Interfacial rheology of stable and weakly aggregated two-dimensional suspensions

The interfacial rheological properties of stable and weakly aggregated two-dimensional suspensions are studied experimentally using a magnetic rod interfacial rheometer. Particle monolayers with well controlled structures were prepared. Charged polystyrene particles create two-dimensional colloidal crystals at the water-decane interface over a wide range of concentrations. Under similar conditions a predominantly liquid structure is obtained at the water-air interface for the same particles. The addition of appropriate combinations of the anionic surfactant sodiumdodecylsulfate (SDS) and sodium chloride (NaCl) to the aqueous subphase leads to a destabilization of these monolayers with the formation of fractal aggregates at low concentrations and a heterogeneous gel forming as the surface coverage is increased. After the structures have been built up a reproducible structure can be obtained, of which the interfacial rheological properties can be investigated using a magnetic rod stress rheometer. In all cases, numerical calculations were used to assess the importance of instrumental artifacts and the effect of the coupling between surface and subphase flows. The rheology of aggregated suspensions was compared to the reference case of a colloidal crystal. The two-dimensional aggregated suspensions display rheological features which are similar to their three-dimensional counterparts. These include an elastic response with small linearity limits, a power law dependence on surface coverage and a dependence on the strength of attraction. The results shed some light on the possible role of interfacial rheology on the stability of particle laden high interface systems. Additionally, the 2D suspensions could present fundamental insights in the rheological properties of dense colloidal suspensions.     

Simultaneous Control of pH and Ionic Strength during Interfacial Rheology of β-Lactoglobulin Fibrils Adsorbed at Liquid/Liquid Interfaces

Proteins can aggregate as amyloid fibrils under denaturing and destabilizing conditions such as low pH (2) and high temperature (90 °C). Fibrils of β-lactoglobulin are surface active and form adsorption layers at fluid-fluid interfaces. In this study, β-lactoglobulin fibrils were adsorbed at the oil-water interface at pH 2. A shear rheometer with a bicone geometry set up was modified to allow subphase exchange without disrupting the interface, enabling the investigation of rheological properties after adsorption of the fibrils, as a function of time, different pH, and ionic strength conditions. It is shown that an increase in pH (2 to 6) leads to an increase of both the interfacial storage and loss moduli. At the isoelectric point (pH 5-6) of β-lactoglobulin fibrils, the maximum storage and loss moduli are reached. Beyond the isoelectric point, by further increasing the pH, a decrease in viscoelastic properties can be observed. Amplitude sweeps at different pH reveal a weak strain overshoot around the isoelectric point. With increasing ionic strength, the moduli increase without a strain overshoot. The method developed in this study allows in situ subphase exchange during interfacial rheological measurements and the investigation of interfacial ordering.     

Shear and dilatational relaxation mechanisms of globular and flexible proteins at the hexadecane/water interface

Proteins adsorbed at fluid/fluid interfaces influence many phenomena: food emulsion and foam stability (Murray et al. Langmuir 2002, 18, 9476 and Borbas et al. Colloids Surf., A 2003, 213, 93), two-phase enzyme catalysis (Cascao-Pereira et al. Biotechnol. Bioeng. 2003, 83, 498; 2002, 78, 595), human lung function (Lunkenheimer et al. Colloids Surf., A 1996, 114, 199; Wustneck et al.; and Banerjee et al. 2000, 15, 14), and cell membrane mechanical properties (Mohandas et al. 1994, 23, 787). Time scales important to these phenomena are broad, necessitating an understanding of the dynamics of biological macromolecules at interfaces. We utilize interfacial shear and dilatational deformations to study the rheology of a globular protein, lysozyme, and a disordered protein, beta-casein, at the hexadecane/water interface. Linear viscoelastic properties are measured using small amplitude oscillatory flow, stress relaxation after a sudden dilatational displacement, and shear creep response to probe the rheological response over broad experimental time scales. Our studies of lysozyme and beta-casein reveal that the interfacial dissipation mechanisms are strongly coupled to changes in the protein structure upon and after adsorption. For beta-casein, the interfacial response is fluidlike in shear deformation and is dominated by interfacial viscous dissipation, particularly at low frequencies. Conversely, the dilatational response of beta-casein is dominated by diffusion dissipation at low frequencies and viscous dissipation at higher frequencies (i.e., when the experimental time scale is faster than the characteristic time for diffusion). For lysozyme in shear deformation, the adsorbed protein layer is primarily elastic with only a weak frequency dependence. Similarly, the interfacial dilatational moduli change very little with frequency. In comparison to beta-casein, the frequency response of lysozyme does not change substantially after washing the protein from the bulk solution. Apparently, it is the irreversibly adsorbed fraction that dominates the dynamic rheological response for lysozyme. Using stress relaxation after a sudden dilatational displacement and shear creep response, the characteristic time of relaxation was found to be 1000 s in both modes of deformation. The very long relaxation time for lysozyme likely results from the formation of a glassy interfacial network. This network develops at high interfacial concentrations where the molecules are highly constrained because of conformation changes that prevent desorption.     

