Dilational surface rheology

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... 

Drop and bubble methods have been developed significantly during the last years. These techniques provide access to dynamic properties of liquid interfaces. The drop and bubble shape technique as well as the fast oscillating drops and bubbles are described essentially as tools for dilational surface rheology. 

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).

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 d)mamics of adsorption of emulsifiers at fluid interfaces have been determined by tensiometry and surface rheology (Figure 14.3) that is, from the time dependence of surface pressure and surface dilatational modulus (E). We found that tt and E increase with time (9), which should be associated with emulsifier adsorption (Patino and Nino, 1999 Nino et al., 2003 Carrera et al., 2005). 

First of all, surface rheology is completely described by four rheological parameters elasticity and viscosity of compression/dilatation and of shear. In every case surface flow is coupled with the hydrodynamics of the adherent liquid bulk phase. From interfacial thermodynamics we know that the integration over the deviation of the tangential stress tensor from the bulk pressure represents the interfacial tension y (after Bakker 1928). 

In respect of the classical mechanics, E is an "ideal" coefficient, like the elasticity modulus in Hooke s model. Most of the practical compressions/dilatation experiments carried out with adsorption layers are comparable to the screening of elastic properties in material science. In analogy to the coefficients of the 3D-elasticity theory, we have to consider complex coefficients in surface rheology. The surface elasticity coefficient written as a complex modulus therefore has the form... 

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... 

For the description of surface rheological properties as we have said we need to consider the surface stress caused by dilatation/compression as well as by shear. As in the case of area changes two coefficients of surface shear flow exist. Using the same symbols for the shear and stress tensors, as given in Fig. 3.5., we obtain... 

Unresolved problems include overall theory and related experiments on the contribution of chemical reactions at interfaces to the relaxation spectrum for a better theoretical description of surface rheological parameters. As discussed above each chemical reaction is characterised by at least one relaxation time. On the other hand surface rheological properties, for example dilational elasticity, are connected to relaxations taking place at interfaces. 

Additional information on interfacial layers can be gained from rheological and ellipsometry experiments. There is quite a number of different experimental setups used to determine surface rheological parameters (27). New possibilities to determine surface dilational parameters arise from oscillating-drop experiments. Using axisymmetric drop shape analysis (ADSA) the change in interfacial ten-... 

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. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

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]... 

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