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PRINCIPLE OF OPERATION

As the pipes are lowered into a tub of water the water displaces air out through the tubes at the top which inflates the

Rig1

Test apparatus

bubbles. The volume of the bubble is easily controlled/measured by how far the tubes are lowered into the water. An indicator (at the top left of the pic) shows the volume in litres, but since I took this picture I've changed the scale to read the height in cm.

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THE 'LOOP'

To get good reproducibility the loops should be as near as possible to identical - not just dimensions but also material

Loop

Loop - top of soda bottle

and surface roughness which will both affect the amount of juice held by the loop. Also every bubble should be blown from a clean loop. I tried all sorts of tubing, but cutting identical ends proved problematical. I ended up using the tops from soda bottles which are standardised. The tops are a push fit onto the ends of the tubes (see the pic) so are easily replaced with a clean one after each bubble. For people in USA: 1/2 inch schedule 40 pipe is a snug fit in a soda bottle top. (If you use soft drink or water bottle tops then be careful because there is a variety of sizes, and even ones that appear to be the same may have different threads or different roughnesses).

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DIPPING THE LOOP

It's important that each bubble starts with the same amount of juice. The best method I've come up with so far is to

Loadloop

Standardising the dip

add a standard volume of mix to a standard size container and then dip the loop all the way into the container. The container must be wettable so that the mix spreads out evenly. The ones I'm using are 'solo' brand 0.75 oz souffle pots. A pack of 250 only costs a few dollars so you can do a lot of bubbles and wash the pots later.

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MEASURING BUBBLE LONGEVITY

The bubbles are inflated to a standard size and the time until they burst is recorded. I record every test on video and then take the times off the video to the nearest second, starting from the time at which the bubbles start to inflate. I cut

Longev00:37

Longev

Measuring bubble longevity

the video so it starts at time 'zero'. One bubble is always a standard control mix so I can compare the others to the control. For this test I used 1 ml of juice in the pots and inflated the bubbles to 2 litres. Temperature was 29C (84F) and relative humidity 55%. Bubbles 1 - 6 were Joy pro at 8% by volume (a ratio of 1:11.5) and a pH of approximately 8.5 with guar gum at concentrations from 0 (bubble 1) to 2.5 g/l (bubble 6). Bubble 7 was the control (12% Joy, 2 g/l guar, pH unadjusted).

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MEASURING MAXIMUM BUBBLE SIZE

This test measures how large each bubble can be inflated before it bursts. I cut the video so it starts at volume zero

Size00:23

Size

as the bubbles are just starting to inflate. The marker is at 4cm on the scale. The first bubble to burst is number 7 (the control) at about 41cm, which is between 3 and 4 litres. For this test I used 0.5 ml of juice in the pots. Temperature was 28C (82F) and relative humidity 39%. Bubbles 1 - 6 were Joy pro at 8% by volume (a ratio of 1:11.5) and a pH of approximately 7.5 with guar gum at concentrations from 0 (bubble 1) to 2.5 g/l (bubble 6). Bubble 7 was the control (12% Joy, 2 g/l guar, pH unadjusted). It's worth noting that the maximum bubble size is not independent of longevity since the water is evaporating off as the bubble is being blown.

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