(499b) On the Surface Shear Viscosity of Soluble Surfactants

The
stability and flow behavior of multiphase materials like foams and emulsions
are closely linked to their interfacial properties, which can be greatly
affected by the presence of surfactants. 
One such important property is the surface shear viscosity, as it is
generally thought to enhance stability via reduced film drainage. The
understanding of surfactant effects on surface shear viscosity is thus crucial
to the study of such materials. However, for a common anionic surfactant used
to make foams, sodium dodecyl sulfate(SDS),
the reported values of measured surface viscosities vary by several orders of
magnitude (0.01-100μNás/m). 
In this study, we use ferromagnetic microdisks
deposited at the SDS solution - air surface, as active, microrheological
probes of the interface. Our oscillatory shear measurements reveal that the
values of surface shear viscosity are on the order of 0.01μNás/m or lower,
at which point the probe becomes Boussinesq limited.
We therefore question the interpretation of previous measurements, and thus of
the reported values of the surface viscosity of SDS solutions.  Instead, we suggest that the non-viscometric flow (including compression and dilation) gives
rise to enhanced drag forces that do not reflect shear viscosity.  Rather, we propose that the enhanced
drag coefficients in the literature are likely due to Marangoni
forces and not the surface shear viscosity.  Our results suggest that the generally-believed correlation
between the surface shear viscosity of surfactants, and the stability of foams
and emulsions made using them, is not correct, but instead reflects other physico-chemical processes.