Shear-induced Structure Formation Research Articles

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

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

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.

Simultaneous determination of micellar structure and drag reduction in a surfactant solution flow using the fluorescence probe method

As widely known, the addition of a specific type of surfactant to water reduces drag in a pipe flow. This effect is considered to be a result of the suppression of turbulent transition caused by the ordered structure of rod-like micelles that is referred to as a shear-induced structure (SIS). However, it is typically difficult to determine the SIS since it is necessary to noninvasively detect the SIS with several hundred nanometers in the actual moving flow. In this study, we used the fluorescence probe method to locally determine the SIS in a pipe flow. When hydrophobic fluorescence molecules are added to the surfactant solution, the fluorescence molecules are trapped in micelles. Thus, fluorescence intensity varies based on the change in the micellar structure. We verified the applicability of the fluorescence probe method to the SIS detection and determined the relationship between the micellar structure and the drag reduction in the pipe flow by simultaneously measuring the fluorescence intensity and pipe friction factor. The experimental result demonstrates that the SIS formation in the near-wall region is closely correlated with the drag reduction and suggests that the near-wall SIS suppresses the turbulent transition.

Flow Behavior of Native Corn and Potato Starch Granules in Aqueous Suspensions

The flow behavior of native corn and potato starch granule suspensions prepared in a concentrated sucrose solution has been investigated. Measurements were performed using a rotational rheometer with a concentric cylinder geometry. Starch suspensions were dilute to semi-concentrated (1 % to 25 % by volume). Shear and dynamic viscosity were measured by shear flow and dynamic oscillatory testing at 20, 50 and 80 °C. The starch suspensions exhibited essentially Newtonian behavior at all solid contents, although at higher solid volume fractions there was evidence of slight shear thickening. The relative viscosity of suspensions increased with increasing starch granule content, and the data conformed well to Maron-Pierce’s equation. An increase in maximum packing fraction and gravitational depletion of the starch granules with increasing temperature resulted in lower relative viscosities at higher temperatures. Also, the relative viscosities of potato starch granule suspensions with bigger, more oval and anisometric particles were lower than those of corn starch suspensions where granules were closer to sphericity but were angular in shape. Oscillatory shear testing results showed the presence of viscoelastic properties at intermediate solid volume fractions at low frequencies; in addition, the relative shear viscosity was higher than the relative dynamic viscosity, probably due to the formation of shear-induced structures during the shear flow test.

Shear-induced fibrous structure formation from a pectin/SPI blend

Abstract Well-defined shear flow can be applied to create fibrous, anisotropic samples from biopolymers when present at sufficiently high dry matter contents. Anisotropic biopolymer structures are of high interest especially when made from plant-based polymer blends due to novel food applications, like meat replacers. We investigate shear-induced structuring of a pectin/soy protein isolate (SPI) blend under heating. Scanning Electron Microscope analysis revealed that shear-induced structuring resulted in elongated pectin filaments, oriented in the direction of the shear flow, being entrapped in a continuous protein phase, inducing anisotropy in the blend. The length of the pectin filaments increased upon higher pectin concentrations and shearing temperatures, leading both to higher anisotropy, as measured with the tensile strength analysis. The fibrous appearance of samples became more evident when deforming the product by tearing, which effect was thought to be caused by detachment through or along the long side of the pectin filament. Industrial relevance The efficient preparation of fibrous products based on plant materials is of interest, because these products can be a starting point for the development of meat replacers. Meat replacers made from plant material are a promising, innovative, and sustainable source of protein for human consumption. With an increasing world population, creation of innovative sources of protein are needed to be able to feed everyone (United Nations - Department of Economic and Social Affairs, 2015). Proteins from plant sources, such as soy, are preferred over proteins from animal origin because plant based materials, for example, have lower environmental impact (Day, 2013; Mogensen, Hermansen, Halberg, Dalgaard, R., Vis, & Smith, 2009). In order to replace meat by a plant-based meat replacer, it is important that a similar product in terms of structural properties is developed to improve consumer acceptance (Hoek et al., 2011). Meat replacers are often produced with extrusion cooking, which is a process that has been applied for texturization of plant materials in the application of meat replacers for decades (Campbell, 1981; Harper & Clark, 1978). Previous research showed that a novel technique based on well-defined shear flow can also be used to create fibrous, anisotropic structures from plant-based biopolymers at sufficiently high dry matter contents, with a cone–cone device (Shear Cell) or a concentric cylinder device (Couette Cell) (Grabowska, Tekidou, Boom, & van der Goot, 2014; G. A. Krintiras, Gobel, Bouwman, van der Goot, & Stefanidis, 2014; Manski, van der Goot, & Boom, 2007). This novel technique uses milder conditions for structure formation, due to lower applied shear forces, and has therefore a lower specific mechanical energy input (Grabowska et al., 2016; G.A. Krintiras, Gadea Diaz, van der Goot, Stankiewicz, & Stefanidis, 2015).