Nonlinear viscoelasticity and shear localization at complex fluid interfaces

Foams and emulsions are often exposed to strong external fields, resulting in large interface deformations far beyond the linear viscoelastic regime. Here, we investigate the nonlinear and transient interfacial rheology of adsorption layers in large-amplitude oscillatory shear flow. As a prototypical material forming soft-solid-type interfacial adsorption layers, we use Acacia gum (i.e., gum arabic), a protein/polysaccharide hybrid. We quantify its nonlinear flow properties at the oil/water interface using a biconical disk interfacial rheometer and analyze the nonlinear stress response under forced strain oscillations. From the resulting Lissajous curves, we access quantitative measures recently introduced for nonlinear viscoelasticity, including the intracycle moduli for both the maximum and zero strains and the degree of plastic energy dissipation upon interfacial yielding. We demonstrate using in situ flow visualization that the onset of nonlinear viscoelasticity coincides with shear localization at the interface. Finally, we address the nonperiodic character of this flow transition using an experimental procedure based on opposing stress pulses, allowing us to extract additional interfacial properties such as the critical interfacial stress upon yielding and the permanent deformation.     

Phase behavior and viscoelastic properties of trisilanolcyclohexyl-POSS at the air/water interface

A trisilanol polyhedral oligomeric silsesquioxane (POSS), trisilanolcyclohexyl-POSS (TCyP), has recently been reported to undergo a series of phase transitions from traditional Langmuir monolayers to unique rodlike hydrophobic aggregates in multilayer films that are different from "collapsed" morphologies seen in other systems at the air/water interface. This paper focuses on the phase transitions and morphology of films varying in average thickness from monolayers to trilayers and the corresponding viscoelastic properties of trisilanolcyclohexyl-POSS molecules at the air/water interface by means of surface pressure-area per molecule (Pi-A) isotherms, Brewster angle microscopy (BAM), and interfacial stress rheometry (ISR) measurements. The morphology studies by BAM reveal that the TCyP monolayer can collapse into different 3D structures by homogeneous or heterogeneous nucleation mechanisms. For homogeneous nucleation, analysis by Vollhardt et al.'s nucleation and growth model reveals that TCyP collapse is consistent with instantaneous nucleation with hemispherical edge growth at Pi = 3.7 mN.m(-1). Both surface storage (Gs') and loss (Gs") moduli obtained by ISR reveal three different non-Newtonian flow regimes that correlate with phase transitions in the Pi-A isotherms: (A) A viscous liquidlike "monolayer"; (B) a "biphasic regime"between a liquidlike viscous monolayer and a more rigid trilayer; and (C) an elastic solidlike "trilayer". These observations provide interesting insights into collapse mechanisms and structures in Langmuir films.     

Mechanical properties of protein adsorption layers at the air/water and oil/water interface: A comparison in light of the thermodynamical stability of proteins

Over the last decades numerous studies on the interfacial rheological response of protein adsorption layers have been published. The comparison of these studies and the retrieval of a common parameter to compare protein interfacial activity are hampered by the fact that different boundary conditions (e.g. physico-chemical, instrumental, interfacial) were used. In the present work we review previous studies and attempt a unifying approach for the comparison between bulk protein properties and their adsorption films. Among many common food grade proteins we chose bovine serum albumin, β-lactoglobulin and lysozyme for their difference in thermodynamic stability and studied their adsorption at the air/water and limonene/water interface. In order to achieve this we have i) systematically analyzed protein adsorption kinetics in terms of surface pressure rise using a drop profile analysis tensiometer and ii) we addressed the interfacial layer properties under shear stress using an interfacial shear rheometer under the same experimental conditions. We could show that thermodynamically less stable proteins adsorb generally faster and yield films with higher shear rheological properties at air/water interface. The same proteins showed an analog behavior when adsorbing at the limonene/water interface but at slower rates.     

