The dynamic air/water (A/W) surface dilational elasticity and
viscosity were measured using a pendant-drop Sinterface PAT1
tensiometer (Sinterface, Berlin, Germany). A drop of the liquid
sample with an area of 20 mm2 was formed automatically at the tip
of a syringe driven by a motor plunger within a thermostatically
controlled glass cuvette set to 20 C. The image of the drop was
captured and digitised by a CCD camera. The interfacial tension (A/
W) was calculated by analysing the profile of the drop and fitting it
to the Laplace equation. After allowing 1000 s to reach equilibrium,
sinusoidal oscillations of the interface occurred by injecting and
extracting volume into and from the drop while the response in
interfacial tension was recorded. The relative amplitude (DA/A) of
the oscillations was set to 5% in order to stay within the linear
viscoelastic region and the frequency ranged from 0.01 to 0.2 Hz
while 0.01 Hz was the frequency chosen as the one relevant to
foams (Schmitt, Bovay, & Rouvet, 2014). The dilatational parameters
were calculated through a Fourier transformation algorithm
implemented in the software package. The dilatational elasticity
and viscosity were calculated from Eqs. 4 and 5.