On the modelling of the shear thickening behavior in micellar solutions

The steady and transient nonlinear rheological behaviors of dilute rod-like micellar solutions are predicted here with a particular case of the generalized Bautista–Manero–Puig (BMP) model that consists of the upper-convected Maxwell constitutive equation and a dissipative power-dependent kinetic equation, which takes into account the formation and disruption of shear-induced structures (SISs). This model has been derived using the extended irreversible thermodynamic (EIT) formalism. In steady shear, the model predicts a Newtonian region at low shear rates and a characteristic shear rate (\( {\dot{\upgamma}}_{\mathrm{c}} \)) at which shear thickening develops. In the shear thickening region, the model predicts either a reentrant zone, which is caused by multi-valued shear stresses when the data are collected with a shear stress-controlled mode or a continuous increase in the shear stress-shear rate flow curve, when the data are collected with a shear rate-controlled mode; in this region, two coexisting phases are predicted. The coexisting phases at the same shear rate in the two-phase envelope and the spinodal-like region were evaluated from the extended Maxwell equal-area criterion, which was calculated from the equal values of the two minima of the plot of the extended Gibbs free energy versus shear rate. At higher shear rates, the model predicts a transition to shear thinning, and under transient flows, an induction time and a saturation time are obtained from the predicted and the experimental data as detailed in the text. The magnitudes of both, the induction and the saturation times, diminish as the shear rate departs from \( {\dot{\upgamma}}_{\mathrm{c}} \), but only the induction time decreases according to a power law with shear rate. The conditions under which these rheological responses arise are derived and justified in detail with the BMP model. The model predictions are compared with experimental data of two dilute micellar solutions. The model parameters were determined from a set of independent experiments without fitting.

Transient behavior of stress in a wormlike micellar solution under oscillatory shear

Strain hardening and shear-induced structure (SIS) formation of a wormlike micellar solution under oscillatory shear is studied by separately controlling strain and shear rate amplitudes. Only when the oscillatory shear beyond the critical strain and shear rate amplitudes are applied is significant strain hardening behavior, which is attributed to the chain extension, observed. We find that the strength of the strain hardening, i.e., the chain extension, depends on the magnitude of the strain and shear rate amplitudes. As the oscillatory shear is repeatedly applied, the strength of the strain hardening is gradually reduced and approaches a periodic state. In the periodic state, stress response shows a triangular wave form but not an ideal sinusoidal response. Since these characteristic behaviors are observed above the critical conditions needed for the SIS formation, the periodic state will be attributed to the SIS formation. The level of the SIS in the periodic state can also be arbitrary controlled by a combination of these amplitudes. A close relation between the strain hardening and the SIS formation suggests that the degree of extension of the wormlike micellar chain affects the level of the SIS.

Rheological properties of hydroxypropylmethyl cellulose/sodium dodecylsulfate mixtures

Rheological properties of mixtures of hydroxypropylmethyl cellulose (HPMC), a nonionic associative cellulose ether, and sodium dodecylsulfate (SDS), an anionic surfactant, were investigated by viscosity measurements performed at different shear rates (0.1-6000 s-1). HPMC/SDS mixtures containing different concentrations of SDS (CSDS=0.00-3.50 % w/w) and HPMC concentrations which corresponded to the overlap parameter c/c*=3, 6, and 12 were prepared. All HPMC/SDS mixtures were found to be shear-thinning when examined in a low-end-to mid-range of the applied shear rates. The degree of shear-thinning, n, and viscosity of the mixtures were influenced by composition of HPMC/SDS mixtures and HPMC-SDS complex formation. The changes in n ranged from values typical for highly shear thinning to almost perfectly Newtonian liquids, and were more pronounced as c/c* was increased from 3 to 6 and 12. A change in flow profile and a buildup of the first normal stress difference (N1) was observed in HPMC/SDS mixtures with c/c*=6 and 12 and CSDS 0.55-1.00 % and 0.55-2.50 %, respectively, when a critical shear rate, crit. was exceeded, suggesting that a shear-induced structure formation in the mixtures took place.