Rheological properties of immiscible polymer blends under parallel superposition shear flow

The small amplitude oscillations can be superimposed parallelly on steady shear flows. The resulting moduli provide information about time‐ and shear‐dependent microstructure. For this purpose, model blends composed of polydimethylsiloxane and polyisobutylene with the viscosity ratio of 7.9 and 0.25 are investigated. The resulting moduli are compared with the results derived from numerical calculation as well as analytical solutions, developed here by introducing the conditions under parallel superposition flow field into MM model. Good agreement is found in the interfacial contribution of the storage moduli for blend with low volume fraction. Moreover, detailed analysis on hydrodynamic interaction between droplets is given to explain the discrepancies. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 431–440, 2008     

Sorbitan tristearate layers at the air/water interface studied by shear and dilatational interfacial rheology

The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.     

流变技术在高分子表征中的应用:如何正确地进行剪切流变测试

流变学是高分子加工和应用的重要基础,流变学表征对于深入理解高分子流动行为非常重要,获取的流变参数可用于指导高分子加工.本文首先总结了剪切流变测试中的基本假设:(1)设置的应变施加在样品上,(2)应力来源于样品自身的响应和(3)施加的流场为纯粹的剪切流场;之后具体阐述了这些假设失效的情形和所导致的常见的实验错误;最后,通过结合一些实验实例具体说明如何培养良好的测试习惯和获得可靠的测试结果.     

Synthesis, characterization, and interfacial properties of an oligomeric, cationic fluorooxetane

The synthesis and characterization of a cationic oligo(fluorooxetane) surfactant with pendant -C4F9 groups are reported. Molecular area demand at saturation was determined to be 55.6 +/- 0.3 angstroms2/molecule and characteristic of an oligomer. The adsorption of the cationic oligo(fluorooxetane) to the air-water interface appears to be diffusion-limited, and dilational rheological properties of the adsorbed molecules are representative of a "soluble" monolayer. Adsorption dynamics have been measured yielding diffusion coefficients that are dependent on concentration and in the 10(-7)-10(-8) cm2/s range. Complex moduli from dilational interfacial rheological measurements as a function of oscillation frequency were well fitted to the Lucassen-van den Tempel equation, providing an estimate of the Gibbs elasticity. The combination of the oligomeric nature of the fluorosurfactant, short perfluoroalkyl chain and its interfacial properties suggests that this synthetic approach is an attractive route to the development of fluorinated surfactants that avoid the environmental concerns of small-molecule, long perfluoroalkyl-chain surfactants.     

Liquid drainage in single plateau borders of foam

This paper reports on an investigation of the influence of the interfacial shear viscosity on the liquid drainage in single Plateau borders of foam. The simplified Navier-Stokes equation governing the liquid flow is solved for the liquid velocity by the numerical computational method. The numerical results show significant influence of the interfacial shear viscosity on the liquid velocity in the Plateau border. Comparison of the numerical results for the average velocity over the cross-section area of the Plateau border to the available analytical solution shows that the available analytical solution underestimates the average velocity. New, simple yet accurate correlations for the dependence of the average velocity on the radius of the cross section of the Plateau border, the pressure gradient, and the interfacial shear viscosity are obtained using the asymptotic analysis and the numerical data.     

Tailoring the interfacial assembly of colloidal particles by engineering the mechanical properties of the interface

Fluid interfaces can be used as a platform for promoting the direct and spontaneous self-assembly of colloidal particles, where the driving force is the reduction in interfacial energy. In addition, fluid interfaces allow fine-tuning of the particles ensemble by an external force, such as the presence of an imposed interfacial flow, or by engineering the interparticle interactions dictated by the interplay of interfacial forces. As a consequence, a wide-ranging set of interfacial structures can be achieved from liquid-like layers, which can flow under stress, to amorphous solids that are able to sustain static stress. Here, far from a comprehensive overview of the interfacial assembly of colloidal particles, different ways of tailoring it by rationally designing the rheological properties of the interface are provided, with a focus on experimental and theoretical methods and model systems that have been recently exploited. In particular, ligand-coated nanoparticles, with a strong emphasis on the effect of the ligands on the interfacial structure and the rheological properties, and soft nanogel particles, in which an environmental factor, such as the temperature, drives to different interfacial structures and mechanical responses will be further discussed.     

