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Casein finds application as an eco-friendly adhesive for paper, wood, glass, etc. Casein being a protein can undergo conformational and microstructural changes during various processing steps involved in interfacial bonding. This study aims at understanding the multiscale contributions of these changes in casein to its adhesion to cellulose pressboards. Investigations spanning from molecular structure to macroscopic adhesion characteristics have been used in this work. The lap shear strength of casein bonded cellulose pressboards is found to increase with the increase in casein concentration. It was observed from Fourier transform infrared spectroscopy (FTIR) investigations along with microscopy and rheological studies that casein dispersions result in more α-helical conformations during the preconcentration process of casein dispersions. This results in increased hydrophobicity of the casein particles/aggregates, which in turn affects the wetting characteristics and the adhesion behavior. Casein compositions lacking α-helices were found to enhance the bonding strength of casein with cellulose. The present study shows that the adhesion between casein and microporous cellulose substrate has contributions at the multiscale originating from the polar-polar interactions of casein and cellulose molecules, conformational changes in the protein structure of casein during drying, microstructure of casein particles in the dispersion, and the microporous nature of the cellulose boards. These interactions at multiple scales can be tuned to suit different adhesive applications using casein.

Plant cell walls undergo multiple cycles of dehydration and rehydration during their life. Calcium crosslinked low methoxy pectin is a major constituent of plant cell walls. Understanding the dehydration-rehydration behavior of pectin gels may shed light on the water transport and mechanics of plant cells. In this work, we report the contributions of microstructure to the mechanics of pectin-Ca gels subjected to different extents of dehydration and subsequent rehydration. This is investigated using a pectin gel composition that forms ‘egg-box bundles’, a characteristic feature of the microstructure of low methoxy pectin-Ca gels. Large Amplitude Oscillatory Shear (LAOS) rheology along with Small Angle Neutron Scattering and Near Infrared (NIR) spectroscopy on pectin gels are used to elucidate the mechanical and microstructural changes during dehydration-rehydration cycles. As the extent of dehydration increase, the reswelling ability, strain-stiffening behavior and the yield strain decreases. These effects are more prominent at faster rates of dehydration and are not completely reversible upon rehydration to the initial undried state. Microstructural changes due to the aggregation of egg-box bundles and single chains and the associated changes in the water configurations lead to these irreversible changes.

In several applications, a protein such as casein in dispersion form undergoes multiple processing steps including drying. In this work, the rheological and microstructural features of casein dispersions concentrated by evaporation of the solvent (drying dispersions) were studied in comparison with those of equal concentrations of the as-prepared dispersions without drying. The molecular assembly of casein is affected by drying along with the conformational composition changes in the secondary structures such as α-helix, β-sheets, turns and random structures of the protein. Modeling of the rheological data indicates that these changes also affect the packing of casein molecular assemblies and these molecular assemblies in alkaline dispersions can behave as soft deformable particles. During drying, casein dispersions show prominent shear thinning for concentrations higher than 20 wt% along with the prevalence of α-helices and β-sheets. In comparison, the as-prepared dispersions show different microstructural features, and therefore different rheological responses. A detailed analysis shows that alkalinity changes during drying is the crucial factor controlling the microstructural changes of the soft casein particles and hence the rheology.

Pluronic F-127 (Pl), a triblock copolymer that exhibits thermoreversible sol-gel transition, is being used with additives such as polysaccharides for enhanced gel stability and mechanical strength for controlled drug delivery applications. This study aims at elucidating the gelation behavior and structure-property relationship (at physiological temperature) of pluronic in the presence of hyaluronic acid (HA) using linear and nonlinear rheology. Small amplitude oscillatory shear of the gels indicated weak frequency dependence of G ′ and G ′ ′. Large amplitude oscillatory shear (LAOS) response indicated flow induced yielding with both the moduli following power law decay beyond the critical strain amplitude. Stress waveform analysis, Fourier analysis, and Chebyshev decomposition indicated similar qualitative features for gels with different HA content. Interestingly, there is a non-monotonic variation in the dynamic light scattering timescales, G ′ (linear regime), and the critical stress values (at corresponding critical strains) with HA content. The physical insights obtained were understood in terms of a cage picture of soft glassy materials. Relaxation times corresponding to yielding transition obtained from strain rate frequency superposition measurements showed similar qualitative variation as those obtained from elastoviscoplastic modeling of the yielding processes occurring during an LAOS cycle. Based on these analyses, we obtain quantitative description of nonlinear rheological response of the Pl-HA gels.Pluronic F-127 (Pl), a triblock copolymer that exhibits thermoreversible sol-gel transition, is being used with additives such as polysaccharides for enhanced gel stability and mechanical strength for controlled drug delivery applications. This study aims at elucidating the gelation behavior and structure-property relationship (at physiological temperature) of pluronic in the presence of hyaluronic acid (HA) using linear and nonlinear rheology. Small amplitude oscillatory shear of the gels indicated weak frequency dependence of G ′ and G ′ ′. Large amplitude oscillatory shear (LAOS) response indicated flow induced yielding with both the moduli following power law decay beyond the critical strain amplitude. Stress waveform analysis, Fourier analysis, and Chebyshev decomposition indicated similar qualitative features for gels with different HA content. Interestingly, there is a non-monotonic variation in the dynamic light scattering timescales, G ′ (linear regime), and the critical stress values...

