How to use it

A soft solid you can push on.

Grab it anywhere and pull. Use two fingers to stretch it from two points at once. Let go and watch the wobble ring out. Hover near the right edge, or tap the tab, for firmness and damping.

What it is modelling

A real gel is a sparse network of protein chains holding a great deal of water. Mechanically that network does three things, and this specimen does the same three.

It keeps its volume

A gel is mostly water, and water will not compress. Squeezing a jelly would mean pushing water out through pores far too fine to let it pass, so the volume stays put no matter how the shape changes. The simulation enforces this directly: the enclosed volume is measured every step and any deviation becomes a force pushing the surface back out.

This is the property that is easiest to get wrong and most obvious when it is missing. A model that lets volume drift slowly deflates into a bag, however convincing the surface looks.

It resists shear, but only just

Sliding one layer past another only requires stretching the loose chains between junction points, which is easy. A soft gel is something like a million times easier to shear than to compress. Here, the resistance comes from springs along every edge of the mesh plus a constraint that pulls the whole body back toward its rest shape.

It loses energy on every cycle

Jelly is viscoelastic: partly a spring, partly a damper. Without the damper it would oscillate forever, which no real gel does. Each step here discards a small fraction of the velocity, which is the same thing the loss modulus describes in a rheometer.

What the two controls change

Control	Physical analogue	What you will see
Firmness	Shear modulus, the axis the Bloom test probes	Firmer means a faster shear wave and a quicker, tighter wobble. Softer means it squashes further under its own weight and stretches much more before springing back.
Internal damping	Loss modulus, the energy lost per cycle	Low values ring on for several seconds like a well-set dessert jelly. High values settle almost at once, closer to a paste than a gel.

Notice that they change different things. Firmness changes the frequency of the wobble. Damping changes how long it lasts. In a real gel those are also separate properties, which is why rheologists report both.

The size effect

Zoom out and the specimen looks smaller but wobbles at the same rate, because zooming only moves the camera. In reality the frequency scales with the inverse of size: double the jelly and the wobble roughly halves in frequency. A banquet jelly heaves slowly, a teaspoon of the same mixture quivers fast, and nothing about the material has changed. That scaling is worked through here.

What it does not model

No surface tension. At this scale it would change nothing visible: for a centimetre-sized gel the length at which surface tension starts competing with elasticity comes out in the micrometres. The blobs article explains why.

No fracture either. Pull a real jelly hard enough and it tears, permanently. This one stretches and always recovers, which is convenient but not honest. Real gels have a strain at which they fail, and that limit is a large part of what separates chewy from brittle.

No melting. A gelatin gel melts in the low thirties Celsius, below body temperature, which is why it dissolves on the tongue. Temperature does not exist in this model at all.

Read on

Start with what jelly actually is for where the network comes from, or why jelly wobbles for the arithmetic behind the frequency.