Dilational rheological

I. C. Callaghan, C. M. Gould, R. J. Hamilton, and E. L. Neustadter. The relationship between the dilatational rheology and crude oil foam stability 1. Preliminary studies. Colloids and Surfaces, 8(1) 17-28, November 1983. 

Lucassen-Reynders, E.H., Benjamins, J. (1999). Dilational rheology of proteins adsorbed at fluid interfaces. In Dickinson, E., Rodriguez Patino, J.M. (Eds). Food Emulsions and Foams Interfaces, Interactions and Stability, Cambridge, UK Royal Society of Chemistry, pp. 195-206. 

Wijmans, C.M., Dickinson, E. (1998). Simulation of interfacial shear and dilatational rheology of an adsorbed protein monolayer modeled as a network of spherical particles. Langmuir, 14, 7278-7286. 

G. Garofalakis and B. S. Murray, Surface pressure isotherms, dilatational rheology, and brewster angle microscopy of insoluble monolayers of sugar monoesters, Langmuir, 18 (2002) 4765-4774. 

The dilational rheology behavior of polymer monolayers is a very interesting aspect. If a polymer film is viewed as a macroscopy continuum medium, several types of motion are possible [96], As it has been explained by Monroy et al. [59], it is possible to distinguish two main types capillary (or out of plane) and dilational (or in plane) [59,60,97], The first one is a shear deformation, while for the second one there are both a compression - dilatation motion and a shear motion. Since dissipative effects do exist within the film, each of the motions consists of elastic and viscous components. The elastic constant for the capillary motion is the surface tension y, while for the second it is the dilatation elasticity e. The latter modulus depends upon the stress applied to the monolayer. For a uniaxial stress (as it is the case for capillary waves or for compression in a single barrier Langmuir trough) the dilatational modulus is the sum of the compression and shear moduli [98]... 

The dilatational rheology of the poly(vinylacetate) monolayer onto an aqueous subphase has been studied between 1°C and 25°C by Monroy et al. [59], These authors have used the combination of several techniques. By this way, the exploration of a broad frequency range was possible. The relaxation experiments have shown multiexponential decay curves, whose complexity increases with decreasing the temperature. A regularization technique has been used to obtain the relaxation spectra from the relaxation curves and the dilatational viscoelastic parameters have been calculated from the spectra. The shapes of the relaxation spectra agree with the predictions of the theoretical model proposed by Noskov [100],... 

When starch is added to cold water (below 29°C, 85°F), only negligible swelling will occur. However, the suspension volume expands, since the insoluble starch replaces water. Addition of starch to water at a concentration of 10% will increase the volume by 13%. The maximum in suspension solids is 40-45%. Various methods are used to determine the solids content of the starch slurry aerometer,92 density cells, densitometer, attenuation of vibration (Dynatrol) or a radiation-type density meter. Concentrated starch slurries have high viscosity and shear thickening (dilatent) rheology. Settling of starch from the slurry produces densely packed sediments that are difficult to disperse. 

In this section we introduce the matter of equivalent mechanieal circuits on an elementary level. First we restrict ourselves to linear viscoelastic behaviour. Second, to show the basic elements, idealized cases will be emphasized (mainly strain retardation and stress relaxation), ignoring for the time being the problem of how to carry out such experiments. As a rule, however, we keep in mind that stress-wise the monolayer is always at equilibrium, and strain has to adjust to it. In this section only dilational rheology will be considered, but this is not a real restriction because for shear the formalism is the same mutatis mutandis. 

Surface dilatational rheology is a very sensitive technique to analyze the competitive adsorption/displacement of protein and LMWE emulsifier at the air-water interface (Patino et al., 2003). A common trend is that the surface dilatational modulus increases as the monolayer is compressed and is a maximum at the highest surface pressures, at the collapse point of the mixed film, and as the content of LMWE in the mixture increases. At higher TT, the collapsed protein residues displaced from the interface by LMWE molecules have important influence on the dilatational characteristics of the mixed films. The mechanical properties of the mixed films also demonstrate that, even at the highest tt, the LMWE is unable to displace completely protein molecules from the air-water interface. 