Shear-induced Crystal Structure Formation in Milk Fat and Blends with Rapeseed Oil

The effect of shear on the rheological properties of anhydrous milk fat (AMF) and blends added 10–30 w/w% rapeseed oil (RO) was studied. Shear was applied during early crystallization, and the complex modulus |G*| was measured during the final isothermal crystallization. The firmest and most rapidly formed network was achieved when an intermediate shear of 50 s−1 was applied, and at this shear rate, up to 20 % RO could be added without lowering the final |G*| compared to pure AMF. A similar effect was not obtained at lower or higher shear rates. Solid fat content was unaffected by shear, while fat crystal size decreased upon increasing shear rate. The study indicates that intermediate shear produces a continuous and strong crystal network, while high shear breaks down the microstructure to such an extent that it is difficult to rebuild during subsequent crystallization, thus resulting in lower |G*|.

Alignment of particles in sheared viscoelastic fluids

We investigate the shear-induced structure formation of colloidal particles dissolved in non-Newtonian fluids by means of computer simulations. The two investigated visco-elastic fluids are a semi-dilute polymer solution and a worm-like micellar solution. Both shear-thinning fluids contain long flexible chains whose entanglements appear and disappear continually as a result of Brownian motion and the applied shear flow. To reach sufficiently large time and length scales in three-dimensional simulations with up to 96 spherical colloids, we employ the responsive particle dynamics simulation method of modeling each chain as a single soft Brownian particle with slowly evolving inter-particle degrees of freedom accounting for the entanglements. Parameters in the model are chosen such that the simulated rheological properties of the fluids, i.e., the storage and loss moduli and the shear viscosities, are in reasonable agreement with experimental values. Spherical colloids dispersed in both quiescent fluids mix homogeneously. Under shear flow, however, the colloids in the micellar solution align to form strings in the flow direction, whereas the colloids in the polymer solution remain randomly distributed. These observations agree with recent experimental studies of colloids in the bulk of these two liquids.

Self-similar shear thickening behavior in CTAB/NaSal surfactant solutions

The effect of salt concentration Cs on the critical shear rate γ̇c required for the onset of shear thickening and apparent relaxation time λ of the shear-thickened phase has been investigated systematically for dilute Cetyl-trimethylammonium bromide/sodium salicylate solutions. Experimental data suggest a self-similar behavior of γ̇c and λ as functions of Cs. Specifically, γ̇c∼Cs−6 whereas λ∼Cs6 such that an effective Weissenberg number We≡λγ̇ for the shear-thickened phase is only weakly dependent on Cs. A procedure has been developed to collapse the apparent shear viscosity versus shear rate data obtained for various values of Cs into a single master curve. The effect of Cs on the elastic modulus and mesh size of the shear-induced gel phase for different surfactant concentrations is discussed. Experiments performed using different flow cells (Couette and cone-and-plate) show that γ̇c, λ and the maximum viscosity attained are geometry independent. The elastic modulus of the gel phase inferred indirectly by employing simplified hydrodynamic instability analysis of a sheared gel-fluid interface is in qualitative agreement with that predicted for an entangled phase of living polymers. A qualitative mechanism that combines the effect of Cs on average micelle length and Debye parameter with shear-induced configurational changes of rod-like micelles is proposed to rationalize the self-similarity of shear induced structure formation.

Importance of Intrinsic Properties of Dense Caseinate Dispersions for Structure Formation

Rheological measurements of dense calcium caseinate and sodium caseinate dispersions (> or =15%) provided insight into the factors determining shear-induced structure formation in caseinates. Calcium caseinate at a sufficiently high concentration (30%) was shown to form highly anisotropic structures during shearing and concurrent enzymatic cross-linking. In contrast, sodium caseinate formed isotropic structures using similar processing conditions. The main difference between the two types of caseinates is the counterion present, and as a consequence, the size of structural elements and their interactions. The rheological behavior of calcium caseinate and sodium caseinate reflected these differences, yielding non-monotonic and shear thinning flow behavior for calcium caseinate whereas sodium caseinate behaved only slightly shear thinning. It appears that the intrinsic properties of the dense caseinate dispersions, which are reflected in their rheological behavior, affect the structure formation that was found after applying shear. Therefore, rheological measurements are useful to obtain an indication of the structure formation potential of caseinate dispersions.

