structure healing back to a homogeneous flow in the ultimate steady state after long-lived periods. In order to explain the diversity of observations, Moorcroft and Fielding have established general criteria for the onset of banding in time-dependent flows of complex systems, ranging from polymeric fluids to soft glassy materials [Moorcroft et al., Phys. Rev. Lett. 110, 086001 (2013)]. The proposed criteria are based on the time evolution of the bulk rheological response function of the system to a given time-dependent flow protocol and are associated with a specific signature in the mechanical response. In this contribution, we test the validity of these criteria in the case of two common time-dependent flow protocols: a step stress and a shear startup. Two types of fluids are examined. On the one hand, a wormlike micelles system exhibiting steady shear banding is studied experimentally, using rheometry coupled with direct visualizations and particle image velocimetry. On the other hand, we analyze previous literature on yield stress fluids exhibiting transient shear banding. Under conditions of a constant imposed stress, for both types of fluids, the onset of banding arises in a time window compatible with the Moorcroft–Fielding criterion. However, the mechanical signature, i.e., the shape of the bulk mechanical signal as a function of time, is not the one expected within some of the specific models with which the general Moorcroft–Fielding criteria were tested numerically. Under shear startup, both types of fluids behave differently. The criterion holds for yield stress fluids, the onset of banding arising just after the stress overshoot, as expected. On the contrary, for wormlike micelles, the window of instability is delayed, even if the overshoot clearly plays a crucial role in the nucleation of shear-induced structures. Regardless of the flow protocol or the system, wall slip seems to go hand in hand with banding indicating that it is a key ingredient to take into account.

Self-Poled Poly(vinylidene fluoride)/MXene Piezoelectric Energy Harvester with Boosted Power Generation Ability and the Roles of Crystalline Orientation and Polarized Interfaces.

Micropiezoelectric devices have become one of the most competitive candidates for use in self-powered flexible and portable electronic products because of their instant response and mechanic-electric conversion ability. However, achievement of high output performance of micropiezoelectric devices is still a significant and challenging task. In this study, a poly(vinylidene fluoride) (PVDF)/MXene piezoelectric microdevice was fabricated through a microinjection molding process. The synergistic effect of both an intense shear rate (>104 s-1) as well as numerous polar C-F functional groups in MXene flakes promoted the formation of β-form crystals of PVDF in which the crystallinity of β-form could reach as high as 59.9%. Moreover, the shear-induced shish-kebab crystal structure with a high orientation degree (fh = ∼0.9) and the stacked MXene acted as the driving force for the dipoles to regularly arrange and produce a self-polarizing effect. Without further polarization, the fabricated piezoelectric microdevices exhibited an open-circuit voltage of 15.2 V and a short-circuit current of 497.3 nA, under optimal conditions (400 mm s-1 and 1 wt % MXene). Impressively, such piezoelectric microdevices can be used for energy storage and for sensing body motion to monitor exercise, and this may have a positive impact on next-generation smart sports equipment.

Constitutive modeling of dilute wormlike micelle solutions: Shear-induced structure and transient dynamics

We present a reformulation of the 'reactive rod model' (RRM) of Dutta and Graham [Dutta, Sarit and Graham, Michael D., JNNFM 251 (2018)], a constitutive model for describing the behavior of dilute wormlike micelle solutions. The RRM treats wormlike micelle solutions as dilute suspensions of rigid Brownian rods undergoing reversible scission and growth in flow. Evolution equations for micelle orientation and stress contribution are coupled to a kinetic reaction equation for a collective micelle length, producing dynamic variations in the length and rotational diffusivity of the rods. This model has previously shown success in capturing many critical steady-state rheological features of dilute wormlike micelle solutions, particularly shear-thickening and -thinning, non-zero normal stress differences, and a reentrant shear stress-shear rate curve, and could fit a variety of steady state experimental data. The present work improves on this framework, which showed difficulty in capturing transient dynamics and high-shear behavior, by reformulating the kinetic equation for micelle growth on a more microstructural (though still highly idealized) basis. In particular, we allow for micelle growth associated with strong alignment of rods and breakage due to tensile stresses along the micelles. This new formulation captures both steady and transient shear rheology in good agreement with experiments. We also find good agreement with available steady state extensional rheology.