Effect of binding of an oligomeric cationic fluorosurfactant on the dilational rheological properties of gelatin adsorbed at the air-water interface

The effect of binding of an oligomeric cationic fluorooxetane surfactant on the interfacial properties of adsorbed gelatin-fluorooxetane complexes has been studied using dynamic surface tension and dilational rheological measurements. Adsorption kinetics of gelatin-fluorooxetane complexes are reminiscent of a mixed (barrier/diffusion limited) process, while the dilational rheological properties of the interface exhibit a strong dependence on surfactant concentration. At low surfactant concentrations, dilational surface moduli as well as phase angles are relatively insensitive to the presence of the fluorooxetane. However, at the critical aggregation concentration of the polymer-surfactant system, there is a sharp increase in the complex modulus. Further increase in the fluorooxetane concentration does not significantly affect the complex modulus. The phase angle, however, does increase with increasing fluorooxetane concentration due to the transport of bound fluorooxetane from the subsurface to the solution-air interface. These results indicate that, at fluorooxetane concentrations exceeding the critical aggregation concentration, the polymer-surfactant complexes adsorb to form cross-linked multilayers at the solution-air interface.     

Interfacial slip at the thermotropic liquid‐crystalline polymer/poly (ethylene naphthalate) interface

The unique rheological properties of a thermotropic liquid‐crystalline polymer (TLCP) were first studied. The thermal and shear history of the TLCP was found to play a critical role in its rheological properties. Crystallites were observed in the TLCP melt even above the melting temperature detected by differential scanning calorimetry. Because interfacial slip had long been suggested as an important reason for viscosity reduction in TLCP/thermoplastic blends, for the first time, interfacial slip at the TLCP/poly(ethylene naphthalate) (PEN) interface was investigated with an energy model. The model quantified the degree of interfacial slip at the TLCP/PEN interface by an energy factor. The calculated energy factors revealed a high degree of interfacial slip at the TLCP/PEN interface. It was proposed that the high rigidity of rodlike TLCP chains and their alignment parallel to the interface prevented mutual entanglements at the TLCP/PEN interface. The lack of mutual entanglements promoted the interfacial slip. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 302–315, 2004     

Methods of measuring rheological properties of interfacial layers (Experimental methods of 2D rheology)

Current methods of studying the rheological properties of interfacial layers at the interfaces of fluids are reviewed. This area of research includes two-dimensional 2D rheology. Regardless of the similarities between the parameters of rheological properties of two-dimensional and bulk (three-dimensional) systems, when measuring surface properties, it is necessary to reformulate the main experimental methods to allow for the different dimensions of surface and bulk characteristics of material. Parameters of shear and dilational (measured upon expansion-compression) properties of interfacial layers are distinguished, and the latter are considered to be independent parameters of a system. The most attention was given to the rotational methods of measuring shear viscosity and the components of the complex 2D elastic modulus, as well as to measuring surface tension upon harmonic changes of the bubble (droplet) surface area, which allows characteristics of the dilational behavior of thin liquid films to be determined. Both groups of methods are widely used in laboratory practice and realized in the form of a number of original and commercial instruments. Dilational measurements of interfacial layers can also be performed with oscillations of a movable barrier on a Langmuir trough. In addition, methods based on the propagation of capillary waves across the surface of a liquid, as well as rarer methods of capillary flow in thin channels forced by either a surface tension gradient or the motion of the interface, are considered.     

Interfacial rheology of an ultrathin nanocrystalline film formed at the liquid/liquid interface

We report the interfacial properties of monolayers of Ag nanoparticles 10-50 nm in diameter formed at the toluene-water interface under steady as well as oscillatory shear. Strain amplitude sweep measurements carried out on the film reveal a shear thickening peak in the loss moduli (G") at large amplitudes followed by a power law decay of the storage (G') and loss moduli with exponents in the ratio 2:1. In the frequency sweep measurements at low frequencies, the storage modulus remains nearly independent of the angular frequency, whereas G" reveals a power law dependence with a negative slope, a behavior reminiscent of soft glassy systems. Under steady shear, a finite yield stress is observed in the limit of shear rate .gamma going to zero. However, for .gamma > 1 s-1, the shear stress increases gradually. In addition, a significant deviation from the Cox-Merz rule confirms that the monolayer of Ag nanoparticles at the toluene-water interface forms a soft two-dimensional colloidal glass.     

