Presets
Playback
Heat
The bulb heats the base; the room cools the top.
Wax
Expansion and density set the neutral temperature T*.
Liquid
Thicker liquid means slower blobs.
Layers
Model checks
The science
Why the wax rises
The wax is slightly denser than the liquid when cold, and expands when heated: ρw(T) = ρ0(1 − β(T − 20 °C)). Above the neutral temperature T* where ρw = ρliquid the blob is buoyant. Its vertical motion follows (ρw + ρl/2) V dv/dt = (ρl − ρw) V g − 6πμr v, with the added-mass term ρl/2, and relaxes to the Stokes terminal speed v = 2Δρ g r2/(9μ). The Reynolds number stays below one, so Stokes drag is a fair model.
Heat and the cycle
The fluid is hot at the bulb and cooler higher up, Tf(z) = Troom + (Tbulb − Troom) e−z/L. A blob warms or cools toward the local fluid temperature with time constant τ = r2/(3α): big blobs react slowly. Warm blobs rise, cool on the way up until they drop below T*, then sink back to the heater. When two blobs touch they merge (volume and heat are conserved); a hot blob larger than the pinch-off radius splits in two, so the lamp keeps a lively population.
Honest simplifications: blobs are spheres seen in a vertical cross-section (not a real free-surface fluid); no flow field or wake interaction, only drag toward the Stokes speed with a little random sway; the fluid temperature profile is fixed and not changed by the wax; an effective thermal diffusivity (10−6 m2/s) stands in for convective stirring; the time scale is sped up (14 simulated seconds per real second by default); real wax shapes (strands, necks, bridges) are not modelled.
What to try
- Run the Classic lamp and watch one blob grow at the base, lift off, cool and sink.
- Choose Barely buoyant wax: T* approaches the bulb temperature and the blobs hardly move.
- Choose Thick liquid and compare the terminal speeds on the last graph.
- Lots of small blobs shows how smaller blobs equilibrate faster and merge less.