Archive for September, 2009

Polyacrylamide

Saturday, September 26th, 2009

dry-sphere

Several months ago, I ordered some “growing spheres” from Educational Innovations , since a) I had an idea I wanted to try out and b) they sounded cool.  It turns out that the idea didn’t work, so we can add this particular article to the “learning experiences” category.

The spheres are made of polyacrylamide.  I freely admit that chemistry is one of my weak points, but even I can understand that this stuff is very hydrophilic, that is, it likes water.  It likes water so much that it can absorb up to 300 times its volume in water.  This means that the spheres, when hydrated, are almost all water, and are therefore a bit unusual (read: interesting) to fiddle around with.

See (or not) what I mean?

See (or not) what I mean?

In the picture above, there is a ~1″ diameter polyacrylamide sphere on the wire stand.  If you brush the dust from your screen and squint a bit, you might see the outline where the light is bent slightly by the “skin” of the sphere.  This is not photographic trickery, incidentally.  I put a half dozen of the spheres into a pot of water and came back the next day – it is an unusual sensation putting your hand into a container and encountering invisible objects in the water.  Their presence is  easier to spot by shining a light through the container and noting that focal points show up opposite the lamp as the polyacrylamide focuses the light passing through it.

Traditionally, these spheres are used in gardening to slowly release water into soil, or in science class to act as a planting medium, with seeds placed among (not inside of ) the spheres so students can watch the roots grow.  I thought it would be interesting to germinate seeds inside this material.  If the roots could penetrate the polyacrylamide, one could get the illusion of a plant hanging in midair, and with a root structure growing through a homogenous medium rather than growing around a pile of spheres.  Alas, it didn’t work that way.

I am not a botanist, nor a horticulturist, nor a floriculturist, nor even a gardener.  I have a Pothos growing across my shelves, and that’s it.  Once at the Agway, I picked flower seeds on the basis of germination speed and time of year.  I ended up with dwarf petunias and Cardinal Climbers.  Both would grow, but not well.

The first question was: how to insert the seeds into the spheres.  It turns out that the behavior of the spheres is rather like that of water balloons: heavily informed by surface tension.  If you throw a hydrated sphere against a hard surface, it shatters quite dramatically.  My first attempt at making a cut into a sphere resulted in it splitting open along the axis of the cut like a grape.  Small cuts seemed more stable as long as they didn’t describe an arc of more than 15 degrees or so.

sphere-sizeNote that the spheres on the left are smaller.  This is a result of my submerging them in a mixture of liquid plant food and water; the polyacrylamide has a lower absorbtive capacity for the plant food than for straight water.

The problem with this attempt is that the spheres, as they dried and contracted, tended to spit out the seeds.

I wanted to put the seeds deeper into the medium, so I needed to make a tunnel of sorts.  Piercing the sphere with a probe left no perceptible tunnel, and pushing a seed into the sphere burst it.  I tried removing material with drill bits, and this worked but was still not very stable.  The best way I found was to use a drinking straw.  This left a cleaner cut with fewer stress risers than the drill bit.  Try not to inhale the plug.

seed-implant

This is a Cardinal Climber seed pushed into the tunnel left by the straw.  The tunnel shortly filled with water, since the interior structure of the sphere had been damaged and began weeping water into the “wound”.  The seeds would float out, if not pushed in to a “press fit”.

This method failed, as well, though it worked better.  Before the UV light and evaporation degraded the spheres, I saw shoots on most of the seeds, but they would run out of the sphere and then down the side, not penetrating the medium but just getting water from contact with it.  Ultimately, I was left with a combination synthetic and organic mess as the sphere contracted around the seed.

Seeds that can grow in clay might work for this, but I don’t think so; the polyacrylamide structure seems to fail when anything forces its way through it.

So I’m stymied.  The minimum order was 1250 spheres.  I still have more than a few left, enough, in fact, to fill a few cubic feet.   I’m keeping them in a dark drawer to prevent degradation, but I feel that there are potential uses for these beyond as projectiles.

(As a note to the more … creative … sorts, no, you don’t want to feed these to the dog, and you don’t want to put them down the drain, either.  Stomach acid and drain cleaner might destroy them, but I wouldn’t bet Fido’s life on it.)