Bulk Rheological Properties Research Articles

Bulk and interfacial viscoelasticity in concentrated emulsions: The role of the surfactant

The aim of this study was to investigate and compare bulk and interfacial viscoelasticity of emulsions stabilized with two surfactants of a markedly different nature: the ionic phospholipids mixture Epikuron 145V and the steric emulsifier Pluronic F68. Emulsions of olive oil in water were prepared by high-pressure homogenization using only one of these surfactants. The impact of the surfactant used on the final linear viscoelastic properties was investigated by means of small-amplitude dynamic oscillatory shear tests, obtaining the values of the storage and loss moduli as a function of frequency, as well as by retardation and relaxation essays. Our results show that for concentrated emulsions, the viscoelastic properties are considerably different depending on which surfactant is used, i.e., the employed surfactant influences to a major extent the bulk rheological properties. In order to explain these differences, an interfacial study of the adsorbed surfactant layers has been carried out, measuring interfacial tension and dilatational viscoelasticity by means of the pendant drop technique. A correlation between the results obtained in bulk and interface has been found.

The influence of side-chain halogenation on the self-assembly and hydrogelation of Fmoc-phenylalanine derivatives

Fmoc-protected aromatic amino acids, including Fmoc-phenylalanine (Fmoc-Phe), Fmoc-tyrosine (Fmoc-Tyr), and Fmoc-pentafluorophenylalanine (Fmoc-F5-Phe), have been shown to undergo efficient self-assembly and to promote hydrogelation in aqueous solvents. In order to probe the electronic and steric role of the benzyl side-chain in hydrophobic and π–π interactions during self-assembly, the hydrogelation behavior of monohalogenated (F, Cl, Br) Fmoc-Phe side-chain derivatives was assessed. Incorporation of single halogen substituents on the aromatic side-chain of Fmoc-Phe dramatically enhances the efficient self-assembly of these amino acid derivatives (relative to Fmoc-Phe) into amyloid-like fibrils that promote hydrogelation in aqueous solvents. The position of halogen substitution (ortho, meta, para) and the halogen itself (F, Cl, Br) exert a strong influence on the self-assembly rate and on the bulk rheological properties of the resultant hydrogel. These results demonstrate that minimal atomic substitutions can be used to tune self-assembly and gelation of small molecule hydrogelators.

The influence of flow confinement on the rheological properties of complex fluids

The rheological properties of plastic and viscoplastic complex fluids have been investigated using constant velocity squeeze flow rheometry at small gaps in order to examine the influence of the resulting flow confinement. The fluids investigated were aqueous carbopol suspensions and three commercial products (Tesco English mustard, Tesco value lemon curd and Heinz tomato ketchup (HTK)). The bulk rheological properties were measured using parallel plate rheometry. Rough plates were used to eliminate wall slip in both rheometric configurations. The commercial products are Herschel–Bulkley fluids and, for gaps less than a critical value of ~200 μm, the yield stresses tended to large values. There was a corresponding trend to small values for the flow consistencies except for HTK, which has a negligibly small bulk value. A possible explanation is that the micro-structural correlation lengths of these fluids are of the order of that of the critical separation. The yield stresses of the carbopol suspensions also increased sharply at the critical gap but then decreased gradually with decreasing gap separation. The transition may correspond to the formation of a jammed state with the subsequent plastic flow at smaller gap separations arising from localised shearing.

Dependence of Self-Assembled Peptide Hydrogel Network Structure on Local Fibril Nanostructure.

Physically cross-linked, fibrillar hydrogel networks are formed by the self-assembly of β-hairpin peptide molecules with varying degrees of strand asymmetry. The peptide registry in the self-assembled state can be used as a design element to generate fibrils with twisting, nontwisting, or laminated morphology. The mass density of the networks varies significantly, and can be directly related to the local fibrillar morphology as evidenced by small angle neutron scattering (SANS) and in situ substantiation using cryogenic transmission electron microscopy (cryo-TEM) under identical concentrations and conditions. Similarly, the density of the network is dependent on changes in the peptide concentration. Bulk rheological properties of the hydrogels can be correlated to the fibrillar nanostructure, with the stiffer, laminated fibrils forming networks with a higher G' as compared to the flexible, singular fibrillar networks.