Crosslinked polymers are three-dimensional network structures formed by interactions that range from weak physical associations to strong covalent bonding. The inherent complexity in their structures results in rich and diverse rheological response at large deformations. In the present work, we investigate two crosslinked materials based on poly(vinyl alcohol) (PVA) using large amplitude oscillatory shear (LAOS). Physically crosslinked PVA-borax exhibited Giesekus-like linear response, and a gradual intra-cycle strain stiffening and shear thinning at large strains. These signatures are attributed to temporary junction points and stretching of chains between crosslinks in the system. In contrast, chemically crosslinked PVA-hyaluronic acid (PVA-HA) gels exhibited a weak frequency dependence of linear moduli and intra-cycle strain stiffening and shear thickening at large strains. PVA-HA contains permanent crosslinks that move apart, along with stretching of crosslinked and uncrosslinked chains, and these features are responsible for such LAOS signatures. Based on these insights, we propose modifications to existing models and obtain quantitative description of LAOS response of both PVA-borax and PVA-HA crosslinked systems.

The rheological behavior such as yielding of fat crystal networks are dictated by many variables. Among these variables, the shape of the constituent fat cluster is important yet relatively unexplored. In this work, we describe the rheological investigations of a fat-oil system which can be formulated to either contain bundles of needles or spherical clusters by controlling the cooling rate and fat concentration. Fat-oil mixtures containing high-fat concentrations exhibited weak frequency dependence of storage modulus (G ′) and loss modulus (G ″). The yielding behavior of the mixtures were investigated by large amplitude oscillatory shear (LAOS) rheology using strain and stress controlled modes. Lissajous-Bowditch plots and orthogonal set of Chebyshev polynomials were used to analyze the non-linearities associated with the yielded fat-oil mixtures. For a given fat concentration, the yield stress of fat networks obtained at low cooling rates (bundles of needles) were similar to that of networks obtained at high cooling rates (spherical clusters). However, after yielding, Lissajous-Bowditch plots suggested that the mixtures comprising of bundles of needles exhibited viscous-like behavior while the spherical clusters exhibited a plastic-like behavior. This was further supported by microscopy images of yielded fat-oil mixtures. Overall, for a given fat concentration, the two different shapes of fat clusters can give rise to networks of similar yield stress values but different behaviors after yielding.

The mechanical properties of fat–oil mixtures are dictated by the morphology of the fat crystal network, which in turn is strongly affected by the processing conditions. In this work the effect of uniform (linear) cooling rates on the size and shape of tribehenin (fat) clusters in isopropyl myristate (oil) was studied. The state of the fat–oil mixtures was comprehensively analyzed by establishing pseudo-phase diagrams via visual observations, brightfield and polarization microscopy at various cooling rates and fat concentrations. The microstructure of the constituent fat clusters undergo a shape transition from bundles of needles at low cooling rates to spherical clusters at high cooling rates. The network structures formed with these different shaped fat clusters were examined using rheology. At a given cooling rate, the storage modulus (Gʹ) of fat–oil mixtures versus fat concentration followed a power law relationship. For a given fat concentration, the fat crystal networks comprising of bundles of needles showed higher Gʹ values as compared to fat networks made of spherical clusters. While the fractal dimension of networks cooled at different cooling rates varied over a small range of 2.78–2.86, considerable change in the pre-exponential factor (γ) was observed. The results show the critical importance of processing conditions on the shape and size of fat clusters and its impact on the rheological properties of the fat crystal network.

