Writing is an age-old cultural technique. Thousands of years ago, humans were already carving signs and symbols into stone slabs. Scripts have become far more sophisticated since then but one aspect remains the same: Whether the writer is using cuneiform or a modern alphabet, a solid substrate, such as clay or paper, is required to fix the written structures in place.
However, researchers at Johannes Gutenberg University Mainz (JGU), TU Darmstadt, and Wuhan University asked themselves how to write in a bulk fluid like water without fixing substrates. The concept would not be unlike the way aircraft leave three-dimensional vapor trails behind them when they cross the sky—compared to two-dimensional writing with a pen on dry paper.
When you dip the nib of a fountain pen in water and try to write something with it in the water, you will, of course, have little success. The movement of the relatively large nib through the water creates turbulence that will eventually eradicate any ink traces left behind. But as the Reynolds number, i.e., the factor used to calculate fluid flow, indicates: The smaller the moving object, the lower the number of vortices it will create.
However, to take advantage of this, a truly minute pen would be needed and this would require a massive reservoir of ink that would cancel out the effect of the tiny pen.