The problem of capillary transport in fibrous porous materials at low levels of liquid saturation has been addressed. It has been demonstrated that the process of liquid spreading in this type of porous material at low saturation can be described macroscopically by a similar super-fast, nonlinear diffusion model to that which had been previously identified in experiments and simulations in particulate porous media. The macroscopic diffusion model has been underpinned by simulations using a microscopic network model. The theoretical results have been qualitatively compared with available experimental observations within the witness card technique using persistent liquids. The long-term evolution of the wetting spots was found to be truly universal and fully in line with the mathematical model developed. The result has important repercussions for the witness card technique used in field measurements of the dissemination of various low-volatility agents in imposing severe restrictions on collection and measurement times.

中文翻译：

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低饱和度的纸质多孔材料中的毛细管传输：正常、快速还是超快？

已经解决了在低液体饱和度下纤维多孔材料中的毛细管传输问题。已经证明，液体在这种低饱和度的多孔材料中的扩散过程可以通过类似于先前在颗粒多孔介质的实验和模拟中确定的超快速非线性扩散模型进行宏观描述。宏观扩散模型以使用微观网络模型的模拟为基础。理论结果已与使用持久性液体的证人卡技术中的可用实验观察进行了定性比较。发现湿点的长期演变是真正普遍的，并且完全符合所开发的数学模型。