As for pure LMWE films, the surface dilatational modulus is higher for protein-saturated-LMWE than for protein-unsaturated-LMWE mixed films at every surface pressure. The surface dilatational properties of mixed protein-emulsifier films also depend on the presence of some food components (ethanol and sucrose) in the aqueous phase. In general, a decrease in the dilatational rheological properties on the addition of ethanol was found for protein-water-insoluble LMWE. That is, the static and... 

While dilatational rheology plays an important role in short-term stability of dispersions shear viscosity may contribute appreciably to the long-term stability (Murray and Dickinson, 1996 Murray, 1998, 2002). The shear characteristics of the interfacial film are governed by the composition and structure of the adsorbed material. In addition, surface shear viscosity is a very sensitive technique to analyze the competitive adsorption of protein and water-soluble LMWE at the air-water interface (Murray and Dickinson, 1996 Murray, 1998, 2002 Bos and van Vliet, 2001). 

Kovalchuk VI, Miller R, Fainerman VB and Logho G (2005). Oscillating bubble pressure experiments for dilational rheology studies. Adv Colloid Interface Sci 114-115 303-313... 

Loglio G, Pandolfini P, Tesei U, and Noskov B (1998b) Measurements of interfacial properties with the axisymmetric bubble-shape analysis technique effects of vibrations. Colloids Surfaces A 143 301-310 Loglio G, Pandolfini P, Miller R, Makievski AV, Ravera F, Ferrari M and Liggieri L (2001) "Drop and Bubble Shape Analysis as Tool for Dilational Rheology Studies of Interfacial Layers", in "Novel Methods to Study Interfacial Layers", Studies in Interface Science, Vol. 11, D. Mobius and R. Miller (Eds.), Elsevier, Amsterdam, pp 439-485... 

The software of the PAT1 allows the programming of a complete sequence of experiments, such as transient and harmonic relaxations after the equilibrium of adsorption has been established. The oscillations were usually performed at various frequencies, so that sufficient data for the dilational rheology were available. A typical result, as an example, is shown for / -LG in Figure 5, where... 

Let us now look into the surface dilational rheology of the adsorbed protein... 

Another recently developed method for determining surface rheological properties is the damping of a radial oscillating bubble, firstly described by Lunkenheimer Kretzschmar (1975) and established theoretically by Wantke et al. (1980). This technique is described in more detail in Chapter 6. It is based on damping effects and yields dilational rheological... 

In the book by Joos [16] as well as in original papers, some special cases of this general approach have been discussed. It was shown that such stress relaxation experiments are well suited for studying the dilational rheology of interfacial layers, which yield the dilational elasticity as a function of the effective surface age teff... 

As mentioned above relaxation techniques are additional methods suitable to get insight into the mechanism of adsorption processes. Moreover, these methods represent the experimental tools to determine the dilational rheology of interfacial layers. The general principle of relaxation methods is the small disturbance of the interfacial layer, which has reached the equilibrium state beforehand. Particular methods are suitable to detect characteristic times of relaxations processes as they work each in a specific frequency range. This paragraph discusses briefly the most frequently used and very recently developed methods. 

The present chapter gives also detailed introduction to a large number of experimental methods, suitable for studying dynamic interfacial tensions. The methods are discussed in terms of the available time window. There are methods which complement each other such that a time interval from less than 100 microseconds up to hours and days of adsorption time can be covered (about ten orders of magnitude). The relaxation methods, also suitable for detecting the adsorption mechanism of surfactant s adsorption provide in addition the dilational rheology of interfacial layers. It is discussed that in particular these dilational rheological studies are most informative in respect to adsorption mechanisms, as the data interpretation includes the thermodynamic model as well as the adsorption dynamics. 

Many experiments have been proposed for measuring the interfacial shear viscosity and elasticity and interfacial dilatational viscosity and elasticity at gas/liquid and liquid/ liquid interfaces [22]. Interfacial shear viscosities of different oil/aqueous systems have been studied worldwide. Some experimental results indicate that low interfacial shear viscosities do not necessarily imply that an emulsion will be unstable [23]. The dilatational rheology is based on area changes due to an expansion or compression of a fluid surface and stress relaxation experiments. The experiment results show that the interfacial dilatational properties can be much higher than the interfacial shear properties for the same system [15,24-27]. This makes researchers believe that interfacial dilatational viscosity and elasticity may have a better relationship with the stability of the emulsion than with interfacial shear properties. 