Water added to a solution of microcrystalline cellulose (MCC) in 1-allyl-3-methylimidazolium chloride (AmimCl) reduces the solvent quality and causes significant changes in the flow properties and microstructure due to restructuring and aggregation of cellulose molecules. We report an experimental investigation by means of polarization optical microscopy (POM) and rheology of the distinct phases formed in 5-20 wt% MCC/AmimCl solutions due to the addition of water. With increase in the cellulose concentration, the MCC/AmimCl/water mixtures showed different morphologies such as the non-aligned cholesteric liquid crystalline (LC) domain, the coexistence of spherulite-like structures within the LC domain and a space-spanning network of spherulite-like structures at high concentrations of water. In situ microscopy during shear and POM observations pre and post shear revealed a significant increase in the size of the birefringent domains as the shear rate is increased, which continued to exist even after the cessation of shear. With an increase in the concentration of water, the zero shear viscosity of the MCC/AmimCl/water mixtures was found to go through a minimum, beyond which the aggregation of cellulose commenced. The corresponding oscillatory shear response showed a sol-gel transition with an increase in water concentration. Moreover, at high cellulose concentrations (12-20 wt%), the MCC/AmimCl/water gels exhibited self-similarity and followed the Chambon-Winter (CW) criterion. The similar phase behavior and rheological response observed for MCC dissolved in 1-butyl-3 methylimidazolium chloride (BmimCl) indicated the generality of the presented results.

Squeeze flow of fluids between surfaces is important in rheology, material processing, lubrication and biomedical applications. The surfaces for squeezing can be solid and/or porous. In this work, the squeeze flow between a porous surface and a solid surface is examined with Newtonian model fluids and model fabrics used in composite processing. Given that the fluid is expected to impregnate the fabric, the permeability and the wettability of the fluid-fabric combination are of utmost importance. Constant velocity squeeze flow experiments were carried out with polyester and polyol resins, while glass fabrics with different sizings were used as the porous surfaces. To understand the variation of the normal force during squeeze flow, its scaling with squeeze gap is evaluated. The scaling is observed to vary from -3 to -1 for different squeeze flow experiments. Squeeze flow theories are used to examine the normal force variation for different fluid-fabric combinations. It is shown that a slip based squeeze flow model and a permeability based squeeze flow model can be used to understand the wettability and the permeability of a fabric with a specific fluid. � 2014 Elsevier Ltd.

Thixotropic materials belong to the important class of time dependent, evolving, and aging systems. Their characteristics are currently understood through rheological measurements of steady and step shear wherein the steady states reached at large deformations and the transition from one steady state to another are monitored. Small deformation protocols are not well explored for such systems. This is contrary to the common approach of probing equilibrium state and small perturbations from the equilibrium state initially by focusing on linear viscoelasticity. This work addresses the question of how to understand thixotropic materials by focusing on both the small and large deformations. A model suspension of fumed silica in polyisobutylene/paraffin oil is characterized using steady shear, transient shear, and oscillatory shear rheology. The physical processes occurring due to structural changes during each of the tests are interpreted from the corresponding response. Several structural kinetics and elastoplastic/viscoelastic phenomenological models are evaluated, and a novel combination of models is proposed to obtain the description of the responses to various tests. The advantage of this strategy is highlighted in terms of possible difference in the mechanisms at small and large deformations. Kramers–Kronig relations are used to examine the experimental and simulated dataset obtained from oscillatory shear to identify possible violation of linearity or time independence. We demonstrate that the analysis of the oscillatory response using Kramers–Kronig relations leads to important insights about these systems.

Large amplitude oscillatory shear [LAOS] helps to investigate non-linear rheology and the dynamic behaviour of materials when subjected to large deformations. Qualitative characterization of non-linear rheology can be easily done but quantification and physical interpretation of non-linear rheological characteristics is one of the most challenging aspects. In this work, LAOS tests are done on different materials generally known as 'living polymers', with similar linear response, in order to study their non-linear response using both quantitative and qualitative analysis. The effect of living polymer networks on LAOS was examined using different concentrations of 2,4 bis(2-ethyl hexyl ureido)toluene [EHUT] gels. Cetyl trimethyl ammonium bromide [CTAB] wormlike micellar solution is prepared to compare and contrast with EHUT gels since both these materials show similar linear viscoelastic response. Elastic loading/unloading and plastic deformations are identified as important processes that lead to the distinguishing behaviour for different materials. An equivalence is observed for Lissajous plots in the concentration frequency plane for EHUT. Giesekus model and Upper Convected Maxwell model coupled with transient network model [TNM], two well established models, are solved numerically for comparison with the experimental results. A modification of the Giesekus model to incorporate the network breakage/reformation as a linear function of strain amplitude, and nonlinear function of strain rate within an oscillation period is suggested. The proposed model is able to capture the competition between reptation, network breakage and reforming during LAOS. Moreover, the modified model suggests that the secondary loops during LAOS for EHUT gel occur soon after yielding due to competition between the network breaking and reforming at high shear rates and reptation at low shear rates within a cycle. � 2014 Elsevier B.V.

