Interfacial shear rheology

Current Opinion in Colloid & Interface Science 15 (2010) 246–255

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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s

Interfacial shear rheology Jürgen Krägel a,⁎, Svetlana R. Derkatch b a b

Max Planck Institute of Colloids and Interfaces, D-14476 Potsdam-Golm, Am Mühlenberg 1, Germany Murmansk State Technical University, Murmansk, Russian Federation

a r t i c l e

i n f o

Article history: Received 7 December 2009 Received in revised form 2 February 2010 Accepted 2 February 2010 Available online 8 February 2010 Keywords: Interfacial rheology Adsorption layers Proteins Monolayers Particles

a b s t r a c t This review summarises the interfacial shear rheology in the context of problems occurring during the measuring process. The main areas covered are surfactants, proteins, macromolecules, monolayers, particles or mixed systems at the gas/liquid and liquid/liquid interface. New developments in measuring techniques, in data analysis, modelling and theory will be discussed, while micro-rheological techniques using optical or magnetic tweezers are not in the scope of this contribution. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction The shear rheology of interfacial layers at gas/liquid or liquid/ liquid phase boundaries is relevant in a wide range of technical applications, especially in colloidal systems which comprise large interfaces, such as foams and emulsions. The interfacial flow behaviour of such systems is controlled by the presence of surfactants, proteins, insoluble monolayers, lipids, macromolecules or particles in the interfacial layer which will be formed due to the adsorption of interfacial active molecules and attachment of particles or by spreading or layer formation of insoluble amphiphilic substances. Applying shear deformations to interfacial layers gives indirect access to information on inter- and intramolecular interactions at interfaces. The understanding of these interactions is relevant for all applications in which the mentioned substances play a role, i.e. have the control over the structure and hence govern the properties of the systems. Therefore the characterisation of interfacial layers under shear deformation becomes more and more a subject of research interest and is reviewed periodically, for example for protein adsorption layers [1•,2•]. A just published book on “Interfacial Rheology” [3••] describes both its history as well as the current, most frequently used experimental techniques for studying dilational and shear rheology of layers at gas/liquid and liquid/liquid interfaces. In a chapter by Benjamins and Lucassen-Reynders [4•] the interfacial rheology of adsorbed protein layers has been reviewed. In another chapter by Erni et al. [5•] the interfacial rheological aspects in relation to food science and

⁎ Corresponding author. E-mail address: [email protected] (J. Krägel). 1359-0294/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2010.02.001

technology is summarised. The viscoelasticity of mixed surfactant– polymer monolayers is reviewed by Langevin [6•]. For interfacial shear rheology several devices and measuring probes have been suggested in the past, which have their certain sensitivity and measuring range, each having specific advantages and disadvantages. This review focuses only on very recent progress, and therefore, older publications will not be cited here. An overview of the many different measuring techniques and their applications to various systems has been recently summarised by the authors elsewhere [7•]. It is often discussed that interfacial shear rheological data suffer from a bad reproducibility, most often caused by the measuring technique and the system under study. The deformation is transferred by the movement of measuring probe to the interface which generates a certain deformation profile in the interface, therefore the contact of the interface by the measuring probe and the geometry of rheological flow field plays an important role. Hence, care has to be taken to understand the measuring process completely. In the last decade several commercial instruments have been marketed and now two main trends can be observed in literature. There are some publications just using the instruments and trusting the results without any criticism. Especially with instrument having an inappropriate measuring geometry and hence a not well defined interfacial shear field, only qualitative measurements can be performed. However, in the majority of publications individual designs or commercial instruments are used and it is tried to really understand what is going on during the measuring process. In some cases also complementary technique is used to visualize the shear field. The problems which can arise during the measuring process are that the flow profile generated by the interfacial shear field depends strongly on the shear rheology which just has to be measured. This review summarises recent trends in

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understanding the different states of molecules in interfacial layers during their shear deformation. Due to the different problems arising during the measuring process, small surfactant molecules, proteins, macromolecules, insoluble monolayers and particles are discussed separately. 2. Characterisation of interfacial shear rheological properties In general interfacial rheology describes the functional relationship between the deformation of an interface, the stresses exerted in and on it, and the resulting flows in the adjacent fluid phases. A variety of measuring techniques have been proposed in literature to measure interfacial shear rheological properties. The different techniques can be classified in indirect and direct methods. Indirect techniques analyze by image analysis the displacement of tracer particles placed at the interface, while direct techniques measure directly the displacement or torque of a probe located in the interface. One point of importance in the design of such instruments is to provide adequate sensitivity to detect stresses in the interfacial layer in presence of stresses in the adjacent subphase. The contact ratio of the measuring probe with the interface and the ratio of the interfacial to the bulk rheological properties for a continuous shear flow defines a characteristic length, often called the dimensionless Boussinesq number. Such characteristic length can be derived for Newtonian and non-Newtonian interfaces. In oscillatory measurements of viscoelastic interfaces this dimensionless number turns into a complex number. From a theoretical point of view it is expected that above and below this characteristic length different flow profiles and relaxation dynamics of the non-equilibrated two-dimensional liquid exist. Therefore, one of the main challenging problems in designing an interfacial shear rheometer is the coupling of flow at the interface with that in the adjacent bulk phases. The drag at the measuring probe is the sum of the force caused by the interfacial shear stress as the response of the complex liquid interface and in addition by the bulk phases due to velocity gradients. The analysis of hydrodynamic flow fields in different measuring techniques has been a subject of many experimental and theoretical studies. Especially for very thin measuring probes, such as thin circular rings, such analysis is very difficult. Interfacial layers are usually weak. Therefore direct interfacial measurement techniques require high resolution force and displacement sensors. Today's standard rotational rheometers equipped with suitable interfacial accessories can be used for direct interfacial measurements. These instruments set and measure the torque over the motor current whereas the angular displacement is measured by a high resolution optical encoder. The motor is supported by a low friction bearing and the build in normal force sensor allows the accurate positioning of the measurement geometry. Limiting factors for interfacial rheology on rotational rheometers are the minimum torque given by the residual friction of the bearing and the resolution of the motor current. In oscillatory tests the detectable phase shift can be very close to 90° and therefore the inertia of the moving parts needs to be taken into account. The idea of minimizing the inertia by reducing the weight of the measuring probe and the minimization of the characteristic length might have forced some instrument manufacturer to use a small thin Du Noüy ring, which was originally designed for interfacial tension measurements. It is known from surface tension measurements that the wetting conditions at the ring play an important role. Therefore, not only the precise placement of the ring in the interfacial layer, but also the wetting conditions and adhesion at the ring has a tremendous influence in rheological applications. In addition the flow field along the ring is difficult to calculate. Therefore a point of criticism is that in some publications [8–11] the ring is just placed in a Petri dish like for interfacial tension measurements without taking any care for a certain measuring geometry. In such cases only phenomenological studies are possible. Recently Spigone at al. [12•] take into consideration that for the

