Rheology, surface

Because of the mobility of molecules in the surface of a pure Uquid, such surfaces have very little elasticity. For that reason, pure liquids cannot support a foam. In the presence of an adsorbed monolayer film, however, the rheological properties of the surface can change dramatically. By analogy with bulk phases, the physical state of a surface film can be distinguished by its viscosity. 

If talc powder is gently placed on a liquid surface, gently blowing on the surface causes the particles to move relatively freely. If the surface is covered with a low density monolayer film movement becomes more restricted, but relatively free movement is still evident. If the film pressure is increased (i.e., more molecules per unit area of surface), at some point the particles become fixed in place—the surface viscosity has increased substantially and the film behaves as if it is in a condensed state (see text below). 

It is the increase in surface viscosity produced by adsorbed films (insoluble and Gibbs monolayers, adsorbed polymers, etc.) that leads to the production of persistent foams, helps stabilize emulsions, and explains the role of spread monolayers in dampening surface waves, among other important interfacial phenomena. 

Like bulk materials, monolayer films exhibit characteristics that can (sometimes with a bit of imagination) be equated to the solid, liquid, and gaseous states of matter. For films, the equivalent states are roughly defined as 

Condensed (solid) films, which are coherent, rigid (essentially incompressible), and densely packed, with high surface viscosity. The molecules have little mobility and are oriented perpendicular (or almost so) to the surface (Fig. 8.13a). 

More detailed descriptions are given in Refs. [295,408,409]. Further details on the principles, measurement and applications to dispersion stability of interfacial viscosity are reviewed by Malhotra and Wasan [408], and Miller et al. [410]. 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. Thus, the dynamic surface tension experienced during a rate-dependent surface expansion, is the result of the surface dilational viscosity, the surface shear viscosity, and elastic forces. Often, the contributions of shear and/or the dilational viscosities are neglected during stress measurements of surface expansions. Isolating interfacial viscosity effects is difficult because, since the interface is connected to the substrate on either side of it, the interfacial viscosity is coupled to the two bulk viscosities. 

Therefore, it becomes laborious to determine purely interfacial viscosities without the influence of the surroundings. 

To the extent that viscosity and surface viscosity influence foam stability, one would predict that stability would vary according to the effect of temperature on the viscosity. Thus some petroleum industry processes exhibit serious foaming problems at low process temperatures, which disappear at higher temperatures. Ross and Morrison [25] cite some examples of petroleum foams that become markedly less stable above a narrow temperature range that may be an interfacial analogue of a melting point. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110], 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. 

The bulk viscosity will most influence the thinning of thick films, while the surface viscosity will be dominant during the thinning of thin films. 

Conventionally the snrfaee viscous stress T, is written in a slightly different form (Scriven, 1960 Slattery, 1990)  

The last two terms on the right-hand side provide the effects of the surface viscosities. The effects of the componoit a n have been ignored for simplicity. If there is no local expansion or contraction of the interface, one has 

FIGURE 7.5 The deep channel viscometer. The longitudinal section of a radially symmetric figure is shown. 

When piglpip = 0 (i.e., when the upper fluid is a gas), the int adal velocity is given by 

Combining the resulting equations for surfactant covered and surfactant free surfaces, one obtains the equation 

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The discussion of surface viscosity and other aspects of surface rheology is deferred to Section IV-3C. 

Phospholipids are amphiphilic compoimds with high surface activity. They can significantly influence the physical properties of emulsions and foams used in the food industry. Rodriguez Patino et al. (2007) investigated structural, morphological, and surface rheology of dipalmitoylpho-sphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers at air-water interface. DPPC monolayers showed structural polymorphisms at the air-water interface as a function of surface pressure and the pH of the aqueous phase (Fig. 6.18). DOPC monolayers showed a... 

The dynamic behavior of fluid interfaces is usually described in terms of surface rheology. Monolayer-covered interfaces may display dramatically different rheological behavior from that of the clean liquid interface. These time-dependent properties vary with the extent of intermolecular association within the monolayer at a given thermodynamic state, which in turn may be related directly to molecular size, shape, and charge (Manheimer and Schechter, 1970). Two of these time-dependent rheological properties are discussed here surface shear viscosity and dynamic surface tension. 

