How to Write on Water

Everyone’s used a Sharpie—from the classroom to the office, the ubiquitous permanent marker is a mainstay for producing waterproof labels and signage. But the same physics that gives this ink its permanence also makes it possible to remove it all at once, with nothing more than ordinary water. The trick, according to new research from the laboratory of one of fluid mechanics’ most prolific authors, is to take it S-L-O-W.

Dipping a glass slide into water at an extremely slow rate (1 µm/s) results in a transfer of the ink on the slide to the water’s surface.
Image Credit: S. Khodaparast, et al.

Sharpie and other permanent markers work by forming a thin, water-repellent elastic layer on the surface of whatever material they’re scribbled on—a bit like a sticker. This is what makes the marker waterproof but, paradoxically, also makes water its greatest weakness.

Water’s unique chemical properties create a range of interesting and useful effects; among the most famous of these is capillary action—the force that lets water fight gravity to creep up the walls of a thin tube, or the paper of a coffee filter. Capillary action results from water’s tendency to stick to itself as well as to other surfaces, properties known as cohesion and adhesion, respectively.
When a piece of glass with a thin film of Sharpie ink on it is dipped into water, the liquid creeps up beyond the water’s level surface, forming a meniscus. At the very upper edge of this meniscus, the water forms a very thin layer—a sharp enough edge to slide under the Sharpie’s film, peeling it off the glass with surface tension.

If the water can cling to the surface well, it will creep upwards, as shown on the left—but if the surface is hydrophobic, or water-repellent, the cohesion of the water dominates and surface tension causes it to curve downward.
Image Credit: UT Knoxville

If the water could adhere to the outer side of the ink layer, it would simply climb up the outside as the slide is lowered into the liquid. But the ink’s chemistry makes it hydrophobic, so the water has a much easier time adhering to the glass behind it—creating the peeling effect reported in this work.

If the “dipping” process happens too fast, the effect is spoiled and the ink layer ends up submerged, but at the right rate the water will flow behind it, transferring the entire layer to the water’s surface.
Image Credit: S. Khodaparast, et al.
While the physics behind this experiment is cool enough for its own sake, the potential applications of the new “capillary peeling” technique are also exciting. Showing that marks from a Sharpie can be transferred to various surfaces like that of a contact lens, the study’s authors suggest that the technique will find applications in printing and manufacturing—it might be a useful workaround for adding designs to surfaces that are ordinarily too fragile or unstable to write on.
What else could this new technique help accomplish? Post your ideas in the comments below!
Stephen Skolnick

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