Soft matter
Why jelly wobbles
and steel does not.
A dessert jelly is about ninety percent water, held in a net of protein so sparse you could not see it. That one fact explains the wobble, the melt in the mouth, the chew of a gummy bear, and why a large jelly shakes more slowly than a small one.
Stretch one yourselfStart here
Jelly is one of the few places where you can watch soft matter physics happen at human speed. A shear wave crosses a soft gel at roughly one to three metres per second. The same wave crosses steel at over three thousand. Identical physics, a thousand times slower, which is exactly why one of them jiggles where you can see it and the other rings at a pitch you can only hear.
The articles below work through what a gel actually is, how its firmness is measured, why it wobbles at the frequency it does, and where the line falls between a blob that holds a round shape and one that spreads into a puddle.
The science
What jelly actually is
Collagen unwound into gelatin, then re-tangled as it cools. Why it melts just below body temperature and agar does not.
How firm is a gummy bear
The Bloom test, and why gelatin, pectin, agar and carrageenan give four completely different textures.
Why jelly wobbles
Viscoelasticity, shear wave speed, and the scaling rule behind the frequency of the wobble.
Round blobs, flat puddles
The capillary length, why it sits near three millimetres for water, and what changes once the blob is elastic.
The specimen
Reading about a wobble is a poor substitute for causing one. The interactive jelly is solved in your browser a hundred and twenty times a second, and it is built to obey the three rules a real gel obeys: it keeps its volume, it resists being sheared out of shape, and it loses a little energy on every oscillation. That last one is why the wobble dies away instead of continuing forever.
Open the specimen