Liquid poured on a fabric sheet makes the world INVALID; cloth takes up no water

What is wrong

Two separate limits.

1. A stream landing on a fabric sheet breaks the world. This happens with self-contact on, which is the default. Within a few hundredths of a second after the stream lands, health flags ENERGY_JUMP, and often SPEED_FAIL too. The world goes INVALID and is frozen (sim.check()["ok"] is False). The same scene with add_cloth(..., self_contact=False) stays OK, and so does a film sheet (film_ldpe, route cloth_film; paper sheets not measured). In those runs the water rests on the sheet and none passes it.
2. Cloth takes no water. There is no absorption, no wetting and no seepage. A sheet gains no weight from the water on it, and its friction and drape do not change when wet. Real untreated cotton wets at once, soaks up about 1-3 times its own weight and lets water through once saturated. In the simulation every fabric acts as a watertight sheet.

Number

The scene: a 0.2 m cotton_woven sheet (2 mm, about 12 g, 20 mm cells) resting on the rim of a static open box. A 20 ml add_pour at 0.5 m/s starts 5 cm above it; the stream lands at about 0.06 s. CPU, Scene(dt=0.005), one run per row (the CPU repeats bit for bit).

variant	first INVALID
as above (self-contact on, the default)	0.080 s, ENERGY_JUMP + SPEED_FAIL (21 m/s)
self_contact=False	none to 0.6 s; all 244 particles rest on the sheet
material="film_ldpe", 20 mm or 5 mm cells	none to 0.6 s
sheet mass 4 g, 120 g or 360 g (mass=)	0.075-0.115 s
dt 2.5 ms or 1 ms	0.077 s / 0.087 s
cells 10 mm or 40 mm; cloth_accuracy="super"; liquid_accuracy="super"	0.075-0.115 s
stream 0.2 m/s	0.105 s
sheet on a full block top instead of a rim	0.39 s, SPEED_FAIL

• A towel draped over a static block and a 2 m/s stream also went INVALID soon after the stream landed.
• A world that fails does not recover: run to 3 s, the default row stays frozen from 0.080 s.
• In one development run with the freeze turned off, 1.2 % of the poured particles ended below the sheet, 9 cm from its border. They were carried through within one 2.5 ms step while the sheet under the stream moved at 3-4 m/s.
• Only this rim scene was measured with self_contact=False, for 0.6 s. A drape, a fold, a long pour and a robot holding the cloth were not.

Why

The failure needs both the sheet's self-contact and liquid on the sheet. The same sheet with no liquid, or with the pour landing beside the box, stays OK with self-contact on. Changing mass, time step, mesh, accuracy settings or the number of coupling passes does not remove it; turning self-contact off does. The exact mechanism is not yet found.

Separately, the liquid sees the cloth as a zero-thickness two-sided wall. Nothing sweeps a fast particle's path against the sheet, and a particle that has crossed stays on the far side. That is how a sheet moving fast lets particles through.

There is no absorption model, so limit 2 is simply missing.

Workaround

• Pour onto a sheet that never folds with add_cloth(..., self_contact=False). It is also about 1.4x cheaper per step on a GPU (docs/guide/deformables.md).
• Use a film sheet (material="film_ldpe") where a watertight sheet is what you want.
• Check sim.check()["ok"] after any pour onto cloth: a frozen world looks calm.
• To show a wet towel's weight, add the water's mass to the cloth (add_cloth(mass=...)) and leave the liquid out: the added mass stands in for the water.

Planned fix

Not in this release. In this order: 1. Find and fix the self-contact failure under a stream, with laws: a pouch of fabric holds a poured volume for 10 s, and a drop landing on one cell does not pass. The stream's momentum on the sheet must match rho Q v. 2. No pass-through: the cloth's thickness and a swept test for the liquid against the sheet. 3. An opt-in absorption model: particles taken into a per-vertex water mass up to the fabric's measured capacity, with that mass added to the cloth.

Wet friction and wet bending need measurements of a named fabric wet and dry before they get a model. No date yet.