When perfectly pure, the interface between fluids usually displays only surface tension.
[1] The stress within a fluid interface can be affected by the adsorption of surfactants in several ways: The mechanical properties (rheology) of dispersed media such as liquid foams and emulsions is strongly affected by surface rheology.
The measurement of surface rheological properties is described by storage and loss moduli.
In the case of a linear response to a sinusoidal deformation, the loss modulus is the product of the viscosity by the frequency.
The responses of adsorbed layers to deformations are frequently non-linear, making this variation measurement relevant to rheological studies.