Wet fiber: k climbs, ρcₚ climbs — which way does α go?

Sorbed water raises conductivity (it replaces air in the voids) and raises volumetric heat capacity at the same time. Thermal diffusivity is their ratio, so its direction is not universal — it depends on which responds faster in that species and structure. Set the response ratio for your material and watch α cross from rising to falling; both regimes are physically reachable, which is exactly why a bio-fiber number without a stated humidity is unusable.

Schematic teaching curves: water uptake follows an illustrative power law and both properties respond linearly to uptake, with the user setting their relative rates. Real fibers have species-specific isotherms, hysteresis and swelling that also changes the cross-section entering the model.