Paper is a renewable, low-cost, and degradable material with an intrinsic hierarchical porous structure. Due to its porous structure, paper shows an intrinsic water uptake which enables multiple application opportunities in the field of point of care diagnostics, sensor technology, or lab-on-a-chip implementations. These applications require a targeted control of the fluid transport through the paper devices. To date, the wettability or the fluid transport trough papers is often achieved by fluorination, wax coatings or shaping of the paper device itself which either involves rather large technical effort or the use of environmentally unfriendly chemicals. Furthermore, the imbibition of fluid in paper is not fully understood, especially at the fiber level. Here, we present the control of the wettability along all three spatial dimensions of the paper sheet via a silica-based coating. We are able to gradually adjust the wettability of paper sheets from hydrophilic to highly hydrophobic depending on the coating amount and to locally control wettability gradients along all three dimensions of the paper sheet. The fluid imbibition as well as the fluid distribution in the fiber fleece and the influence of the silica-coating on the fluid behavior is systematically investigated by using confocal laser scanning microscopy (CLSM). Using CLSM we identified three different imbibition regimes depending on the coating amount and the coating composition which differs in their fluid distribution and in the occurrence of fiber swelling.

Investigation and control of fluid imbibition in silica-coated cotton linter paper sheets

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