Hyaluronic acid (HA)-based materials are being investigated because of their role in biological fluids and tissues. Poly(vinyl alcohol) (PVA) when blended with HA at different compositions leads to superior mechanical properties compared to pure HA. The PVAHA blend hydrogels are potential candidates for pharmaceutical, biomedical, and cosmetic applications. It is essential to understand the structure, gelation time, and morphological properties of these hydrogels. In this work, a blend system of PVA crosslinked with glutaraldehyde in the presence of HA is studied. Semidilute solutions of PVA and HA are blended, followed by gelation due to crosslinking. The crosslinked gels as well as the gel cast membranes were examined. The effect of HA on the gelation process is investigated using rheological characterization. It is shown that kinetics of gelation is influenced by HA content, though storage modulus of the gels is influenced marginally. The structural features of PVAHA gels were also probed with scanning electron microscopy and dynamic light scattering. It is argued that there is a complex interplay between intra- and intermolecular crosslinking of PVA and PVA–HA interactions during the gel formation. Based on the insights obtained from various probing techniques for PVAHA gels with different HA content, three broad structural features were identified. It is shown that the hydrogel is semi-interpenetrating network at lower HA content ( 20% HA). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41081.

There is an increasing demand for designing controlled drug delivery systems with materials which are more biocompatible, economical and materials which can be processed easily. Poly (vinyl alcohol) (PVA) and hyaluronic acid (HA) are promising polymers for applications in drug delivery. PVA forms gel based on the acetal bridges when crosslinked with glutaraldehyde (GA). On the other hand, HA a natural polymer, forms gel with divinyl sulfone (DVS) as a crosslinker. PVA and HA blends upon crosslinking PVA with GA or HA with DVS, in the presence of the other polymer, form gels that are more adaptable to the drug delivery systems. In this work, the mechanical properties and swelling behaviour of PVAHA gels were characterized. The effect of composition on viscoelastic moduli and degree of swelling was determined. The storage modulus (G") of various gels made of PVA, HA and PVAHA blends were measured using rheology and compared with the values available in the literature. Swelling properties were measured and compared among various PVA and HA gels. Collagen is added to PVA solution and the rheological properties were measured in the gel state. Based on the values of storage modulus, gels of various compositions of PVA, HA and collagen might be selected as potential biomaterials for drug delivery system depending on careful understanding the type of application. � 2014, DESIDOC.

Standard laboratory ageing methods of bitumen only take into account the effect of thermo-oxidation during the service life of a pavement but the effect of high energy cosmic radiation on site is not simulated in these procedures. The aim of the present work is to compare the laboratory simulated short term bitumen ageing (rolling thin film oven test) with ageing produced by short exposures of bitumen samples to Ultra Violet and gamma radiation. The influence of ageing agents on the thermal properties and rheological performance of the pristine and modified bitumen binders has been evaluated in this study. The thermal behavior of various aged bitumens is characterized by both isothermal as well as non-isothermal thermogravimetric analysis. The thermoanalytic investigations on bituminous samples are carried out to evaluate the thermal stabilities and activation energies of the binders and the life time prediction of the materials is made with the help of the kinetic information. It is found that modified bituminous binders are more resistant to heat and radiation. Different rheological tests are conducted by dynamic shear rheometer to examine the effect of ageing in terms of bitumen oxidation and polymer phase degradation which has a major consequence on high temperature rutting or low temperature cracking. Type of modifier is found to be of decisive importance. Creep and recovery tests show that the structure-time dependency of pristine aged bitumen is influenced much by stress and temperature than in the case of modified aged bitumens. The study has revealed that the elastomeric modifier protects the bituminous binder more than plastic modifier or nano filler. Finally, a fair correlation has been made between standard RTFO ageing and radiation aging. � 2012 RILEM.

