  Hypothesis Saponins are natural surfactants which can provide highly
viscoelastic interfaces. This property can be used to quantify precisely the
effect of interfacial dilatational elasticity on the various rheological
properties of bulk emulsions. Experiments We measured the interfacial
dilatational elasticity of adsorption layers from four saponins (Quillaja,
Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water
interfaces. In parallel, the rheological properties under steady and
oscillatory shear deformations were measured for bulk emulsions, stabilized by
the same saponins (oil volume fraction between 75 and 85 %). Findings Quillaja
saponin and Berry saponin formed solid adsorption layers (shells) on the
SFO-water interface. As a consequence, the respective emulsions contained
non-spherical drops. For the other systems, the interfacial elasticities varied
between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very
significant impact on the emulsion shear elasticity, moderate effect on the
dynamic yield stress, and no effect on the viscous stress of the respective
steadily sheared emulsions. The last conclusion is not trivial, because the
dilatational surface elasticity is known to have strong impact on the viscous
stress of steadily sheared foams. Mechanistic explanations of all observed
effects are described.

Soft Condensed Matter;Materials Science