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oscillating ring technique certain geometry is needed and equipped their instrument with a second outer ring. To overcome the geometry and wetting problems with the oscillating ring technique Franck et al. [13] proposed a double wall-ring geometry and different ring cross section geometries. The proposed ring formed by a bicone shaped wire could perhaps have the potential to overcome the wetting, positioning and adhesion problems. The double gap approach allows the calculation of the geometry factors. Due to the biconical shape of the ring it requires also a precise hydrodynamic analysis of the rheological flow field. Other techniques which try to minimize the characteristic length as well, is the oscillating needle and the torsion pendulum technique. Both techniques count as most sensitive in interfacial shear rheology due to the very small torques applied to the interface. However, they are still not sensitive enough to detect the rheological behaviour of low molecular weight surfactants. Both techniques suffer from the disadvantage of a limited dynamic range of any individual measuring probe. Therefore, a frequency sweep over a broader range requires the complete exchange of probe which destroys the interfacial structure under study. The oscillating needle stress rheometer is a device capable of sensitive interfacial rheology measurements. Yet even for this device, when measuring interfaces of low elastic and viscous moduli, the system response itself contributes significantly to the measured response. To determine the operation limits of the oscillating needle technique, Reynaert et al. [14•] analyzed the relative errors introduced by the instrument. The analysis of the fluid mechanics demonstrates the intimate coupling between the flow fields at the two-dimensional interface and in the bulk phase at low Boussinesq number. The effect of a non-zero Reynolds number is of a similar order of magnitude. The resulting non-linear interfacial deformation profiles lead to an error, which depends on the magnitude of the interfacial modulus, as well as on the phase angle. The analysis identifies the experimental conditions under which reliable measurements can be obtained and show that with small modifications of the measuring probe the sensitivity of the instrument can be improved. All measuring techniques try to transfer low torques or very low deformations to the interface in order to influence the interfacial structure as less as possible or not to destroy them. Therefore, continuous shear experiments under steadystate flow conditions are not recommended. 3. Characterization of Gibbs adsorption layers In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer–surfactant mixtures. A broad range of systems have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte–ionic surfactant mixtures. Rich pattern in adsorption layers were observed in oppositely charged polyelectrolyte–surfactant mixtures. The strong interfacial electrostatic attractions in these systems have a very pronounced effect on the interfacial polymer–surfactant complex formation, pointing to significant interfacial ordering. It is often evidenced, that in some cases the interfacial tensions remain almost unchanged, while the interfacial viscoelastic properties change significantly. Hence, dynamic interfacial rheological properties are more sensitive to conformational changes in adsorbed polymer–surfactant layers. Surfactants and proteins, natural biopolymers, adsorb spontaneously from aqueous solution at interfaces where their free energy is lower than in the solution state. During the adsorption, proteins unfold and establish intermolecular interaction with other protein molecules at or close to the interface leading to the formation of an interfacial film. In contrast to low molecular weight surfactants, proteins are less effective to reduce the dynamic interfacial tension but the formed interfacial film can exhibit strong viscoelastic properties. Therefore adsorption layers of low molecular weight surfactants,

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polymeric surfactants and individual proteins have quite different interfacial viscoelasticities, so that the interfacial rheology can be used as an indirect method to monitor changes in the structure and composition of adsorbed layers. Interfacial shear rheology is particularly sensitive to changes in the strength of molecular interactions in the adsorption layer and can give useful information to predict long term stability to coalescence. The interfacial rheology of protein layers depends on different factors including the intrinsic molecular properties, the physicochemical conditions of the solution and some controlled structural modifications. For example, globular proteins like β-lactoglobulin or lysozyme form interfacial layers with remarkable higher viscoelasticities than the more flexible β-casein. The rheological parameters have generally higher values near the iso-electric point where the net charge of a protein is minimal and the degree of intra- and intermolecular interaction is higher. The ionic strength of the solution influences the rheological properties due to the screening of charges in the proteins which are concentrated at the interface. Proteins or other biosurfactants are commonly used as emulsifiers and foaming agents in food applications. A lot of research activities are related to improve their functional properties by chemical, physical or enzymatic modification. Often complex formation between proteins and polysaccharides, modified polysaccharides and lipids are used leading to highly complex interfacial layers. Such modifications influence the kinetics of adsorption at the interface, i.e. the time needed to transport the protein by diffusion to the interface and to rearrange the molecule at the interface. Also the intermolecular interaction between molecules is influenced due to the higher complex state of the adsorption layer. Vessely et al. [15] studied the influence of calcium (Ca) on the interactions of β-casein at the air–water interface by several techniques, including interfacial shear rheology, atomic force microscopy (AFM), infrared reflectance-absorbance spectroscopy (IRRAS), and zeta potential measurements. In absence of Ca, adsorbed β-casein films form a weak interfacial gel after about 2.5 h, and also some degree of intra- and intermolecular structural organization is observed. For example, in the IRRAS spectra a measurable amount of α-helix content is determined, and AFM images indicate the presence of interfacial aggregates with a characteristic lateral length scale of 20–30 nm, which is interpreted as hemi-micelles. Upon the addition of Ca, in particular at Ca:β-casein molar ratios above 5:1, a stronger interfacial gel are formed more quickly. For example, the interfacial shear moduli increase two times faster, and there is only little evidence for structural organization; i.e., the α-helix peaks are very weak, and AFM images show a continuous but disordered film without distinct hemi-micelles. From these findings, it was concluded that Ca binding destabilizes the coupled intra- and intermolecular structural organization, and that this loss in organization permits a more rapid interfacial gelation. These phenomena are characteristic to the air–water interface but do not appear in the solution bulk. Arboleya and Wilde [16] investigated the competitive adsorption of proteins with methylcellulose and hydroxypropyl methylcellulose. These mixtures are common in many applications, which cause the interest in quantifying the competitive adsorption of these components. The interfacial shear rheology, using a bicone equipped rheometer, interfacial tension and foam stability of the mixed protein–polysaccharide systems (β-lactoglobulin and β-casein) have been determined to elucidate the mechanism and consequences of competition. In contrast to low molecular weight surfactants, it has been determined that both methylcellulose and hydroxypropyl methylcellulose form highly elastic interfaces, more elastic even than β-lactoglobulin alone. It was shown that both methylcellulose and hydroxypropyl methylcellulose are more interfacial active than the proteins, therefore at higher concentrations, the polysaccharides began to dominate the interfacial properties. While low molecular weight surfactants reduce the elasticity of protein adsorption layers, the elastic properties of the polysaccharides en-