Background.- Non-Newtonian Fluids: An Introduction.- Fundamentals of Rheology.- Mechanics of Liquid Mixtures.- Rheology.- Oscillatory Shear Rheology for Probing Nonlinear Viscoelasticity of Complex Fluids: Large Amplitude Oscillatory Shear.- PIV Techniques in Experimental Measurement of Two Phase (Gas-Liquid) Systems.- An Introduction to Hydrodynamic Stability.- Applications.- Statics and Dynamics of Dilute Polymer Solutions.- Polymer Rheology.- Active Matter.- Mathematical Modelling of Granular Materials.

Gelation of crosslinking polymers, with ionic groups, is of interest in membrane processing of these materials. The gelation of polyvinyl alcohol in the presence of an ionic crosslinker, sulfosuccinic acid, was monitored through rheological measurements. The evolution of rheological material functions during film formation, which involves solvent evaporation and crosslinking, was observed at different polymer and crosslinker concentrations. Relative effect of water evaporation leading to physical gelation and the chemical crosslinking was examined. To understand the effect of crosslinker type, the rheology of nonionic crosslinker, glutaraldehyde was also examined. The gel points as well as network parameters for these covalent networking systems were different and depended on crosslinker type and polymer/crosslinker concentrations. However, qualitative evolution of rheological behavior during film formation was largely similar, demonstrating the dominating effect of solvent evaporation. Crosslinking in the absence of solvent evaporation was examined at different temperatures. It can be concluded that similar mechanism was involved, independent of temperature, in the temperature range of interest. The effect of crosslinker concentration on evolution of rheological properties near gelation was insignificant for ionic crosslinker though marginally significant for nonionic crosslinker. � 2010 Springer-Verlag.

Linear and non-linear rheological responses of complex fluids are of great interest. Oscillatory techniques are commonly used to analyze the complex fluid rheological behavior. In this chapter, the approach based on large amplitude oscillatory shear (LAOS) is reviewed. Initially, oscillatory shear based on small strain amplitudes is presented along with a brief discussion on relaxation time spectrum. Subsequently, key observable features of LAOS are shown with experimental observations on selected materials. Various applications for which LAOS is being investigated have been described through these examples. Complex fluids such as polymer solutions and melts, emulsions, blends, biological gels, micellar solutions, etc. are being investigated for their non-linear response at large amplitudes during LAOS. In addition to the general response during LAOS, specific material functions being proposed in the literature are discussed. Finally, an example of bulk oscillatory flow is discussed in the context of LAOS behavior of complex fluids.

The rheology of poly(sodium acrylate) hydrogel cross-linked with N, N? -methylene (bisacrylamide), during gelation is studied. The validity of the Winter-Chambon criterion is examined for gelation. Cellulose microfibrils (CMF) of an aspect ratio ranging from 20 to 100 are used as fillers. The effect of filler CMF (0-1 wt %) on the rheological properties during gelation is also investigated. The WC criterion is satisfied and the gel time increased with an increase in the filler content. This suggests the presence of a self-similar structure at the gel point. The storage (G?) and the loss (G?) moduli crossover is observed in the case of unfilled system. Whereas no crossover is observed when the filler is incorporated and from the start of the reaction, G? is always greater than G?. With the incorporation of the filler, there is a plateau initially in G? and G?. This is attributed to the physical network of fibrils in the sample mixture. Scaling behavior of rheological properties and fractal dimensions near the gel point are also examined. � 2009 The Society of Rheology.

Rheological behavior of asphalt is strongly affected by loading conditions, temperature and environment. One of the main challenges in understanding the rheology of asphalt is to relate the chemical constituents and the micro-structure of asphalt on one hand to its rheological behavior on the other hand. In this work, nonlinear rheological behaviour of asphalt was investigated using a structural rheological model. A first order kinetic equation to describe structural changes in asphalt has been incorporated with the nonlinear rheological model of White- Metzner. The resulting set of governing equations was solved numerically to describe the rheology of asphalts. Different modes of rheological testing and asphalts with different compositions were considered. An analysis and comparison of model behaviour with experimental data from the literature is carried out in both stress growth at constant shear rate and oscillatory shear modes. A strategy is proposed for the estimation and tuning of the model parameters based on available experimental data and literature. Qualitatively, the model can capture the rheological behaviour of non-Newtonian fluids such as asphalt under different modes of rheological testing. Quantitative analysis from this work shows that the model describes the rheological behaviour of asphalt for the temperature range of 20 – 60ºC. It is demonstrated that a single set of equations tuned with the steady shear experimental data can be used to predict the nonlinear rheological behaviour of asphalts. In addition, it is shown that the model parameters can be related to the chemical composition of asphalts.