Saturation of Shear-Induced Isothermal Crystallization of Polymers at the Steady State and the Entanglement−Disentanglement Transition

To provide insight into the formation of shear-induced precursor structures, three apparently unrelated subjects are analyzed and discussed: the saturation of crystallization from sheared polymer melts, evaluated with a new shear DTA instrument, the steady state in steady shear, and the entanglement-disentanglement transition. It is shown that the same large strains that saturate crystallization also lead to a reversible steady state in steady shear, where the viscosity of the sheared melt is constant in time, and their magnitude is only determined by the temperature of the sheared melt. Features of the melt morphology at this state are discussed, and their importance is highlighted to understand the possible mechanisms behind the formation of shear-induced precursors. Measurements of the reptation time for unsheared samples, and samples sheared up to the steady state, allowed the quantification of the entanglements loss during the transition between these two states (around 2/3), which is interpreted as an entanglement-disentanglement transition. The relevance of this result on assumptions of flow models, particularly the constant number of topological constraints, convective constraint release process, and chain stretch, is discussed.

Rheological characteristics of trimethylolethane hydrate slurry treated with drag-reducing surfactants

Rheological characteristics of trimethylolethane (TME) clathrate–hydrate slurry treated with drag-reducing surfactants were investigated. Friction coefficients and apparent viscosities were measured when the concentration of TME and its hydrate fraction treated with and without drag-reducing surfactants were changed in several steps. From the results, it is found that the surfactant addition causes effective drag reduction in a pipe flow when the hydrate fraction becomes high, while effective drag reduction disappears in the cases of low hydrate fraction. The results of viscosity measurements indicate that the TME molecules disturb the formation of shear-induced structures (SIS) causing drag reduction phenomena. To investigate this interaction between TME and surfactant micelles, the effect of TME concentration on viscosity and relaxation time of solutions was discussed. From this, it was found out that there exists a critical concentration of TME on the formation of SIS and that it becomes larger as shear rate increases. Thus, we conclude that this interaction between TME and micellar structures causes less drag reduction for the cases of low hydrate fraction, while the drag reduction appears in cases of high hydrate fraction because TME concentration in liquid phase becomes small.

Effects of shear flow on a semidilute polymer solution under phase-separating condition

We studied the effects of a step-up shear flow from zero shear rate to the given shear rate, γ ˙ , on formation of shear-induced structures for a semidilute polystyrene (PS)/diethyl malonate (DEM) solution below its cloud point temperature where the solution undergoes phase separation via spinodal decomposition (SD) in quiescent state. We elucidated that the effects of step-up shear can be divided into two regions: below and above a critical shear rate, γ ˙ c,SD . At γ ˙ < γ ˙ c,SD , growing phase-separated domains via SD are found to be deformed under the flow, so that FFT spectra of the shear-microscopy images become elliptical with the wave number q m x at the maximum intensity parallel to the flow being smaller than the corresponding wave number q m z parallel to the neutral axis. However, strikingly enough, the aspect ratio q m z / q m x of the elliptic spinodal ring observed for this system was much smaller than that observed for binary fluids. The unique feature was proposed to be the elastic effect inherent in this system. When γ ˙ is larger than γ ˙ c,SD , however, initially phase-separating structures via SD are strongly deformed and distorted. Interestingly enough, the light scattering pattern was transformed from the isotropic ring pattern into the butterfly pattern. This is interpreted as follows: when γ ˙ > γ ˙ c,SD , there may not be enough time for the domains composed of elastically deformed swollen-network chains to relax, and consequently the domains are cooperatively disrupted. The disrupted domains tend to squeeze solvent in order to release the elastic free energy stored in the deformed swollen-network chains, resulting in anisotropic domain more extended to neutral axis than flow direction and hence giving rise to the butterfly pattern.

Dynamics of shear-induced structure formation in high molecular weight aqueous solutions

Shear-induced structure formation in solutions of drag-reducing polymers has been studied using several rheo-optical methods including scattering dichroism, turbidity, and small-angle light scattering. Aqueous solutions of high molecular weight partially hydrolyzed polyacrylamide, polyacrylamide, and poly(ethylene oxide) have been examined in the concentration range of 100 to 4000 parts per million by weight. Under high shear flow, increasing turbidity is accompanied by growing lozenge- or butterfly-shaped scattering patterns. With increasing shear, dichroism increases and the orientation angle approaches zero, i.e., scattering centers are aligning with the flow. Fits of the turbidity and structure factor independently demonstrate the presence of micron-sized structures forming under shear flow.