Rheological study of the effect of shearing process and κ-carrageenan concentration on the formation of whey protein microgels at pH 7

Abstract Gelation under shear during phase separation is evaluated as potential process to prepare microstructured functional ingredients with tunable rheological properties. The physical and chemical reactions leading to gelation have been extensively studied under quiescent conditions. However, a clear understanding of the correlation between microstructure and bulk rheological properties under shear is still lacking. Our work reports a number of interesting new findings, regarding WPI–κ-car gel behaviour. Steady shear flow behaviour and dynamic viscoelastic properties of WPI–κ-car (pH 7.0) were evaluated at 80 °C. The steady viscosity was determined at shear rates from 0.02 to 200 s −1 . Frequency sweep measurements of the dynamic viscosity were performed in the angular frequency range of 0.015–15 Hz at a strain of 0.5%. Confocal microscopy was used to look at the gels microstructure after gelation and demixing process. The effect of shear treatment applied during heating was studied on 5% WPI mixes with varying κ-car concentrations (0%, 0.2%, 0.3%, 0.5% and 0.6%). A steep increase in viscosity was observed at κ-car concentration more than 0.3% and was related to segregating conditions. It was concluded that the low shear Newtonian viscosity for κ-car in isolation changed linearly with κ-car concentration. However, in mixtures, the low shear Newtonian viscosity changed non-linearly with κ-car concentration for non-sheared as well as for sheared systems. The effect of varying shear rates (3.6, 14.4, 43.2 and 86.4 s −1 ) was also studied on a 5% WPI–0.5% κ-car mixture. A competition arises between the gelation and the phase separation depending on whether shear is applied during or after gelation. Applying shear during gelation modifies the microstructure and the viscoelastic properties of the microgels. This work brings further understanding on the segregating interactions taking place between whey proteins and κ-car when used as functional ingredients in dairy products.

Rheological Properties of Chlorinated Polyethylene Blended with Low-cost Grade Natural Rubber

Abstract A low-cost grade natural rubber (STR20 NR) was used for partly substituting elastomeric chlorinated polyethylene (CPE) and then cured by sulfur. Rheological properties of blends with various blend composition ratios under oscillatory and steady shear flows were investigated. Cure behavior is found to be promoted with increasing NR content. The viscoelastic behavior of CPE/NR blends, as determined from the Rubber Process Analyzer (RPA2000), is controlled strongly by blend composition. Uncured blends show relatively poor storage modulus associated with high damping factor probably due to the thermal degradation of NR phase. By contrast, after curing the NR phase in blends, bulk rheological properties change remarkably. Blends with NR as a major component reveal frequency-independent behavior with a broad linear viscoelastic (LVE) region. Flow properties under capillary shear of uncured blends agree well with those under oscillatory shear, i.e., the greater the NR content, the lower the apparent shear viscosity. Surprisingly, even without the elimination of elastic effect by the Bagley correction, it is still possible to superimpose plots of complex viscosity against test frequency over those of apparent shear viscosity against wall shear rate, which are different from the rheological properties of CPE blended with premium grade of NR (STR5L).

Non-linear Rheology Of Collagen Type I Gels Probed On The Length Scale Of A Migrating Cell

Recent investigations showed that the dimensionality of the environment in which living cells are cultured - flat 2D culture wells versus 3D biopolymer networks - has a strong effect on cell morphology, metabolism and migration. The reason for these differences are unclear. What is lacking is a fundamental understanding of the mechanical and morphological properties of 3D matrices at varying length scales.We probe the local microrheology of a series of reconstituted collagen gels with different concentrations (1.2 - 2.4 mg/ml) by applying a calibrated force on embedded magnetic particles (O4.5μm) using magnetic tweezers. The resulting strain field within the matrix is visualized by tracking the positions of polystyrene spheres (O1μm) embedded in the collagen gels. This strain field is compared to expectations from continuum theory. In addition, the local microrheology is compared to bulk rheological properties measured in a cone-plate rheometer. At low forces and strains below 3%, local and bulk rheological properties agree closely, and the strain field follows that of a continuum linear elastic material. At higher strains, marked non-linear strain stiffening occurs, showing an increase in modulus of nearly 20-fold until the material eventually yields. Because of the non-uniform shear conditions around magnetic beads in the local microrheology experiments, the non-linear stiffening appeared to be less pronounced, but the strain field spread much farther out than expected from continuum theory. These data suggest that the strain stiffening behavior of collagen gels, together with the well-documented ability of cells to sense the stiffness of their surroundings, could account for the differences in cell behavior seen in 2D versus 3D culture conditions.