hanced the overall strength of the interfacial layer, resulting in more stable foams. Due to dry-heat treatment of protein and polysaccharide mixtures, the protein solubility and emulsification properties can be improved. There is an increasing interest of such Maillard-type conjugates as they are much more interfacially active than the polysaccharide alone. Wooster and Augustini [17] used the oscillating ring technique to examine the interfacial shear rheology of whey protein isolate– dextran Maillard conjugates with different levels of dextran at the water/air interface. The changes in film properties have been assessed in relation to changes in protein unfolding induced by various levels of attachment of dextrans of different molecular weight. It has been observed that the attachment of about 1 dextran per mole of whey protein isolates (low conjugation) did not change the interfacial shear modulus. Due to the low level of conjugation there is only a small effect on protein structure, as supported by fluorescence emission and circular dichroism spectra. With the increasing number of dextrans attached, up to about 5 dextrans per mole WPI (moderate conjugation), a substantial protein unfolding and consequently a remarkable decrease in the interfacial shear modulus was observed. At present, no direct relations exist between protein physicochemical properties, their interfacial properties, and foaming [18] or emulsification 19••] properties. The influence of the thermal treatment of β-lactoglobulin (heating the solution to 80 °C several times) on its interfacial properties has been determined by Kim et al. [20]. It was found that heating changes the secondary conformation as indicated by a decrease in the content in α-helix combined with a corresponding increase in random coil, leading to an increase in surface hydrophobicity due to the exposure of more hydrophobic groups upon partial unfolding of the protein molecule. Additionally it has been observed that heating results in an increased flexibility of the protein molecule. These changes in the secondary and tertiary structures of β-lactoglobulin upon thermal treatment resulted in increased interfacial rheological properties. Using the oscillating ring technique the increases in interfacial shear elasticity and viscosity of adsorbed β-lactoglobulin layer were measured at pH 5.5 and 7. Interfacial shear elasticity, shear viscosity, stability of β-lactoglobulin stabilized emulsion and average coalescence time of a single droplet at a planar oil–water interface with an adsorbed protein layer exhibited a maximum for proteins subjected to 15 min heat treatment at pH 7. At pH 5.5, the interfacial shear rheology and average single drop coalescence time were at maximum for 15 min heat treatment whereas emulsion stability was at maximum after 5 min heat treatment. At pH 7, thermal treatment was found to enhance foam stability. The analysis of thin film drainage of the aqueous phase between an oil drop and a planar oil–water interface indicated that the interface can be considered immobile and, consequently, interfacial shear and dilatational rheological properties do not influence film drainage before coalescence. Hence the observed correlation between interfacial rheology and emulsion stability is may be due to differences in steric interactions. An interesting field of application is the study of interfacial properties in the marine environment, important for the understanding of air–sea gas exchange processes, especially with respect to the behaviour of entrained air bubbles. Seawater contains surfactant-like material, much of which is thought to originate from the exudation of dissolved organic material (DOM) by phytoplankton. Kuhnhenn et al. [21] studied the influence of different phytoplankton species on the interfacial shear viscosity of an air–water interface using the torsion pendulum technique. Interfacial shear viscosities of stock cultures of Phaeocystis sp., Thalassiosira rotula, Thalassiosira punctigera and Nitzschia closterium and also of F/2 nutrient medium and seawater have been measured. N. closterium was studied during different stages of its growth. The results reveal that the influence of phytoplankton on the interfacial shear viscosity is species specific. An increase in viscosity occurred only for the N. closterium stock culture, while all

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other cultures showed a behaviour similar to the F/2 nutrient medium. The surface shear viscosity increase of N. closterium occurred mainly during the exponential growth phase and depends on the presence of phytoplankton cells in the sample. The formation of compact mechanical structures at the air–water interface originating from the aggregation of DOM released by N. closterium is the reason for the observed increase. Croguennec et al. [22] found that intermolecular disulfide bonds are not necessary for the formation of a high interfacial shear elasticity. The effects of a controlled blocking of free cystein by Nethylmaleimide on the kinetics of adsorption at the air/water interface, the interfacial shear rheology and the foaming properties (density and stability) of β-lactoglobulin have been investigated. Compared to native β-lactoglobulin (unmodified β-lactoglobulin), sulfydryl-modified β-lactoglobulin has a higher surface activity, adsorbs faster at the air/water interface, has the capability to develop rapidly an interfacial layer with a high shear elasticity within the initial adsorption period. It exhibits also better foaming properties especially at short times suggesting that the initial rheology of the interfacial film is at least as much important for the general mechanism of foam stabilization as the potential viscoelasticity the interfacial film could reach upon aging. The spontaneous formations of sulfide cross-links between proteins when adsorbed and unfolded were a subject of discussion. It can be assumed that close intermolecular packing, or the presence of other multiple weak bonds, such as hydrogen bonding, can be the basis of mechanically stable films. Mackie et al. [23] observed differences in the microstructure and rheology of creaming emulsions stabilized by whey protein isolate or low molecular weight surfactants. The zeta potential of the surfactantstabilized emulsion was kept matched to that stabilized by the protein by the added amount of anionic (SDS) and nonionic (Brij 35). 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-stabilized emulsion droplets rearranged more quickly into a well-packed, concentrated cream layer. In situ rheological measurements during creaming and interfacial shear rheology of the continuous phase at the oil/water interface were performed. The rheological analysis of the creams showed that depite the lower dispersed phase volume fraction of the protein-stabilized emulsions, their elastic modulus was approximately 30 times larger than that of a comparable surfactant-stabilized emulsion. These differences are explained by the ability of the protein to form a highly viscoelastic interfacial network around the droplets which may include intermolecular covalent cross-links. The interaction between the film layers contributes to the microstructure and rheology of the concentrated emulsions. Erni et al. [24] studied the shape and interfacial viscoelastic response of emulsion droplets in shear flow using a rheometer-based small-angle light scattering, while the interfacial shear rheology of the adsorbed protein layers was measured by a bicone equipped rheometer. The emulsions were prepared with sodium dodecyl sulfate (SDS) or the globular protein β-lactoglobulin. The SDS concentration was far above the CMC so that interfacial concentration gradients were negligible. On the other hand, the deformation of the protein stabilized emulsion droplets are governed by a solid-like interfacial behaviour, as expected from the measured shear rheology at the water–oil interface. The rheometer-based small-angle light scattering technique allows studying such effects for droplets of micrometer size. For identical capillary numbers, the flow-induced anisotropy was significantly smaller for protein-stabilized droplets as compared to the SDS stabilized droplets. It was concluded that protein-covered emulsion droplets, once broken down to their final micrometer size, will not be deformed anymore, especially if the continuous fluid is of low viscosity. Interfacial shear rheological properties enhance the stabilization of droplets at small deformations and sub-critical

