Thixotropic sand and fluidised bed

Public summary: 

Find out more about quicksand and how sand can behave like a fluid.

Looking at floating and sinking using a bed of sand
Useful information
Kit List: 

# Sand - about 2.5cm in a washing up bowl
# fluidised Bed
# reversible hoover - small variac
# Fluidised bed sand (finer and more uniform than normal)
# Things of a range of densities that can be floated/sunk in the dry sand, similar set to go in water/thixotropic sand.. wood, rocks
# Dustpan and brush
# Container of water

Packing Away: 

Drain out water, retain sand for re-use.

Frequency of use: 

Thixotropic sand

Just add water to sand until it is thixotropic - this is a bit more of a challenge than the cornflour, but if the surface is actually wet it normally works.

Get the kids to build a sandcastle, then shake the bucket and the sand should slowly liquefy

Because the sand is much heavier it squashes the water from between the grains just by its weight, so the only way you can get the lubrication back is to shake them apart so the water can get in between.

The ground liquifying is a big problem during earthquakes, and can often cause more damage than the shaking itself, eg in a 1964 Niigata earthquake in Japan.

Fluidised bed:

Fill bed with sand, attach hoover in blow mode (attach pipe under cover in the middle). The hoover is a bit powerful, so it is a good idea to vent some of the air out the side to avoid blowing all the sand everywhere, we should have a device for doing this, but something may have to be manufactured using gaffer tape.

Get them to feel the sand without air, then get them to feel it as you turn the air on. It will suddenly feel light, will flow easily and almost behave like a liquid - you can float and sink things in it - you should be able to float things in it that won't float in water.

You can get the child to bury things that float in it, then blow air and they will float up, and vice versa

Basically the sand particles can't move past one another normally as they are touching, but if you blow sand between them the bounce around and can joggle past one another. You can look at it like a blown up model of a liquid with all the particles joggling around so it will flow.

It is used to move things like flour around (which is why you can transport it in tankers.) Also as a way of doing reactions where you need a large surface area eg burning things well.

Fluidised beds are used for burning coal in power stations, as you get a really large surface area, and you can add Sulphur and Nitrogen Oxide scrubbers as lumps in the fluidised bed.

This also has a similarity to quicksand, if you have a (water) spring under sand, it can fluidise the sand which you can sink into (the water acts like the air in this demonstration). This is particularly dangerous in big sandy tidal flats, as when the tide comes up, you can get water trapped under an impermeable layer, and when you walk over the top you can release this, liquefying the sand you are standing on, which will then solidify again, leaving you stuck in the sand with the tide coming in!

Risk Assessment
Date risk assesment last checked: 
Fri, 31/01/2020
Risk assesment checked by: 
Beatrix Huissoon
Date risk assesment double checked: 
Sun, 02/02/2020
Risk assesment double-checked by: 
Risk Assessment: 

Blowing air through sand, making it flow like a fluid (and allowing more/less dense items to sink/float through it).


Hazard Risk Affected person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Sand Blowing sand into people's eyes/faces. Public 4 3 12 Set up carefully, using blower on low power, work out correct air flow rate for amount of sand used. Less sand needs lower flow, important to work out correct settings on set-up (without the public sticking their heads over the bed) and to make sure if anything changes (for example some of the sand is tipped out).
Call first aider in event of injury.
If sand gets into eyes, discourage rubbing (sand may scratch), sterile eye wash should be used to flush it out (by a trained first aider).
2 3 6
Sand underfoot Sand on a hard, smooth surface can be a lubricant/slip hazard. All 4 3 12 Use play-pit type sand (less dusty).
Keep an eye on the floor, and use a dust-pan and brush to clean up if necessary.
Call first aider in event of injury.
2 3 6
Spillages Spilt water is also a slip hazard All 4 3 12 Mop up any water immediately. Use wet floor sign if necessary.
Call first aider in event of injury.
2 3 6
Water and electronic components CHaOS committee adding water to electronic components may cause a hazard. All 4 4 16 Do not pour water into the electrical equipment.
See electrical parts RA.
If electrical equipment is soaked, unplug, disassemble and put on a radiator
1 4 4
Sand Wet sand from fluidised bed getting transferred to child’s hands then into eyes. Public 4 3 12 Ensure experiment is set up near sink to wash hands in afterwards, Demonstrator ensure children wash hands after demonstration.
If sand gets into eyes, discourage rubbing (sand may scratch), Call first aider. Sterile eye wash should be used to flush it out, only administer if trained and confident.
2 3 6
Container pressure Explosion due to pressure drop across container. All 3 3 9 Check for cracks before use. Do not use if cracked.
Call first aider in event of injury.
2 3 6
This experiment contains mains electrical parts, see separate risk assessment.
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