Rheological behavior of precursor PPV monolayers

The rheological behavior of different precursor poly(p-phenylene vinylene) (prec-PPV) monolayers at the air-water interface was investigated using an interfacial stress rheometer (ISR). This device nicely reveals a transition of the precursor poly(2,5-dimethoxy-1,4 phenylene vinylene) (prec-DMePPV) monolayer from Newtonian to elastic behavior with increasing surface pressure. The transition is accompanied by an increase in the modulus. This behavior coincides with the coagulation of different 2D condensed domains as revealed by Brewster angle microscopy (BAM). However, partly converted prec-DMePPV monolayers show elastic behavior even at low surface pressures, although a sudden increase of the moduli does occur. This phenomenon is attributed to enhanced hydrophobic interactions between the conjugated moieties in the partly converted polymers. The latter also explains the stretching behavior of the partly converted prec-DMePPV upon transfer in Langmuir-Blodgett-type vertical dipping. The increase of the moduli which is observed is much more gradual in the precursor poly(2,5-dibutoxy-1,4-phenylene vinylene), prec-DBuPPV, a monolayer which is in agreement with the expected expanded state of the latter monolayer.     

Numerical assessment of the analytical models used to determine flexural and shear moduli in I-Beams when the tensile and compressive moduli are different

The different compressive and tensile moduli of fibre reinforced composites have been considered in the analysis of the flexural and shear moduli of I-beams. Firstly, the neutral axis has been determined analytically and then, assuming that location of the neutral axis, the analytical flexural modulus of I-beams has also been obtained. In order to assess the proposed procedure, virtual pure bending and three-point bending tests at different spans have been carried out using the finite element method. The compressive and tensile moduli have been taken into account by defining two parts in the numerical models. The numerical flexural and shear moduli have been determined by reducing the data obtained in the virtual tests. Analytical and numerical results are in good agreement. Therefore, the flexural modulus determined by the proposed analytical approach can be introduced as a material property in the finite element method.     

Rheology and microstructures formation of immiscible model polymer blends under steady state and transient flows

Viscoelastic properties of model immiscible blend were studied here under steady state condition at different initial conditions and transient flow conditions. The flow‐induced microstructure has been studied on these model blends. For this system, the elastic properties of the blend are mainly governed by the interface. Measurement of the dynamic modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to prove the blend morphology. The dynamic moduli after cessation of shear flow, the mean diameter of the disperse phase as generated by the shear flow, have been calculated using the model of Palierne. A procedure based on a direct fitting of the dynamic moduli with the model is compared with the one that uses a weight relaxation spectrum. On the other hand, the steady state normal stress data have been related to the morphology of the blend by means of Doi and Ohta model. The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for the blend. The flow behavior during transient shear flow was also discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3519–3533, 2005     

Interfacial dilational properties of anionic gemini surfactants with polymethylene spacers

The dilational properties of anionic gemini surfactants alkanediyl-α,ω-bis(m-octylphenoxy sulfonate) (C₈C_mC₈) with polymethylene spacers at the water–air and water–decane interfaces were investigated by oscillating barriers and interfacial tension relaxation methods. The influences of oscillating frequency and bulk concentration on the dilational properties were explored. The experimental results show that the linking spacer plays an important role in the interfacial dilational properties. The moduli pass through one maximum for all three gemini surfactants at both water–air and water–decane interfaces. However, the values of moduli at the water–air interface are obviously higher than those at the water–decane interface because the sublayer formed by spacer chains will be destroyed by the insertion of oil molecules. Moreover, with the increase of spacer length, the surface adsorption film becomes more viscous at high concentration, which can be attributed to the process involving the formation of the sublayer. On the other hand, the spacers of the adsorbed C₈C₆C₈ molecules will extend into the oil phase when the interface is compressed. As a result, the interfacial film becomes more elastic with the increase of spacer length at high concentration. The experimental results obtained by the interfacial tension relaxation measurements are in accord with those obtained by the oscillating barriers method.