Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 373

A

C D

C

B

E

F

G

H

J

I

Figure 2 Schematic diagram of the foam stability apparatus. Key: A ¼ external piston
and cylinder for controlling pressure in cell; B ¼ air space; C ¼ glass walls;
D ¼ illumination element; E ¼ digital video camera; F ¼ foam; G ¼ foam
bubbles issuing from inner capillary tube; H ¼ outer capillary tube;
I ¼ peristaltic pump for aqueous phase; and J ¼ peristaltic pump for air phase.

The whole point of this arrangement (Figure 2) is to enable simultaneous
(but independent) concentric flow of the test solution around the exit end of the
inner tube, as air is also made to flow out of the inner tube at a different,
independently controlled rate. This is the basis of hydrodynamic focusing: that
is, under the correct relative flow-rates, the air issuing from the inner tube is
made to break up into small monodisperse bubbles, i.e., much smaller than in
the absence of the outer liquid flow stream. More complex geometries and flow
regimes have been developed and analysed to allow the rapid production of a
wide range of perfectly monodisperse bubbles. 7 Nevertheless, the set-up we
have employed is robust and reproducible, in that variations of 20% in the
flow-rate of the air or aqueous phase, or similar changes to the diameters of the
capillary tubes used, have little effect on the successful operation of the jet or on
the size of the bubbles formed.
The air space in the cell is connected to an external cylinder of larger
capacity. This can be pressurized and expanded by a piston driven by a stepper
motor under computer control of the rate and extent of the expansion. In the
experiments reported here, bubbles were formed at a pressure of 3 bar and the
pressure was reduced to 1 bar in 12 s, as indicated by the pressure gauge on the
instrument. In this way, the bubbles in the foam could be subjected to similar
degrees of expansion/compression as in the single bubble layer experiments.