Some approaches analyzed directly flic influence of flic stabilizing adsorption layers and concluded that diere is a dependence of the stability of an emulsion on flic interfacial concentration and the sum of inter-molecular interactions (8—10). Murdoch and Leng (11) pointed out the role of bulk and interfacial rheological parameters to describe these processes. This concept was further treated by several authors (12—14). A very comprehensive approach was given by Wasan and co-workers (15,16) who considered the surface shear and dilational rheology, and also some hy-drody-namic parameters in their analysis of emulsion films. 

With respect to the rheological parameters fliey come to the conclusion that surface elasticity effects are superior to surface viscosity effects. This, however, apphes to pure surfactant layers and may be different for pure protein or mixed surfactant/protein adsorption layers. It has been stressed also by Langevin (26), in her review on foams and emulsions, fliat studies on the dynamics of adsorption and dilational rheology studies for mixed systems, in particular surfactant-polymer systems, are desirable in order to understand these most common stabilizing systems. 

Dilational rheological experiments are based on area changes by keeping the shape of the interface constant. Models for the exchange of matter, which sets in after a compression or expansion of the interface, are generally applicable to both harmonic and transient types of relaxations (178). Stress-relaxation experiments may yield results different from those obtained from measurements on small disturbances as the composition of the surface layer can vary (179). Overviews on experimental and theoretical aspects of dilational rheology were given recently in Refs 180—182. 

Transient as well as harmonic relaxation experiments give aecess to the dilational rheology of the studied interface or film (211). The definition of the dilational elasticity E is given by the relation ... 

Theoretical models have reached a state that allows a quantitative description of the equilibrium state by thermodynamic models, the adsorption kinetics of surfactants at fluid interfaces, the transfer across interfaces and the response to transient or harmonic perturbations. As result adsorption mechanisms, exchange of matter mechanisms and the dilational rheology are obtained. For some selected surfactant systems, the characteristic parameters obtained on the various levels coincide very well so that a comprehensive understanding was reached. 

Since it is commonly (5) held that the occurrence and magnitude of surface tension gradients (Marangoni effects), whether due to spatial variations in temperature or concentration or compression/ expans ion of the interface, are important to many colloid problems, some consideration has been given to methods of determining the dilatational rheological parameters. 

Dilatant Rheological state in which a fluid s viscosity increases with increasing shear stress. 

Rheology The study of material flow. Used in the ceramic industry to describe the reaction of a fluid to an applied stress. While some slips may increase in viscosity upon stirring (dilatent rheology), others will decrease in viscosity (pseudoplastic rheology). The third major rheological category, Newtonian, describes a fluid whose viscosity does not change with applied shear stress. 

Loglio G, Pandolflni P, Miller R, Makievski AV, Ravera R, Ferrari M, Liggieri L (2001) Drop and bubble shape analysis as a tool for dilational rheological studies of interfacial... 

Drop and Bubble Shape Analysis as a Tool for Dilational Rheological Studies of Interfacial Layers... 

The interfacial tension response to transient and harmonic area perturbations yields the dilational rheological parameters of the interfacial layer dilational elasticity and exchange of matter function. The data interpretation with the diffusion-controlled adsorption mechanism based on various adsorption isotherms is demonstrated by a number of experiments, obtained for model surfactants and proteins and also technical surfactants. The application of the Fourier transformation is demonstrated for the analysis of harmonic area changes. The experiments shown are performed at the water/air and water/oil interface and underline the large capacity of the tensiometer. 

In this chapter first the theoretical basis of the drop and bubble shape method is described, and then details of its practical use are given. The various functionalities of the instrument are demonstrated then in the form of examples, such as dynamic surface and interfacial tensions of surfactant and protein solutions, and the dilational rheology of some selected systems. 

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