Abstract

Surface viscosity, like its three-dimensional counterpart, depends on the experimental conditions used for its measurement. In compression/expansion experiments, different definitions have been used for the “surface dilational viscosity”. Steady-state viscosity coefficients were first obtained for areas expanding at a constant relative rate, by van Voorst Vader et al. (Trans. Faraday Soc., 60 (1964) 1170). This coefficient is not generally equal to the viscous part of the dilational modulus measured for small periodic (wave) deformations at the same relative rate, even if the relaxation mechanism causing the viscous behaviour is the same in both cases (see Lucassen and van den Tempel, J. Colloid Interface Sci., 41 (1972) 491). A quantitative comparison is presented of the two cases if the relaxation mechanism is diffusional interchange of surfactant between the surface and the bulk solution. The relationship with the energy dissipation is evaluated for both cases. The surface viscosity defined for the small-deformation experiment is shown to be a pure viscous loss modulus, because it is directly proportional to the amount of mechanical energy dissipated as heat. The steady-state, large-expansion experiment yields a value which is not a true viscosity in this strict sense, because it also contains an elastic contribution which measures the amount of potential energy stored in the surface.

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