Most analytical techniques used to characterize interfacial dilatational rheology measure the change in interfacial tension as the surface area is increased or decreased in a controlled manner (e.g., trough and overflowing cylinder methods) (Murray and Dickinson 1996). Trough methods measure the surface or interfacial tension of a liquid using a Wilhelmy plate as the interfacial area is varied by changing the distance between two solid barriers which confine the liquid (Figure 5.21). The overflowing cylinder method can be used to measure the dynamic dilatational rheology of surfaces or interfaces. A Wilhelmy plate is used to measure the surface or interfacial tension of a liquid as it is continuously pumped into a cylinder so that it overflows at the edges. The dilatational viscosity is measured as a function of the surface age by altering the rate at which the liquid is pumped into the cylinder.
The capillary wave method described in Section 5.10.8 can also be used to measure the interfacial dilatational rheology of liquids. A laser beam reflected from the surface of a liquid is used to determine the amplitude and wavelength of the surface waves, which can then be related to the dilatational modulus or viscosity of the surface using an appropriate theory. These surface waves are believed to play an important role in the coalescence of emulsion droplets (Chapter 7), and therefore this technique may provide information that has direct practical importance for food scientists.
It should be noted that the dilatational rheology of an interface can be influenced by the adsorption kinetics of emulsifiers (Murray and Dickinson 1996). When a surface undergoes a dilatational expansion, the concentration of emulsifierperunitareadecreasesandtherefore its surface tension increases, which is energeticallyunfavorable.Thedilatationalelasticityor viscosity is a measure of this resistance of the surface to dilation ( 5.2). When emulsifier molecules are present in the surrounding liquid, they may be adsorbed to the surface during dilation and thereby reduce the surface tension. The dilatational rheology therefore depends on the rate at which emulsifier molecules are adsorbed to a surface, which is determined by their concentration, molecular structure, and the prevailing environmental conditions (Section
). The faster the molecules adsorb to the freshly formed interface, the lower is the resistance to dilation, and therefore the lower is the dilatational modulus or viscosity. When an interface undergoes dilatational compression,someoftheemulsifiermoleculesmayleave the interface to reduce the resulting strain, and therefore the desorption rate may also influence the rheological characteristics of an interface.
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