Dilational elastic modulus, interfacial

This is the dilational elastic modulus or the interfacial tension gradient which is in phase with the area change. 

This is a measurement of the resistance that an interface shows to the creation of new regions with higher surface tensions (higher surface area). In a more general case, the response of the interface would have also a viscous response due to relaxation phenomena at the interface. In the case of a sinusoidal perturbation to the interfacial area of frequency v (v = 2nco) and small amplitude, the response of the interface is a complex magnitude the dilatational elastic modulus. 

The dilatational elastic modulus contains important structural information about adsorbed interfacial layers (Benjamins et al., 2006 Maldonado-Valderrama et al., 2005a). Moreover, the elastic... 

For an air/liquid system a measure of the surface-tension variation resulting from the imposed periodic area variation in the Langmuir trough is performed. If both dilational viscous = (f) and dilational elastic j = e (f) data are needed, and if a Langmuir-type trough is used, then one barrier can be oscillated and another barrier can be used to adjust the extent of the interfacial area. The calculation of the complex modulus, , requires complete scans at different frequencies. 

It was discussed quite extensively, that interfacial dynamics and rheology are key properties of liquid disperse systems, such as foams and emulsions. The stability of such systems depends for example on the dilational elasticity and viscosity, however, surely not on the elasticity modulus (Borwankar et al. 1992). Here, the interfacial rheology with its frequency dependence comes into play, and data at respective frequencies will possibly correlate with the stability behaviour. 

A torsion pendulum apparatus was used to get information about the shear rheological properties of the surfactants at the interface. This sensitive instrument was not capable of detecting significant elastic or viscous properties except for Lecithine. The analysis provided data that indicated a low viscous resistance for Lecithine with an interfacial shear loss modulus of about 0.03 mN/m. Hence, we can conclude, that no network-like superstructures were formed, and that the large dilatational elastic response was mainly caused by Gibbs- and Marangoni-effects. 

The tangential bulk-phase stress component evaluated at the interface combines an elastic (interfacial tension gradient) effect, e, and an apparent viscous effect, rxj + tIj) -I- e 7o). One of the most convenient methods of measuring capillary waves is to use light scattering (29), which can yield information on both the tension and dilational modulus of the interface. 

---