Invisible Balls

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Invisible Balls
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At first glance, this jar looks to contain nothing but unassuming water. But tip it up or give it a shake and you’ll see the water ripple, bubble and flow in a strange and mercurial manner. That’s because it contains so-called invisible balls; spheres made of a super-absorbent polymer gel which is almost exactly as transparent as ordinary water, but the balls only become revealed when set in motion. 

The secret to their invisibility is all down to the physical phenomenon of refractive index. This describes how quickly light travels through substances. Although the jar’s glass and the water inside are both transparent, colourless substances, light which passes through them from the surrounding air is slowed down by the light’s interaction with their relatively densely-packed atoms. In everyday life, we’re able to make out the shape and substance of water and glass because of the difference in refractive index between them with the adjoining air. These invisible balls have been formulated to have the same refractive index as water. Without a change of the speed of light at the interface between the balls and water, our brain is tricked into believing the jar contains nothing but plain water. When the jar is shaken, some of the invisible balls break free of the water and are suddenly surrounded by air, and a difference in refractive index is easily discernible to the keen-eyed viewer.

This refractive index matching is the same phenomenon used by jellyfish to stay camouflaged against a transparent watery background. Their bodies are made from a gel which is just 5% proteins, and 95% water. Inside jellyfish – and, indeed, these invisible balls – the long, chain-shaped polymer molecules are bonded together along their length in cross-links to form an open network structure a bit like a 3D spider’s web but on a minute scale. This open molecular arrangement has ample capacity to accommodate lots of water molecules, and its flexibility allows the material to swell to many times its dry starting volume. See the size difference for yourself – another jar contains the tiny, dry balls.

Whilst these balls are generally sold as an optical curiosity, or as 'water beads' to help florists create displays that seem to stand up in a vase unsupported, super-absorbent polymers also play an integral role in personal hygiene, including disposable nappies and sanitary napkins. Somewhat less intimate applications which make use of the supreme absorption of these materials are in spill kits, water retention agents in horticultural soil, and as artificial 'dry' snow for movies and stage productions. 

Sample ID: 931

Particularities

State
Liquid | Solid
Compound
Selections
Categories
Polymer
Curiosities
Optical
Relationships
Camouflage | Cinema | Floristry | Gel | Hygiene | Invisible | Optical | Refraction | Refractive Index | Super absorbent | Swell | Transparent

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