Cornflour Goop

I'm sure most homeschoolers have tried this out, the old favourite- cornflour goop! We've done it a few times now (including today), and although both my kids start off tentatively, it only takes minutes before they're dripping their hands into the ooey gooey goop with reckless abandon.

The cool thing about this is, that if you touch it firmly, it feels firm, but if you touch it slowly/gently, it seems more fluid, and will run through your fingers! Just in case some readers haven't tried it, I thought I would share the ''how to'' along with a few pics, once I figure out how to attach pics to my blog posts.

All you need is 1 packet of cornflour, and 1 cup of water, plus a few drops of food colouring. You can double or triple this amount, depending on the number of kids participating. Just mix all ingredients in a bit bowl or tray- I'd recommend you do it outdoors, with good access to a hose or tap! Thankfully it rinses off really easily.

Here is the physics on the goop, taken from physicsguide.blogspot.com

''On a microscopic level, the goop consists of small starch particles,
packed close together. Seperating the particles is a thin layer of water that
acts like grease- allowing the particles to slide across each other and
move around, as long as they move slowly. So when you push your fingers slowly
into the goop, the starch slides out of the way, allowing your fingers to slide
in easily. In this situation, the fluid applies viscous drag to the grains,
gently slowing their motion. However, if you try to smash your fist in quickly,
the starch tries to move faster than the water can accomodate and grains come
into contact. Now, the much stronger force of static friction acts between the
grains- as long as they are being pushed together, there is force preventing
them from sliding across each other, and the harder they are pushed together,
the stronger the friction force is!

Almost instantly, long columns of starch grains are pushed together- a
chain reaction of jammed particles that are held together by the stress you're
applying and the frictional forces that stop them from slipping sideways under
your hand like they did when you moved slowly. This ''jamming'' leads to ''force
chains'' through the goop. While the stress is applied, these force chains can
last essentially forever because of the static friction. Releasing the stress
allows the structures to break down, returning to it's fluid-like state.