Phys. Rev. Lett. (in the press)

Credit: © ISTOCKPHOTO.COM/ROXANA GONZALEZ

It's simple, really: the basic principle of writing with ink is the delivery of a liquid from a reservoir to an absorbent surface, and then the spreading of that liquid as the source moves across the porous substrate. To study the hydrodynamics in more detail, however, Jungchul Kim and colleagues have used a 'minimal pen' made of a capillary tube, brought close to a highly hydrophilic micropillar array, which served as their model of paper.

The basic hydrodynamics of the writing process is governed by the capillary force that makes the ink flow and by the viscous force exerted by the substrate. Considering the balance of the two forces, Kim et al. have succeeded in reproducing both the dynamics of the blot that is created before the pen starts moving, and the frontal shape and final width of the line produced by their simple pen.