ime. Drainage volume was measured in stability cups (a beaker
attached to a 10 mL graduated cylinder). Fresh foam (200 mL)
was placed in the cup over a bed of fiberglass. The volume of
drained liquid was measured every 5 min or every 30 min, depending
on sample stability. The kinetics was fitted to a linear model; in
this case, the slope corresponds to the drainage rate. The time (ti)
to begin drainage was registered, where a high ti value indicated
high foam stability and a low ti value indicated poor foam stability.
Foam stability was also determined optically by measuring the
light transmission through the foam using a Turbiscan MA2000
(Formulaction, France) (Mengual et al., 1999). The cylindrical glass
tube was filled with fresh foam as described recently. Foam destabilization
was analyzed using backscattering profiles as a response
to applied pulses (near infrared light source, 850 nm). Foam scans
were made every 2 or 30 min. Less stable foams were scanned for
1 h, while more stable foams were scanned for 21 h. Typical
backscattering profiles were reported in a previous study; drainage
of fluid was identified by a negative peak at the bottom of the glass
tube, that showed increasing width or thickness with time; the
increase in bubble diameter (Ostwald ripening) is observed as a
progressive decrease of backscattered light in the center of the
glass tube (Martínez-Padilla et al., 2014).