Dickinson, E. (1999b). Adsorbed protein layers at fluid interfaces interactions, structure and surface rheology. Colloids and Surfaces B Biointerfaces, 15, 161-176. 

Thermodynamically unfavourable interactions between two biopolymers may produce a significant increase in the surface shear viscosity (rf) of the adsorbed protein layer. This change in surface rheological behaviour is a consequence of the greater surface concentration of adsorbed protein. For instance, with p-casein + pectin at pH = 5.5 and ionic strength = 0.01 M (Ay = 2.6 x 10 m3 mol kg-2), the surface shear viscosity at the oil-water interface was found to increase by 20-30%, i.e., rp = 750 75 and 590 60 mN s m-1 in the presence and absence of polysaccharide. These values of rp refer to data taken some 24 hours following initial protein layer formation (Dickinson et al., 1998 Semenova et al., 1999a). 

In considering the impact of thermodynamically favourable interactions between biopolymers on the formation and stabilization of food colloids, a number of regular trends can be identified. One of the most important aspects is the effect of complexation on interfacial properties, including rates of adsorption and surface rheological behaviour. 

Dilatational surface rheology is a less discriminating experimental technique. At air-water and sunflower oil-water interfaces, it is found (Lucassen-Reynders and Benjamins, 1999) that both disordered p-casein... 

Boerboom, F.J.G., de Groot-Mostert, A.E.A., Prins, A., van Vliet, T. (1996). Bulk and surface rheological behaviour of aqueous milk protein solutions a comparison. Netherlands Milk and Dairy Journal, 50, 183-198. 

Ganzevles, R.A., Zinoviadou, K., van Vliet, T., Cohen Stuart, M.A., de Jongh, H.H.J. (2006). Modulating surface rheology by electrostatic protein-polysaccharide interactions. Langmuir, 22, 10089-10096. 

THE RELATIONSHIP OF FRAP MEASUREMENTS OF SURFACE DIFFUSION AND OTHER SURFACE RHEOLOGICAL MEASUREMENTS... 

Figure 24. A comparison of the data obtained from a range of surface rheological measurements of samples of /3-lg as a function of Tween 20 concentration. ( ), The surface diffusion coefficient of FITC-jS-lg (0.2 mg/ml) at the interfaces of a/w thin films (X), the surface shear viscosity of /3-lg (0.01 mg/ml) at the o/w interface after 5 hours adsorption ( ), the surface dilational elasticity and (o) the dilational loss modulus of /3-lg (0.2 mg/ml).

It was of interest to compare the results obtained with the FRAP technique with those obtained with classical surface rheological techniques. Our detailed knowledge of properties of solutions of /3-lg containing Tween 20 made this an ideal system on which to compare the methods. Firstly, surface shear viscosity measurements were performed on the Tween 20//3-lg system [47] using a Couette-type torsion-wire surface rheometer as described previously [3,48]. All the experiments were carried out at a macroscopic n-tetradecane-water interface at a fixed protein concentration of O.Olmg/ml. In the absence of Tween 20, the surface shear... 

The surface rheological properties of the /3-lg/Tween 20 system at the macroscopic a/w interface were examined by a third method, namely surface dilation [40]. Sample data obtained are presented in Figure 24. The surface dilational modulus, (E) of a liquid is the ratio between the small change in surface tension (Ay) and the small change in surface area (AlnA). The surface dilational modulus is a complex quantity. The real part of the modulus is the storage modulus, e (often referred to as the surface dilational elasticity, Ed). The imaginary part is the loss modulus, e , which is related to the product of the surface dilational viscosity and the radial frequency ( jdu). 

The study of surface rheology is useful in connection with the stability of emulsions and foams (Chapter 10) and the effectiveness of lubricants, adhesives, etc. 

Dickinson, E., Murray, B.S., Stainsby, G. 1988. Coalescence stability of emulsion-sized droplets at a planar oil-water interface and the relationship to protein film surface rheology. J. Chem. Soc., Faraday Trans. I, 84, 871-883. 

Golding, M., and Sein, A. (2004). Surface rheology of aqueous casein-monoglyceride dispersions. Food Hydrocolloids. 18,451-461. 

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