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capillary numbers. Erni et al. [25] observed that the physical and engineering properties in drop deformation processes critically depend on different physico-chemical properties, essentially on the interfacial shear rheology. A remarkable interfacial behaviour of class II hydrophobin proteins HFBI and HFBII from Trichoderma reesei and their effect on the stability of air bubbles during disproportionation was observed by Cox et al. [8]. Hydrophobins are a family of highly interfacial active proteins produced naturally by filamentous fungi. They are small proteins (7–9 kDa) and characterized by the presence of eight cysteine residues forming four disulfide linkages. Hydrophobins self-assemble at hydrophobic–hydrophilic interfaces and form a robust, amphipathic layer. Tensiometry data show that hydrophobins reduce the surface tension to approximately 30 mN/m. Interfacial shear rheological data obtained with the oscillating ring technique show that layers of hydrophobins at the air/water surface have a high elasticity, much higher than those for other common proteins used as foam or emulsion stabilizers. The measured interfacial properties are well in line with the stability of bubbles covered by hydrophobin adsorption layers. These hydrophobins have also a dramatic effect on the rate of disproportionation as shown in [26], in contrast to other proteins, although the mechanism is not well understood. It is however emphasised that the hydrophobin does not desorb easily from the air/water surface but acts as a small, amphiphilic protein particle, i.e., a nature-designed “Janus” nanoparticle. How this influences disproportionation is still an open question and requires further work. Azadani et al. [27] investigated the coupling between proteinladen films and bulk flow generating the interfacial shear. The geometry of this experimental set-up is similar to conventional deepchannel viscometers, where the flow in the stationary open cylinder is driven by a constant rotation of the floor. Using the Boussinesq– Scriven surface model for a Newtonian interface coupled to the Navier–Stokes equations for the bulk flow, the authors observed a very low interfacial shear viscosity of the protein-laden films, regardless of the duration of the flow. When the film is intermittently sheared, a significant interfacial shear viscosity appears. In such cases, the interfacial shear viscosity is not uniform across the film. Detailed measurements of the interfacial dilatational elasticity and shear viscosity of systems in the presence and absence of oil droplets have been performed by Murray et al. [28]. The interfacial rheology and stability have been measured as a function of adsorption time and pH by including glucono-δ-lactone as an acidification agent. Commercial sodium caseinate and purified β-lactoglobulin have been used as bubble stabilizing agents. As emulsion oil droplet phase n-tetradecane or 1-bromohexadecane have been used. It was shown that with sodium caseinate at all pH values the interfacial shear viscosity was markedly higher in the presence of tetradecane droplets than in their absence, but particularly when the pH was below pH 5.5 so that the sodium caseinate started to aggregate. The increase in interfacial shear viscosity correlates very well with the increase in coalescence stability. With neutrally buoyant 1-bromohexadecane droplets the determined increase in interfacial shear viscosity was not as large, but it is still significantly higher than in the absence of droplets, which is discussed as an indication that the creaming and packing of tetradecane droplets at the air–water interface is not solely responsible for the observed effects. Interfacial shear rheological experiments, using a bicone equipped rheometer, were performed by Arboleya et al. [29] to get additional information on the coalescence behaviour of aerated palm kernel oil/ water emulsions. The influence of interfacial compositional changes was studied by two different complex model emulsion containing a mixture of methyl cellulose and sodium caseinate. In the first case lecithin was solved in the oil phase while in the second one a mixture of lecithin and the surfactant Tween 60. The interfacial shear rheology helped to understand how the competitive adsorption of ingredients can change the emulsion stability and promote different degrees and

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rates of partial coalescence. During aeration, fat globule interfaces break down, promoting partial coalescence to form a solid network of fat globules, which surrounds and stabilizes air bubbles. Therefore, partial coalescence is a consequence of the inherent emulsion instability. Kotsmar et al. [30] studied the dilational and shear rheology of adsorption layers of β-casein mixed with the nonionic dodecyl dimethyl phosphine oxide (C12DMPO) and the positively charged dodecyl trimethyl ammonium bromide (DoTAB), respectively. The features of the mixed interfacial layers drawn from the dilational rheology have been confirmed by the interfacial shear rheology measured by a torsion pendulum technique. The lowest added surfactant concentrations caused a remarkable increase in both shear elasticity and viscosity. A further increase of the surfactant concentration leads to a decrease of the elasticity and viscosity values, which is an indication of the protein displacement. The measured decrease in shear elasticity and viscosity with C12DMPO is much steeper, showing that this surfactant displaces the protein molecules more efficiently from the surface, which correlates as well with the discussed tensiometry data and is related to its higher surface activity as compared to DoTAB. Moreover, the elasticity and viscosity values at low surfactant concentrations are higher for C12DMPO, obviously caused by small changes in the structure of the β-casein due to interaction with the surfactants. Hence, C12DMPO interacts more efficiently with β-casein than DoTAB. This difference was explained by the types of interaction, i.e. hydrophobic and electrostatic, because the length of the hydrocarbon chains of both used surfactants was identical, so that hydrophobic interactions alone cannot cause the differences. Interfaces play also an important role in biology. The majority of biological events occur at interfaces rather than in bulk. One very important interfacial process is fat digestion, which occurs not only in animals but also in plants and microorganisms. Lipases and phospholipases are the enzymes involved in fat digestion. Reis et al. [31,32] studied the competition between lipases and globular proteins and monoglycerides at interfaces by tensiometry, interfacial shear rheology, and ellipsometry. The effect of polar lipids which will be generated during fat digestion on the behaviour of lipases at the oil–water interface was discussed. Both Sn-1,3 regiospecific and nonregiospecific lipases, and a non-catalytically active protein, β-lacloglobulin, as reference were used. The results obtained from the interfacial rheology studies demonstrated that the Sn-2 monoglyceride is very interfacial active and efficiently expels the enzyme from the interface. Both interfacial shear and dilational rheological techniques were applied for the characterization of film formation and surface gelation of gelatin molecules at the air–water interface by Leick et al. [33]. The shear properties measured by the torsion pendulum technique and a bicone equipped rheometer show a strong adsorption of the polypeptide molecules at the water surface and the formation of densely packed film structures even for highly diluted solutions. All measured dependencies show a steep increase of the rheological parameters. At low gelatine concentrations torsion pendulum with its high sensitivity was used, while due to the formation of such highly viscoelastic interfacial layers at higher gelatine concentrations it was necessary to use a bicone equipped rheometer. The obtained data attest the coexistence of viscous and elastic properties, which point to the formation of gel-like films. Pronounced elastic and viscous properties for more concentrated gelatin solutions are the result of closer packed interfacial layers. After a specific time the elastic properties become more important than the viscous film response, which is an indication of the existence of a sol–gel phase transition. From these observations the authors conclude that a two-dimensional gel layer has been formed at the water surface at conditions while the bulk phase is not yet gelling. It is assumed that the attractive forces between the adsorbed molecules seem to be so strong that the system evidently forms a coherent, two-dimensional gel layer. Therefore it can consequently be assumed that a thin network structure is generated in the first, more concentrated layer of adsorbed molecules.