Turbulent drag reduction of polyacrylamide solutions: effect of degradation on molecular weight distribution

The relationship of turbulent drag reduction to molecular weight and molecular weight distribution has been investigated for aqueous polyacrylamide (PAM) solutions of concentration 200, 500 and 1000 ppm. Drag reduction experiments were carried out by circulating solutions in a rectangular flow channel having a flat wall and a wavy wall. In situ measurements of the velocity field were made using particle image velocimetry (PIV). Rheological and rheo-optical measurements carried out over a range of shear rates were used to determine the extent of shear-induced structure formation in the solutions. Molecular weight and molecular weight distribution were analyzed using gel permeation chromatography (GPC). Results showed that diminution of the drag reduction does not always correlate with loss in molecular weight. Long intervals were observed in which the loss of drag reduction occurred at constant molecular weight distribution. In combination with earlier results reported for HPAM solutions, these results support the notion that molecular aggregation could be an important factor in understanding and applying polymer drag reduction.

Development characteristics of drag-reducing surfactant solution flow in a duct

Development characteristics of dilute cationic surfactant solution flow have been studied through the measurements of the time characteristics of surfactant solution by birefringence experiments and of the streamwise mean velocity profiles of surfactant solution duct flow by a laser Doppler velocimetry system. For both experiments, the concentration of cationic surfactant (oleylbishydroxymethylethylammonium chloride: Ethoquad O/12) was kept constant at 1000 ppm and the molar ratio of the counter ion of sodium salicylate to the surfactants was at 1.5. From the birefringence experiments, dilute surfactant solution shows very long retardation time corresponding to micellar shear induced structure formation. This causes very slow flow development of surfactant solution in a duct. Even at the end of the test section with the distance of 112 times of hydraulic diameter form the inlet, the flow is not fully developed but still has the developing boundary layer characteristics on the duct wall. From the time characteristics and the boundary layer development, it is concluded that the entry length of 1000 to 2000 times hydraulic diameter is required for fully developed surfactant solution flow.

Shear-induced structure formation in solutions of drag reducing polymers

The phenomenon of shear-induced structure formation has been investigated for several drag reducing polymers. Aqueous solutions of high molecular weight partially hydrolyzed polyacrylamide, polyacrylamide, and poly(ethylene oxide) have been examined in the concentration range of 100–4000 ppm by weight. Experimental protocols included rheological and rheo-optical characterization under simple shear flow, and molecular weight analysis by chromatography following various shear histories. All solutions exhibited time-dependent stresses and turbidity patterns under high shear, indicative of structure formation. Two regimes of behavior were found with the partially hydrolyzed polyacrylamide. At concentrations below 1500 ppm, aggregate formation occurred above a critical shear rate on time scales of minutes that relaxed back over similar time scales to an apparent homogenous state on cessation of flow. However, these sheared solutions developed irreversible, large-scale turbidity and eventual precipitation on quiescent storage over several days. At higher concentrations, phase-separated droplets formed under shear that also showed ripening of the turbidity and eventual precipitation upon quiescent storage for days. The kinetics of the semi-reversible aggregate formation and relaxation processes in the lower concentration regime showed a regular variation with shear rate and concentration. The strength of the structure formation and precipitation processes was lessened in the presence of salts. Similar behavior patterns were exhibited by the other systems, however, the strength of the structure formation was also somewhat lessened compared to the partially hydrolyzed polyacrylamide. Correlations with solutions being used in ongoing turbulent drag reduction studies suggest that such structure formation may play an important role in the drag reduction process.

Influence of nucleants on the formation of shear-induced structures in polypropylene

The interaction between the type of nucleating agent and base polymer was investigated for a combination of three commercial nucleants and polypropylenes each. Both the crystallisation behavior under DSC conditions and the mechanical and optical properties were determined for all materials. For both the influences of the nucleant and of the base polymer, the present study has provided qualitative indications as to the triggered crystal morphology and shear-induced superstructure. Especially the latter can be related to the differences between optical and mechanical effects of the nucleation process.