Bubble Column

Introduction
Explore the wonderous shapes made by simple air bubbles in water.
Useful information
Kit List: 

- Large bubble tank with rotating spoon at the bottom (stored wrapped in white cover)
- Large syringe
- Source of water.
- Short bit of hosepipe to act as siphon

Packing Away: 

Empty into some suitable container using a siphon, do not attempt to tip up/move it when full. Once dry (ish), wrap up in protective cover.

Explanation
Explanation: 


Preparation:

Experiment with making the different sized bubbles, so that you know that you can.

First things first, start at small bubbles...

You can ask what shape they think very small bubbles are. (Most people will say circles, which you can gently remind them that they mean spheres.)

You then create some...

Small bubbles are in fact the most difficult to produce. The method I found that worked was...

1 - Turn the spoon so that it doesn't catch your bubbles.

2 - Push and pull the syringe gently a few times until you can see some small bubbles in the tube that the syringe in connected to.

3 - Push the syringe as slowly as you can - probably in small bursts.



You'll tend to get the slightly larger bubbles as well - but they travel faster, so you should be able to point out the small ones quite easily.

If you think they (and you) are up to it, you can have a go at explaining surface tension. Something along the lines of "The water prefers to be next to more water, rather than air, so it makes it so that it moulds the air into a shape where the least water has to touch it." Dave had a pretty good explaination of surface tension I think, or if anyone else has, that'd be useful.

We now have noted that tiny bubbles are spherical, and travel quite slowly (they will have seen this as the larger ones produced at the same time reach the top first).

Now we find out what shape a little bit larger ones are... (They may already have seen them)

Quite often they don't wat to guess here, but that's ok as they are normally into it by the next stage.

So to make the next size up ones, you just put a constant pressure on the syringe and they will be produced.

Get them to tell you what they are observing. In this case it's easier to explain the shape first.

Shape - "They are slightly squashed", I like "They are smarty shaped!", normally gets a smile :-)

Why are they smartie-shaped?

For a bubble to go up in the water, what has to happen to the water just above the bubble? What about the water that has to appear below the bubble as it moves up?

It has to move from on top, to below. How does it do that?

It rushes around the side of the bubble.

- Lots of hand waving is good at this point... and thinking about it, probably a squashy foam ball or something might be useful.

Now we've got the idea of water rushing down the sides of the bubble, refer them to the hovering beachball experiment (if they've seen it), in that fast moving air effectively sucks things from beside the air stream towards the air stream. In this case it's exactly the same, the fast moving water sucks the sides of the bubble out towards the fast moving water round it's edges.

Movement - They're moving from side to side.

There's probably some really very complicated goings on here as well, but this is how I understand and explain it:

The bubble gets some sideways movement somehow (possibly by the syringe pushing it out, possibly just pretty randomly).

If the bubble moves to the left, what happens to the right of it? Water has to rush in to replace the bubble.

How does the speed of the water compare, between the left and right? The water to the right is moving faster.

And what do we know about faster moving water? It pulls things towards it. So the bubble gets pulled over the the right.

Now... what's happened on the left as the bubble has moved to the right?

etc... etc... etc...

Ok, I think that's smarty-bubbles finished now.

Next size up bubbles (Dome bubbles).

To make these, turn the spoon upsidedown, squirt less than 1/4 of the syringe in underneath the spoon. (Every now and again the tube needs pushing in a little bit further or rotating slightly - you can tell this when the spoon leaks.) Once some air is in the spoon, turn the spoon 180 degrees quickly (I think anti-clockwise - whichever direction does NOT hit the tube).

Shape - Dome shaped or like jelly fish!

The same effect as in the smarties causes the curved part of the dome.

However when you look underneath it is nice and flat with pretty ripples on it. Why? For a larger bubble to rise, the water must travel faster to get around it. This faster water drags the sides of the bubble down. The bottom is flat because nice spinning vortexes are formed, effectively the water is going past, then coming back up. It's due to turbulance - a reference to a sailing boat going slowly in water, then the wind picks up and the sailing boat goes really fast, and causes a wake.

Bigger Domes

Fill the spoon up with air until it's overflowing (You'll need more than one syringe full of air). Release quickly. This time it's a huge jelly fish, traveling pretty fast. bubbles get dragged off the edge of the bottom causing a wake (or tenticles!). This sometimes wobbles from side to side. Also, the bubble sometimes splits and rejoins. You can explain all of this as the water has to move so fast that the turbulance is so big that it starts ripping bubble off the edges, and causing the bubbles to interact visciously!

So there we have it, the wonders of air and water... Just wait and see what happens when you restrict the air and water into tubes, in... the other experiment... "Slug bubbles"...

Risk Assessment
Date risk assesment last checked: 
Fri, 30/12/2011
Risk assesment checked by: 
mnd22
Risk Assessment: 
DESCRIPTION Let different amounts of air into column and observe bubble shapes
RISKS
  1. Column heavy and unstable when full
  2. Column extremely heavy when full
  3. Slip hazard in case of spill
  4. Electric risk if water spills out or leaks onto any nearby electric cables/experiments
ACTION TO BE TAKEN TO MINIMISE RISKS
  1. Ensure column is securely anchored e.g. to fixed pillar.
  2. DO NOT ATTEMPT TO LIFT OR TIP WHEN FULL! It is too heavy for someone to lift, and attempting to do so risks cracking the column and causing all of the water to fall out.
  3. Clear up spills promptly. If indoors, ensure that the column is in a waterproof tray in case of leaks, and do not run experiment if it is leaking too badly.
  4. Keep any electronic components off the ground in the vicinity of the column.
ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT Call first aider in event on injury
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