PREDICTION OF TEXTURE PERCEPTION OF MAYONNAISES FROM RHEOLOGICAL AND NOVEL INSTRUMENTAL MEASUREMENTS

ABSTRACT Commercial and model mayonnaises varying in fat content and type and amount of thickener were characterized by sensory analysis, rheological measurements and novel instrumental measurements covering other physicochemical properties and/or reflecting changes of food properties during oral processing. Predictions of texture attributes by rheological measurements were analyzed and compared with predictions by rheological measurements combined with novel measurements. Most of the texture attributes were predicted well by rheological parameters alone. Parameters from other instrumental measurements played a small complementary role, except in the predictions of most of the afterfeel attributes. Most important were rheometry at large deformation and in the nonlinear regime of the dynamic stress sweep and two novel measurements reflecting the effect of saliva: turbidity of rinse water and viscosity with added saliva. Tanδat 500% strain, reflecting the fluid‐like character of the samples during high‐strain dynamic flow, relates best to creaminess and other texture attributes.PRACTICAL APPLICATIONSThis article describes how and how well the texture attributes of mayonnaises can be predicted from rheological and novel instrumental measurements. It shows that many texture attributes can be successfully predicted by bulk rheological properties alone, but that for some the quality of the predictions increases slightly when parameters from other instrumental measurements, such as those from turbidity measurements, those from viscosity measurements in the structure breakdown cell with saliva, or those from friction measurements, are added. These results are relevant not only to those investigating the mechanisms involved in the oral perception of texture in semisolids but also to those who want to perform quick screening of new samples without the use of time‐consuming and expensive sensory panels. Simulations can be performed using the models to predict how texture attributes are influenced by changes in the rheological characteristics of a product. The results also identify rheological measurements and novel instrumental measurements relevant for texture attributes of mayonnaise. This knowledge may help to improve the efficiency of product development in industry.

Topological transition dynamics in a strained bubble cluster

We study experimentally the dynamics of strain-induced T1 neighbour switching in clusters of 3D bubbles. To determine the physico-chemical processes that set the time scale of such rearrangements, foaming solutions of a wide range of well characterized bulk and interfacial rheological properties are used. At low strain rates, the time scale is set by a balance between surface tension and surface viscous forces, and we present a simple physical model that explains these findings, on the basis of previous experimental and theoretical work with 2D foams.1 At higher strain rates, rearrangement dynamics are driven by the applied strain. In a wider context, our study of T1s in bubble clusters provides insight into the structural relaxations that accompany the flow of 3D foams.

Modulation of the release from a w/o/w multiple emulsion by controlling the viscoelastic properties of the two interfaces

This paper describes the main results that we obtained in the study of two different lipophilic surfactants, Dow Corning Emulsifier 10 and Abil EM90, and the influence of their interfacial properties on the fragmentation and release from w/o/w multiple emulsions. Fluid w/o/w multiple emulsions were formulated with an original thermoresponsive polymer. The sol–gel transition at the application temperature of these two different preparations was investigated mainly by rheology. The emulsions were then sheared at 30 °C in order to simulate the release conditions that could be encountered during topical application. Although we obtained similar bulk rheological properties and fragmentation results for both multiple emulsions, the release rates after shear were very different, as we had disappointing results with Abil EM90. In order to explain this phenomenon, rheological properties of the interfacial films formed by the lipophilic surfactant alone, and by the lipophilic surfactant in combination with the thermogelling polymer, were determined and compared. Two release mechanisms were identified and found to be strongly correlated with the viscoelastic properties of the two interfaces.