Frequency sweep experiments, performed after the film formation was completed, at higher gelatin concentrations confirm these observations. All measured interfacial layers show strong viscoelastic properties with moduli nearly constant over the whole frequency range, which supports the idea of the formation of two-dimensional network structures due to cross-linking by physical or chemical processes. Studies on mixtures of pure proteins are already complicated, but some authors even studied real commercial systems, e.g. foodstuffs. The reason for such phenomenological studies is that mixed protein– surfactant systems offer a simple way for optimising foamability and foam stability. Rouimi et al. [34] studied the interfacial shear behaviour of commercial milk proteins and food emulsifiers at the air– water and n-dodecane–water interfaces using the oscillatory ring technique. They concluded that most of the aspects of competitive adsorption of low molecular surfactants and proteins on the interfacial viscoelasticity, foamability and foam stability discussed in literature for purified systems seem to hold also for commercial milk protein–surfactant systems. Similar conclusions were drawn by Eisner et al. [35] for ice cream formulations containing proteins, fat particles and low molecular surfactants. Acacia gum is a hybrid polyelectrolyte containing both protein and polysaccharide subunits. Erni et al. [36] studied the interfacial rheology of respective adsorption layers at the oil/water interface and the results were compared with adsorbed layers of hydrophobically modified starch, often used as a substitute for Acacia gum. While in dilational experiments the viscoelastic response of the starch derivative is slightly weaker than for Acacia gum, pronounced differences in shear deformation were measured. Interfaces covered with the plant gum flow like a rigid, solid material with large storage moduli and a linear viscoelastic regime limited to small shear deformations was observed. Films formed by hydrophobically modified starch are predominantly viscous, and the shear moduli depend only weakly on the deformation. 4. Characterization of Langmuir monolayers The primary classes of insoluble monolayers at the air/water interface are for example fatty acids, fatty alcohols, lipids or polymers. An interesting feature of Langmuir monolayers are their rich phase behaviour depending on the state of compression, with the equivalent of gas, liquid, liquid condensed, and crystalline phases. Often they exhibit rich domain phase coexistence regions in which various microstructures can be observed. Therefore Langmuir monolayers are investigated at the air/water interface already over a long period of time even under the aspect to determine their rheological behaviour under shear deformation. Powerful optical tools as fluorescence microscopy, Brewster angle microscopy, X-ray diffraction and reflectivity measurements have been developed to visualize the monolayers' structure and streamlines in an interfacial shear field. The effect of the monolayer structure on the interfacial shear rheology was studied by Gavranovic et al. [37] for mixtures of straight-chain and branched forms of hexadecanol using the oscillating needle technique, the shear viscosities of which show a maximum in the condensed un-tilted phase. The addition of branched molecules results in a non-monotonic increase in interfacial viscosity, with the maximum occurring near 12% of branched molecules. The visualization of these immiscible mixed monolayers using Brewster angle microscopy in the liquid condensed phase show the formation of discrete domains that first increase in density and then decrease as surface pressure is increased. Kurtz et al. [38] also studied mixtures of straight-chain and branched molecules and found that eicosanol mixtures exhibit rheological and structural behaviour different from the hexadecanol mixtures. Two-phase transitions at certain surface pressure for all mixtures were observed. The shear viscosities of both straight and mixed monolayers show a maximum close to a surface pressure of 5 mN/m. In [39] also X-ray diffraction and reflectivity were

