Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 372

behave as if they are expanding in contact with the surface of a very large
expanding bubble. The bubbles at the interface are observed from above via a
microscope and digital camera, and the images are recorded and stored digitally
for subsequent analysis.
Two alternative methods were used to introduce bubbles into the cell.
When the aqueous phase was of low viscosity, the bubbles were injected into
the cell beneath the barrier with a special syringe. 2 In other experiments
where the aqueous phase was viscosified with sugar, it was not possible to
create and inject bubbles in this way. Therefore, in these systems, bubbles
were pre-formed in the test solution by agitation in a mixer, 1,6 and then a
small volume of the aerated viscous solution was pipetted just beneath the
planar air water interface contained by the barrier. In each method, the
individual bubbles rose up to the interface, though this took 1-2 min longer
in the latter case. The typical size range of the injected bubbles was 10-200
mm. Compared with the bubbles formed by injection, the high viscosity
aerated systems contained smaller bubbles, but the size distribution
was wider. Around 4-6 min after introducing bubbles beneath the planar
interface, the cell was sealed, and then the barrier was expanded while the
pressure was simultaneously lowered and the behaviour of the bubbles was
recorded.
25.2.4 Foam Stability Test
The second technique used to test bubble stability, called the 'foam stability
test', was designed to mimic the behaviour of a real bulk foam undergoing
similar rates and extents of expansion as in the single bubble layer apparatus.
To facilitate clear observation and quantification of individual bubble stability
in the foam, attempts were made to create a monodisperse foam before
expansion via a special jet employing the technique of hydrodynamic focusing. 7
Figure 2 schematically illustrates the test equipment. The test solution (typically
50 mL) was contained within a rectangular stainless steel cell (capacity 250 mL)
possessing two large glass windows in the sides.
The bubble-generating jet consisted of two concentric stainless steel capillary
tubes. The external diameters of the outer and inner tubes were 1.82 and 0.50
mm, while their internal diameters were 1.30 and 0.20 mm, respectively. The
'air exit end' of the inner tube was positioned so that its end was inside the outer
tube but 10-15 mm from the end of the outer tube. The test solution was
circulated from a port in the cell wall and back into the bottom of the cell via a
peristaltic pump. Air was circulated through the inner tube, via another
peristaltic pump and tubing, from a port communicating with the top of the
air space above the test solution and the other 'air entry end' of the inner tube.
To achieve this connection more easily, the 'air entry end' pierces the wall of the
outer tubing at a point some 2 cm away from the 'air exit end', so that the
connection can be made external to the outer tube. Where the inner tube pierces
the outer tube the hole is silver soldered to achieve a perfect seal.