Thermally Induced Gelling of Oil-in-Water Emulsions Comprising Partially Crystallized Droplets: The Impact of Interfacial Crystals

We produced triglyceride-in-water emulsions comprising partially crystallized droplets, stabilized by a mixture of protein and low molecular weight surfactant. The emulsions were emulsified in the melted state of the oil phase and stored at low temperature (4 degrees C) right after fabrication to induce oil crystallization. The systems were then warmed to room temperature for a short period of time and cooled again to 4 degrees C. Owing to this treatment referred to as temperature cycling or "tempering", the initially fluid emulsions turned into hard gels. We followed the bulk rheological properties of the materials during and after tempering. The storage modulus, G', exhibited a dramatic increase when tempering was applied. We showed that the systems evolved following two distinct regimes that depend on the average droplet size and on the surfactant-to-protein molar ratio. Gelling may involve partial coalescence of the droplets, i.e., film rupturing with no further shape relaxation because of the solid nature of the droplets. Alternatively, gelling may occur without film rupturing, and is reminiscent of a jamming transition induced by surface roughness. We discussed the origin of these two mechanisms in terms of the properties (size and protuberance) of the interfacial oil crystals.

Determination of critical micelle concentration with the rotating sample system

A novel experimental approach using the rotating sample system (RSS) is proposed here for the determination of the critical micelle concentration (CMC) of surfactants. The RSS has been conceived in our laboratory as a convection platform for physicochemical studies and analyses in microliter-sized sample drops. The scheme allows for vigorous rotation of the drop despite its small size through efficient air-liquid mechanical coupling. Thus, changes in surface properties of aqueous samples result in corresponding modulation of the hydrodynamic performance of the RSS, which can be utilized to investigate interfacial phenomena. In this work, we demonstrate that the RSS can be used to study the effects of surfactants on the surface and in the bulk of very small samples with hydrodynamic electrochemistry. Potassium ferrocyanide is employed here with cyclic voltammetry to probe the air-water interface of solutions containing Triton X-100. The CMC of this surfactant determined using this approach is 140 ppm, which agrees well with reported values obtained with conventional methods in much larger samples. The results also demonstrate that besides the CMC, variations in bulk rheological properties can also be investigated in very small specimens using the RSS with a simple method.

Suspension properties at finite Reynolds number from simulated shear flow

This work examines the role of particle-scale inertia in a monodisperse suspension of non-Brownian and neutrally buoyant spherical particles subjected to simple-shear flow. The dimensionless parameters governing the problem are the solid-volume fraction ϕ and the Reynolds number defined Re=ργ̇a2∕μ, where a is the sphere radius, γ̇ is the shear rate, and μ and ρ are the viscosity and density of the fluid, respectively. Using numerical simulations in a wall-bounded domain via the lattice-Boltzmann method, the bulk rheological properties of relative viscosity, normal stress differences, and particle pressure are reported for 0.01⩽Re<5 and 0.05⩽ϕ⩽0.3. The anisotropy in microstructure at finite Re is studied through the pair distribution function g(r). Also presented are the probability density functions of particle velocity fluctuations in gradient and vorticity directions. Comparisons to low Reynolds number theory and simulations are provided wherever possible.

Rheology and microrheology of a microstructured fluid: The gellan gum case

Particle tracking microrheology is used to study the effect of a constant applied shear during gelation of aqueous gellan gum with a monovalent salt. Shear modifies the gellan gum hydrogel microstructure and the bulk rheological properties of the system, depending on whether shear is applied during gelation or afterwards. The microstructure determines the linear elastic response of the gel, as well as the critical strain and stress above which the response becomes nonlinear. Bulk oscillatory rheology is used to study microstructured gellan gum hydrogels at different polymer and salt concentrations. The similarity between our system and concentrated microgel particle suspensions can be explained by considering the microstructured gellan system to be composed of microgel particles whose size is set by the applied shear stress magnitude during gelation. Polymer concentration and ionic strength control the individual microgel particles’ elastic properties. We also find the gellan system exhibits an isoenerget...