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used to provide a better understanding of the monolayer structure in terms of phase segregation and localisation of branched chains, which provided valuable hints to interpret the differences in rheological behaviour between the two fatty alcohols. The branched fractions have a greater effect on eicosanol mixtures, since the branched chains are only weakly associated with the hexadecanol films. It was also observed that for both hexadecanol and eicosanol monolayers the interfacial shear viscosity increases with increasing in-plane coherence length. Larger monolayer regions or domains are more viscous. An interesting aspect to combine macro- and micro-rheological techniques to study Langmuir monolayers was proposed by Walder et al. [40] using a specially equipped Langmuir monolayer trough. The central elements are the trough itself with a full range of optical tools (fluorescence microscope and optical tweezers) accessing the air– water interface from below and a portable knife-edge torsion pendulum that can access the interface from above. The ability of simultaneous measurements of the mechanical response of Langmuir monolayers on very different length scales is useful to understand the mechanical response of two-dimensional viscoelastic networks and can provide valuable information to understand and control the relative contribution to the drug from the interfacial layer versus the contribution from the underlying bulk phase. The apparatus was tested with dipalmitoylphosphatidylcholin (DPPC) monolayers. Lucero Caro et al. [41] studied the pH effect on the shear rheological characteristics of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers using an oscillatory Du Noüy ring apparatus. It was observed that the monolayer structure and, especially, the conditions at which the monolayer collapses, determine the viscoelastic behaviour under shear deformations. The non-linear viscoelastic behaviour of the interface was associated with the phospholipid monolayer collapse. It turns out that interfacial shear rheology is more sensitive to the effect of pH on surface rheology of phospholipid monolayers than surface dilatational rheology. The interfacial shear characteristics for DPPC and DOPC monolayers were higher at pH 5, due to the attractive van der Waals interactions between phospholipid molecules, and lower at pH 9 due to electrostatic repulsions between phospholipid molecules weakening the monolayer structure. Both the interfacial dilatational and shear moduli were also higher for DPPC than for DOPC at any pH. In [42] the interaction between dipalmitoyl phosphatidylcholine (DPPC) and β-casein in mixed monolayers spread at the air–water interface was investigated by means of surface pressure Π/A isotherms (monolayer structure, elasticity, and miscibility), interfacial dilatational and shear properties, and Brewster angle microscopy, as a function of surface pressure, pH, and β-casein/DPPC interfacial mixing ratio. At higher surface pressure the protein collapsed and was displaced from the interface by DPPC. The existence of weak interactions between β-casein and DPPC and/or the repulsion between these components on a basic aqueous subphase facilitates the displacement of β-casein by DPPC from the air–water interface. In a study by Nishimura et al. [43] model tear film lipid layer composed by a binary mixture of cholesteryl myristate (CM) and 1,2dipalmitoyl sn-glycero-3-phosphocholine (DPPC) was characterized via surface tension measurements, Brewster angle microscopy and interfacial shear rheology. The shear rheology confirms the observations made in the isotherms that DPPC and 30:70 CM/DPPC films are fluid at all pressures and more compliant than films with higher CM content. Films containing more than 50 mol% CM became elastic at higher surface pressures. The increasing CM content reduced the surface pressure at which the mixed film became elastic. The adsorption of added Lysozyme into a CM film increased the compressibility which results in a more expanded film. It was also observed that added Lysozyme increases the ductility of CM/DPPC films and prevents film breakdown up to the highest surface pressures of 40 mN/m. It was concluded that CM increased the elasticity of the lipid films, but also made them brittle and incapable of expansion following compression.

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Another interesting topic is the interfacial rheology of polymer layers which have been recently reviewed by Monroy et al. [44•] as adsorbed or spread monolayers at liquid interfaces. Most of the water soluble polymers are polyelectrolytes or designed as block copolymers with hydrophilic and hydrophobic groups. The majority of existing studies were performed on aqueous solutions of surfactants and polyelectrolytes as model systems [6•]. The reason for extensive studies on polymer layers is the possibility to effectively confine them in two-dimensional systems, which is important from both theoretical and technological points of view. However, especially the dynamics of polymers in two dimensions is rather unexplored as compared to the polymer dynamics in bulk phase and yet not well understood. Gavranovic et al. [45] performed both interfacial rheological experiments and computer simulations to understand the conformation of polymers in 2D systems. The oscillating needle technique was applied to Langmuir monolayers of poly(tert-butyl methacrylate) (PtBMA) of different molecular weight. The measured rheological properties of the films are substantially different above and below a plateau surface pressure Πp. Below Πp, the monolayers are primarily viscous, and the interfacial viscosity increases linearly with the molecular weight, while above Πp, the films are more elastic, and the interfacial viscosity is independent of the molecular weight. Respective computer simulations produced qualitatively similar Π/A isotherms. The observed transition at Πp marks the change over from polymer chains existing in a single layer to multilayers. Further studies on PtBMA Langmuir monolayers on the influence of temperature and effect of small amounts of carboxylic acid groups in the polymer were reported by Gavranovic et al. [46]. Both temperature variation and chemical modification affect the relative importance of polymer–polymer and polymer–subphase interactions. Montreux et al. [47] investigated the interfacial rheology of the thermo sensitive polymer poly(N-isopropylacrylamide) (PNIPAM) adsorbed at the air–water interface by the oscillating needle technique. A steep increase in the interfacial shear elasticity and viscosity was observed as a function of temperature around the lower critical solution temperature due to the increased amount of adsorbed molecules as the solvent quality decreases for PNIPAM chains. The layers undergo a transition from a Newtonian liquid like state to an elastic state due to increased intermolecular entanglements. The PNIPAM bulk concentration and molecular mass influence the interfacial rheological properties at the air–water interface. Maestro et al. [48•] studied the molecular weight dependence of poly(methyl methacrylate) (PMMA) Langmuir films as a function of polymer concentration (Γ) and molecular weight (N) by using two different measuring techniques (based on free damped oscillations of a sharp edged ring and a forced oscillation of a biconical disk, respectively). Both instruments were used in the oscillatory mode at comparable oscillation frequency and amplitude, which gave access to the viscoelastic shear modulus (G*) and work in complementary viscosity ranges. The results obtained for four PMMA samples of molecular weight between 8 × 104 and 2.7 × 105 g/mol show power law like behaviour of the viscoelastic shear modulus (Γ10 and N4, see Fig. 1). These strong dependences suggest a structural scenario based on a 2D percolation of the polymer pancakes. Polyglycerol fatty acid esters (PGE) are a class of surfactants which are commonly used in different applications. The molecular structure of PGE, especially the long hydrocarbon chains, provides the surfactant a potentially high interfacial affinity if molecularly dispersed in aqueous solution. This fact, however, results simultaneously in a very low solubility in water. As a consequence, PGE mainly self-assembles into multilamellar vesicles even at extremely low surfactant concentrations. In the vesicular form, surfactants are significantly less surface-active than their monomers. Similar experimental studies exist for biological membrane lipids and pulmonary surfactants and different mechanisms of transport of monomers to the interface have been derived, however are still not well understood.

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Fig. 1. Interfacial shear elasticity Gi′ as a function of the polymer concentration in the semidilute regime; each graph corresponds to a sample with different molecular weight, from left to right: 2.708 × 105, 1.59 × 105, 1.043 × 105 and 8.0 × 104 g/mol; measurements were performed with two different interfacial rheometers: (○) torsion pendulum technique and (▲) bicone equipped rheometer; the straight lines present power law fits of the elastic modulus against the polymer concentration. Gi′ is proportional to the polymer concentration with an exponent of 10 ± 1; according to [48•].