On the consistent modeling of shear-thickening polymer solutions

During the past few decades, the interest in understanding the peculiar rheological behavior of shear-thickening fluids has increased due to their potential use in various commercial applications. In such an endeavor, the optimal design of these fluids is essential, which necessitates our in-depth understanding of their properties from a modeling perspective. We herein introduce a continuum model to predict the rheological behavior of shear-thickening polymer solutions using non-equilibrium thermodynamics that guarantees, by construction, consistency with the laws of thermodynamics as extended to handle non-equilibrium systems. This is made possible by using a scalar structural variable that characterizes the formation of the shear-induced structure at sufficiently high shear rates, and a conformation tensor that characterizes the deformation of the polymer segments. The model predicts the exhibition of a shear-thickening behavior for all steady shear flow material functions (shear viscosity and normal stress coefficients), which is then followed by a shear-thinning behavior if finite extensibility or anisotropic effects are considered. We further document that these model predictions are in line with available shear viscosity rheological data for shear-thickening polymer solutions.

Rheological characterization of a physically-modified waxy potato starch: Investigation of its shear-thickening mechanism

This study focuses on the rheological characterization of the time- and shear-dependence of a physically modified waxy potato starch through treatment at 120 °C for 30 min under continuous stirring. The starch sample exhibited time-dependency, where the viscosity increased with time at constant shear rates between 5 and 60 s−1. Shear-induced gelation was also observed when the sample was sheared above 100 s−1. The gel structure remained relatively stable within the experimental timeframe of 72 h at 20 °C. A minimum starch concentration of 1% w/w was required for shear-thickening and anti-thixotropy to be observed. This study showed that the formation of a shear-induced weak gel structure was not caused solely by hydrogen bonding, electrostatic, hydrophobic interactions or a combination of these forces. The formation of a weak gel structure was probably initiated first by shear which orientated the amylopectin molecules in the direction of shear. The ‘stretched’ amylopectin molecules subsequently facilitated closer proximity between polymer chains which promoted intermolecular association between polymer chains to take place.

Drag Reduction in the Flow of Aqueous Solutions of a Mixture of Cocamidopropyl Betaine and Cocamide DEA

The study presents results of rheological tests and measurements of pressure drops occurring during the flow of aqueous solutions of a mixture of drag reducing surfactants: cocamidopropyl betaine (CAPB, zwitterionic surfactant) and cocamide DEA (nonionic surfactant) through straight pipes. Tests were carried out at different CAPB/DEA weight ratios and different total concentrations of surfactants in the solution. Rheological measurements demonstrate the formation of a shear-induced structure (SIS) in the temperature range below 10 °C, which provides evidence for the presence of wormlike micelles in CAPB/DEA solutions. Drag reduction was observed during the flow of CAPB/DEA solutions in the temperature range from 3 to 45 °C, however, above 25 °C the degree of drag reduction was markedly decreased. The lower temperature limit at which drag reduction occurs depends on the CAPB and DEA weight ratio in the solution. In the range of higher temperatures, during the flow of CAPB/DEA solutions (similarly to flexible-chain polymer solutions) the onset of drag reduction is noted above a certain critical value of the Rec,0 number, whose value depends on the temperature of the solution, diameter of the pipe and the weight ratio of surfactants. At the same time, the critical value of wall shear stress τw,c0 corresponding to the critical value of Rec,0 is approximately independent of pipe diameter. The critical value of the Rec,0 number has been linked to the clouding of CAPB/DEA solutions.

Synergistic effect of plasticizer and nucleating agent on crystallization behavior of polylactide during fused filament fabrication

Polylactide (PLA) materials manufactured by fused filament fabrication (FFF) technique are usually in amorphous form and thereby exhibit poor mechanical properties and thermal resistance. Overcoming this issue requires a good knowledge of the crystallization behavior of PLA materials during the FFF process. In this work, a plasticizer (polyethylene glycol, PEG) and a nucleating agent (tetramethylene-dicarboxylic dibenzoyl-hydrazide, TMC-306), were added in PLA matrix, separately and synergistically, to tailor crystallization behavior of FFF-printed PLA parts. At a bed temperature of 60 °C, PEG played a stronger effect on crystallization behavior than the nucleating agent TMC and even the combination of TMC and PEG. This resulted in an increase of the crystallinity from 8% for both neat PLA and PLA/TMC samples to 18% for the samples containing PEG. With the increase of the bed temperature to 90 °C, TMC and PEG separately or synergistically had prominent effects on enhancing the crystallization ability of PLA during the FFF process, leading to the highly crystalline PLA parts with the crystallinities in the range of 30–40%. By means of wide angle X-ray diffraction and scanning electron microscopy measurements, shear-induced crystal structures were indentified at the filament surface as well as the weld interface. Nevertheless, the shear-induced effect had negligible influence on the final crystallinity of PLA parts. Instead, PLA parts with a high crystallinity can be attained, only when the material characteristics and the printing conditions are in favor of cold crystallization of PLA. The unique crystallization behavior of FFF-printed PLA materials offers guidelines for the fabrication of products with controlled crystallinity and hierarchical structures for specific applications.