Effect of the Interfacial Layer Composition on the Properties of Emulsion Creams

We have quantified observed differences in the microstructure and rheology of creaming emulsions stabilized by protein and low molecular weight surfactants. In this study, we made two sets of emulsions from a single parent emulsion, which differed only in their interfacial composition (i.e., either protein or surfactant). The protein studied was whey protein isolate. The zeta potential of the surfactant-stabilized emulsion was controlled by mixing anionic (SDS) and nonionic (Brij 35) surfactants to match the zeta potential of the protein-stabilized emulsion. Despite this, ultrasonic creaming measurements and confocal microscopy showed that the structures within the cream layers were different between the two sets of emulsions. The protein-stabilized emulsions appeared to slow or arrest the packing within the cream, leading to a lower density network of emulsion droplets, whereas the surfactant emulsion droplets rearranged more quickly into a well-packed, concentrated cream layer. Rheological analysis of the creams showed that despite the protein-stabilized emulsions having a lower dispersed phase volume fraction, their elastic modulus was approximately 30 times greater than that of a comparable surfactant-stabilized emulsion. These differences were caused by the ability of the protein to form a highly viscoelastic interfacial network around the droplets which may include intermolecular covalent cross-links. At close range the adhesive nature of the interaction between the layers contributes to the microstructure and rheology of concentrated emulsions. This is the first time that such well-defined emulsion systems have been studied in detail both noninvasively to look at the impact on creaming and also invasively to look at the impact on bulk rheological properties.

RHEOLOGICAL CHARACTERIZATION OF WET FOOD FOAMS

ABSTRACT Incorporating air bubbles into foods is a technical challenge, and in all cases, the fabrication of a foam goes through a “wet foam” stage, where bubbles are diluted in the food matrix and require stabilization. Sometimes, the end product is itself a wet foam, and a popular example is ice cream. This article describes a study of structural aspects of wet foams, where the continuous phase is a fluid, by means of dynamic rheological tests. The effects of formulation and processing conditions on aerated food foams are studied, and an example is presented for ice cream mix. The incorporation of gas bubbles at volume fractions of 50% or less modifies moderately the bulk rheological properties, and their effect can be predicted by the foam limit case. The continuous phase dominates to a great part the bulk's rheological behavior, and in the case of food systems, it stems from the presence of polysaccharide thickeners. PRACTICAL APPLICATIONSThis work presents an alternative approach to study the rheological properties of short life and difficult‐to‐sample products, such as wet food foams, in a rheo‐reactor. Through the analysis of mechanical properties in oscillatory regime, the structure and stability of wet food foams can be characterized immediately after being fabricated in situ. This work presents new insights on the foaming step of ice cream mix (decoupled from the freezing step), and shows how the mechanical properties are affected by the incorporation ofbubbles, by process conditions and by the presence of thickeners and emulsifiers. This work can be a valuable guide to decide on optimal process and formulation to fabricate wet food foams (e.g., ice cream, aerated desserts) with specific mechanical properties and stability.

Electrohydrodynamic instability of a confined viscoelastic liquid film

We study the surface instability of a confined viscoelastic liquid film under the influence of an applied electric field using the Maxwell and Jeffreys models for the liquid. It was shown recently for a Maxwell fluid in the absence of inertia that the growth rate of the electrohydrodynamic instability diverges above a critical value of Deborah number [L. Wu, S.Y. Chou, Electrohydrodynamic instability of a thin film of viscoelastic polymer underneath a lithographically manufactured mask, J. Non-Newtonian Fluid Mech. 125 (2005) 91] and the problem of pattern length selection becomes ill-defined. We show here that inclusion of fluid inertia removes the singularity and leads to finite but large growth rates for all values of Deborah number. The dominant wavelength of instability is thus identified. Our results show that the limit of small inertia is not the same as the limit of zero inertia for the correct description of the dynamics and wavelength of instability in a polymer melt. In the absence of inertia, we show that the presence of a very small amount of solvent viscosity (in the Jeffreys model) also removes the non-physical singularity in the growth rate for arbitrary Deborah numbers. Our linear stability analysis offers a plausible explanation for the highly regular length scales of the electric field induced patterns obtained in experiments for polymer melts. Further, the dominant length scale of the instability is found to be independent of bulk rheological properties such as the relaxation time and solvent viscosity.