Nevertheless, it is a prevailing opinion that vesicles can transform into interfacial films. Duerr-Auster et al. [49•] studied the structure and rheological properties of interfacial layers formed by PGE at the water/air interface by oscillatory interfacial shear rheology. Due to the slow transport from bulk to the interface, films of measurable viscoelastic behaviour were obtained only after a sufficiently long time. The rheological response to a temporary network indicates an intermolecular connectivity at the interface. It is assumed that this temporary network is probably created by hydrophobic interactions of alkyl chains. More detailed information was gained from compare experiments with spread monolayers, the structural features of which were visualized by Brewster angle microscopy (BAM). On the basis of all rheological results, it can be assumed that the structure of the resulting interfacial layer is a composite of a monomolecular layer at the interface and eventually further bimolecular sublayers in the bulk. Another study by Duerr-Auster et al. [50] was devoted to the coalescence behaviour of two isolated bubbles in presence of PGE. It was found that the increase of the ionic strength increases foaming efficiency and stability. The corresponding interfacial rheological properties at different pH values are not strongly influenced. As shown in a previous study [49•] the elastic or solid-like response has been related to multilayer formation in the interface and subsurface. This structuring has a three-dimensional nature, which is extended in the subsurface bulk. The spontaneous formation of such a structure is explained to be related to the molecular structure of the surfactant and its predisposition to form a lamellar phase in aqueous solution. Such kinds of experiments are not able to prove the existence of multiple layers. The magnitude of the interfacial shear moduli are related to the interfacial concentration of PGE and the obtained results in these studies can only be interpreted qualitatively. For a quantitative discussion one has to take into consideration that this extension of the interface in a certain third dimension makes the data analysis for the measuring geometry extremely difficult, as the hydrodynamic analysis of the biconical disk geometry requires a planar, homogeneous interface. 5. Characterization of particles at interfaces In solid particle-stabilized emulsions, named Pickering-Ramsden emulsions, particles accumulate at the interface between two immiscible liquids and stabilize the emulsion drops against coales-

cence by forming a robust mechanical monolayer at the liquid–liquid interface. Particles adsorb to the interfaces to form a dense monolayer only under certain conditions depending on particle size and shape, wettability, and interparticle interactions. Such complex interfacial layers are of fundamental and practical importance and attract therefore attention as an alternative to surfactants or polymers to stabilize interfaces in gas/liquid and liquid/liquid systems. Typically interfacial accumulation is observed when the particles are partly wetted by both liquids. Another strategy is to introduce a chemical anisotropy to the stabilizing particles, to make the particles themselves amphiphilic. Such particles attach or “adsorb” at interfaces more easily than isotropic particles due to their amphiphilicity and more strongly than molecular surfactants due to the larger adsorption energy. Consequently, in contrast to surfactant molecules which are in dynamic equilibrium between interface and bulk phase, particles are irreversibly adsorbed at the interface. The variation of hydrophobicity of particles will tune the interaction between the particles at the interface which influences in turn the structure of the interfacial layer. Recent work demonstrates the importance of interparticle interactions of the particles at the interface to the overall foam and emulsion stability e.g. in [51]. Krishnaswamy et al. [52] studied the interfacial rheology of 10–50 nm silver nanoparticles monolayers at the toluene–water interface under steady and oscillatory shear. Strain amplitude sweep measurements reveal a shear thickening peak in the loss moduli (G´´) at large amplitudes followed by a power law decay of the storage and loss moduli with exponents in the ratio 2:1. In frequency sweep measurements at low frequencies, the storage modulus remains nearly independent, whereas the loss modulus follows a power law dependence with a negative slope, a behaviour reminiscent of soft glassy systems. Under steady shear, a finite yield stress was observed in the limit of low shear rates. For shear rates N1 s− 1, the shear stress increases gradually. In addition it has been confirmed that due to a significant deviation from the Cox–Merz rule the monolayer of silver nanoparticles at the toluene–water interface forms a soft twodimensional colloidal glass. In contrast to foams stabilized by surfactant molecules the lifetimes of which are of the order of few hours, particle-stabilized foams can be stable over long periods of time. It was shown that foam stability depends on the contact angle of the particles at the interface.

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Longer lifetimes were observed for particles of intermediate hydrophobicity. The origin of the stability of the foam is often related to the interfacial rheological properties mostly because the deformations occurring during bubble or droplet collisions produce shear and compression stresses in the surface layers on the bubble or drop surfaces. The effect of particle's hydrophobicity on the properties of silica particle layers at the water/air interface, including the interfacial shear rheological behaviour was studied by Safouane et al. [53] with focus on the rheological properties of silica particle layers at the air– water interface. The shear rheology studies were performed with the torsion pendulum technique for different particle hydrophobicities at a fixed layer density. The measured interfacial shear moduli were small, but much larger than for any densely packed surfactant layers. This corresponds to Boussinesq numbers larger than 105, ensuring that the subphase contribution is negligible. Both G′ and G″ increase with the hydrophobicity, while at low hydrophobicity G′ and G″ are negligibly small, at intermediate hydrophobicity G′ is of the same order of magnitude as G″. At larger hydrophobicity G′ is higher than G″, and the particle layers become more rigid, probably because of the increase in the hydrophobic interactions between the particles. A cross over at around 36% is observed which corresponds to a gel point. The shear rheological measurements show a transition from viscous to elastic behaviour for particles with contact angles close to 90°. It demonstrates that not only the geometric parameter but also the interparticle interaction contributes to the overall foam and emulsion stability. This suggests that the rheological properties of such monolayers of aggregated particles play an important role in the stabilization process [54,55]. Recently it was shown that at a sufficiently high concentration of adsorbed particles the liquid/liquid interfaces lose their mobility and display solid-like properties, a phenomenon which is called “interfacial jamming”. Jamming can arrest interfacial tension driven morphological coarsening in liquid/liquid or gas/liquid systems and therefore stabilize two-phase morphologies with unusual interfacial shapes, for example non-spherical bubbles, drops and bijels. The strategy to tailor aggregate structures and two-dimensional particle networks by the addition of surfactants or salts to weaken the interaction force has been applied by Reyneart et al. [56] to study charged polystyrene particles at the water–decane and water–air interface using the oscillating needle technique. The strength of aggregation has been modified by the addition of appropriate combinations of the anionic surfactant sodium dodecylsulfate (SDS) and sodium chloride. Other ways to modify the structure of interfacial layer is to change the size of particles or to use mixtures of particles. The influence of the shape of particles on the structure and properties of interfacial layers is additionally important. Shape is an important parameter in controlling the maximum packing. The maximal random-jammed packing of non-spherical particles depends on the aspect ratio and can become denser than that of spherical particles. The packing density is important to predict or control interfacial rheological properties. Basavaraj et al. [57] examined the effect of non-spherical particles on the structure and properties of monolayers and it was observed that ellipsoids of a sufficient large aspect ratio display a less abrupt increase in the surface pressure isotherms upon compression than spherical particles. When a certain lateral pressure is reached, jamming of the system is observed and in-plane rearrangements are not anymore possible. Any further compression is relieved by flipping the ellipsoids into an upright position or by expelling particles from the monolayer. Such behaviour was not observed for spherical particles with similar dimensions and surface chemistry. These observations demonstrate that structural transitions in ellipsoidal monolayers are more complex than for spherical particles. Furthermore, the mechanical properties of these monolayers, as measured by interfacial shear rheology, show that these monolayers exhibit a substantial interfacial modulus even at low interfacial coverage and can be used to create more elastic

