Dilational elastic modulus, interfacial tension

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. 

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. 

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. 

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