Morphological analysis of highly filled propylene/ethylene copolymers

A series of particulate composites based on a new family of propylene/ethylene (P/E) copolymers produced by Dow Chemical Co. were studied with microscopy and differential scanning calorimetry (DSC). To understand the good processability exhibited by the composites, we studied the composite microstructure and the bulk rheological properties. Here we report the results of a study of the microstructure and thermal behavior. Electron micrographs of the composites showed a uniform dispersion of the particles in the copolymer matrices at all experimental concentrations with very little particle agglomeration. The images suggested low adhesion between the matrices and the particles. The copolymers were semicrystalline, and their morphologies changed with the ethylene content. An increase in the ethylene content led to a decrease in the crystallinity and changes in the shape and size of the crystallites. Tapping-mode atomic force microcopy images of a 9 wt % ethylene copolymer contained spherulites, lamellae, and a crosshatched structure characteristic of α-isotactic polypropylene. The crosshatched structure was not present at higher ethylene contents. A DSC study of the pure copolymers and their composites revealed that only very small modifications to the thermal behavior of the P/E copolymers were induced by calcium carbonate particles. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Investigation of Short-Time Particle Dynamics near an Interface in the Presence of Nonadsorbed Macro-ions

The optical technique of total internal reflection microscopy was used to study the normal Brownian motion of a single colloidal particle near an interface. The measurements were made using a recently developed technique in which the diffusion coefficient was determined by the variance of the short-time (Deltat --> 0) motion of the particle. Experiments were performed in solutions containing either silica nanospheres or clay platelets (Laponite RD) to investigate the effect of nonadsorbed material on the dynamics of near-contact particle motion. The change in the diffusion coefficient with separation distance between the particle and plate in solutions containing nonadsorbed macro-ions was well-described by the theory developed for simple fluids. These results suggest that, in dilute solutions of nonadsorbed material in which the bulk rheological properties remain similar to those of the pure fluid, the mobility and diffusion coefficient correction factors developed for simple fluids remain valid.

Surface Layer Properties of Dough Liquor Components: Are They Key Parameters in Gas Retention in Bread Dough?

Gas cell stability during bread making is controlled by both surface and bulk properties. This paper is focused on studying the surface properties of the water-soluble phase of the dough, the dough liquor (with and without lipids), as well as the composition of the air/water interface. Using infrared reflection measurements, we showed that in lipid-poor liquor, proteins are the dominant species present at the air/water interface. With complete liquor (including the lipids), a mixed interface of protein and lipids is obtained. However, the presence of lipids in the surface layer did not significantly affect the surface pressure. We also added enzymes to the flour to evaluate in what way the surface-active properties of the liquor components can be affected. These results were compared to the effect of adding a surfactant [diacetyl tartaric esters of mono- and diglycerides (DATEM)]. Biobake 10804, a xylanase that increased the arabinoxylan content of the dough liquor, decreased the surface pressure and increased the dilational modulus in lipid-poor liquor. This effect was not observed with the liquor including the lipids. Lipopan 50 BG, a 1,3-specific lipase, increased the surface pressure of the liquor that included the lipids. Lipopan F BG, which converts polar lipids to their lyso form, strongly increased the surface pressure not only in the lipid-containing liquor but also in the lipid-poor liquor. DATEM, as expected, increased the surface pressure while strongly decreasing the dilational modulus. Results of these studies were used to help explain changes in loaf volume observed in a series of baking tests, using the same enzymes and additives. This led to the conclusion that the effect of surface-active components alone cannot account for the larger loaf volumes observed. Clearly, both the effect of bulk and interfacial rheological properties should be considered together when explaining gas cell stability. © Springer Science+Business Media, Inc. 2006.