Paper‐based microfluidic devices are used in point of care diagnostic, sensor technology or lab‐on‐a‐chip devices. Although a number of studies has been reported, only relatively few paper‐based diagnostic tools are available on the market. A remaining challenge is the mechanistic understanding and precise design of capillary flow in paper. Here, silica coatings are applied to control paper wettability, fiber swelling, and thus fluid transport in all three dimensions of a paper sheet via a simple dip‐coating and post‐treatment process. By adjusting the three‐dimensional silica coating distribution, a three‐dimensional asymmetric wettability gradient within the paper sheet is obtained which controls the fluid distribution and imbibition. The correlation between silica coating amount and silica distribution with the resulting fluid behavior is systematically elaborated by analyzing the interaction between fiber and fluid as well as the fiber swelling by applying confocal microscopy. Three different silica‐amount dependent fluid distribution states are demonstrated. These new insights into the mechanism of fluid imbibition using simple silica coatings enable the specific design of different imbibition mechanisms and thus the adjustment of the microfluidic properties in paper‐based microfluidic devices with control over all three spatial dimensions of a paper sheet in one fabrication step