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monolayers as compared to aggregate networks formed by spherical particles. Also the self-assembling of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. Madivala et al. [58] varied the particle shape, surface charge, and wetting properties. When model monodisperse ellipsoidal particles are deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changes of the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembling process. It was observed, that the initial microstructure of charged ellipsoids at a decane–water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. Compared to particles at an oil–water interface, particles of the same surface chemistry and charge at an air– water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks consisting of ellipsoids connected at their tips in triangular or flower-like configuration. These networks are very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the monolayer of ellipsoids exhibit a substantial surface shear modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties. Madivala et al. [59] investigated experimentally the effect of the aspect ratio of particles on the stability of both water-in-oil and oil-inwater emulsions. A strong dependence of the emulsion stability on the aspect ratio of the particles was observed obviously caused by the pronounced viscoelastic shear properties and high interfacial moduli, both at air–water and oil–water interfaces. It is assumed that this is perhaps due to an increased effective coverage and the occurrence of strong attractive shape-induced capillary interactions. The results demonstrate that interfaces with controlled interfacial rheology, as obtained by using shape-induced capillary forces and packing effects, can be used for the design of particle-stabilized emulsions.

6. Conclusions In conclusion, the field of shear rheology of interfacial layers is extremely rich and divers but relatively undeveloped. This is surprising in view of the large number of applications in which the interfacial shear rheological properties play an important role. No studies of emulsification and emulsion stability, wetting or two-phase flow have been compared to interfacial shear rheological properties. A few attempts to relate foam, foam film stability and foam rheology to interfacial rheology, mainly to dilational properties, were reported. In view of the numerous applications it is clearly desirable to devote further research efforts to this topic. Many publications have shown that interfacial rheological techniques give very useful information of the more or less macroscopic properties of interfacial layers. But these properties depend strongly on the microscopic interfacial structure. Therefore information of such microstructures is very important to better understand the interfacial rheology. It is demonstrated here that it becomes more and more important to study at the same time e.g. with optical techniques structure changes within the interfacial shear field. Therefore it can be concluded, that there is a need to reliable measuring techniques to reveal the rheology interfacial layers both mechanically and micro structurally. The interfacial complexity is directly linked to the interplay of the coupled interfacial flow and the flow of the adjacent bulk phase due to the moving measuring probe.

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Acknowledgements The work was financially supported by a project of the European Space Agency (FASES MAP AO-99-052) and the DFG SPP 1273 (Mi418/16-2).

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This is the first book on interfacial rheology and aims at describing both the history and the state of the art of this scientific field which is of increasing interest. The most frequently used experimental techniques for studying shear and dilational rheology of layers at both liquid/gas and liquid/liquid interfaces are described. All contributions include the theoretical basis for the presented methodologies and selected experimental examples. Therefore it gives an excellent and brief insight into the composition and structure of interfacial layers, essentially when they have been built from complex mixed solutions. [4] Benjamins J, Lucassen-Reynders EH. Interfacial rheology of adsorbed protein layers. • In: Miller R, Liggieri L, editors. Progress in colloid and interface science series vol. 1. Interfacial rheologyLeiden: Brill; 2009. p. 253–302.(And the referenced literature therein.) 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Viscoelasticity of mixed surfactant–polymer monolayers. In: Miller R, • Liggieri L, editors. Progress in Colloid and Interface Science Series Vol. 1. Interfacial RheologyLeiden: Brill; 2009. p. 303–31.(And the referenced literature therein.) This book chapter gives an excellent overview on the interfacial rheology of polyelectrolyte–surfactant systems. It describes the problems to investigate the viscoelasticity of interfacial layer in a fully controlled manner. The relations between interfacial viscoelasticity and practical applications are discussed as well. [7] Krägel J, Derkatch SR. Interfacial shear rheology — an overview of measuring • techniques and their applications. In: Miller R, Liggieri L, editors. Progress in Colloid and Interface Science Series Vol. 1. Interfacial RheologyLeiden: Brill; 2009. p. 372–428.(And the referenced literature therein.) 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properties of spread and adsorbed polymer layers at fluid interfaces. Among traditional characterisation techniques different rheological methods and experimental examples are discussed. Gavranovic GT, Deutsch JM, Fuller GG. Two-dimensional melts: polymer chains at the air/water interface. Macromolecules 2005;38:6672–9. Gavranovic GT, Smith MM, Jeong W, Wong AY, Waymouth RM, Fuller GG. Effects of temperature and chemical modification on polymer Langmuir films. J Phys Chem B 2006;110:22285–90. Montreux C, Mangeret R, Laibe G, Freyssingeas E, Bergeron V, Fuller GG. Shear surface rheology of poly(N-isopropyacrylamide) adsorbed layers at the air/water interface. Macromolecules 2006;39:3408–14. Maestro A, Ortega F, Monroy F, Krägel J, Miller R. Molecular weight dependence of the shear rheology of poly(methyl methacrylate) Langmuir films: a comparison between two different rheometry techniques. Langmuir 2009;25:7393–400.For this experimental study of spread polymer layers of a broad range of molecular weight two different measuring techniques were applied. Both instruments work at different measuring principles and have different measuring ranges. It is demonstrated that both instruments provide complementary and mutually compatible data. Duerr-Auster N, Gunde R, Windhab EJ. Structure and mechanical properties of a polyglycerol ester at the air/water interface. Langmuir 2008;24:12282–9.This is an excellent experimental study of the structure and mechanical properties of interfacial layers formed from vesicle dispersions at the water/air interface. It is shown that more detailed information about network properties can be obtained by the combination of interfacial rheology and optical methods.

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