The history of human writing and drawing dates back at least 30,000 years, and includes traditional techniques such as sculpture, engraving, and printing/inking, as well as more modern methods such as electronic lithography. Now a team of German physicists has discovered a unique way to write in water and other liquid substrates, according to a recent paper published in the journal Small.
According to the authors, most classical writing styles involve the same basic approach, where the line is carved or ink is deposited. On a solid substrate, strong molecular forces help written forms maintain their shape, but this is not the case for surfaces immersed in liquids. Previous research has used scanning probe lithography to “write” on self-assembled monolayers immersed in liquids, or to bring about micron-scale structures using two-photon polymerization. “There are now commercial diving boards available for underwater writing on a substrate,” they wrote.
However, all of these methods are still substrate dependent. The German team wanted to create a way to literally “write fluidly.” Such a method would need to be robust enough to cope with the rapid dispersion of the drawn lines, and would need a very small ‘pen’ that would not create too much turbulence as it moved through the liquid medium. (The smaller the object moving through the fluid, the fewer vortices or eddies it will create.)
This kind of turbulence is not a problem for something like large-scale sky writing, for example, because the “pen” is much smaller than the letters written in the middle (air). However, the authors write: “To write fully reconfigurable lines in liquid at the microscopic level, a radically different approach from underwater ink deposition or line carving and a new type of micropen is required.”
The solution: Put the ink directly into water, according to co-author Thomas Pahlberg of Johannes Gutenberg University Mainz, and use a small bead made of an ion-exchange material as a pen, about 20 to 50 microns in diameter. The bead is so small relative to the “ink” reservoir that it does not generate vortices at all. The pill “writes” by changing the local pH value of the water, attracting ink molecules to those areas. It is possible to “write” a letter in water by tilting the water bath so that the bead moves in a path that follows whatever letter or letter one is trying to draw. The ink particles then accumulate along this path and voila! I wrote a message in the water.
“In our first attempts, we moved the water bath by hand but have since built a programmable rocker,” Pahlberg said. “In a water bath no larger than a single euro coin, we were able to produce a simple house-like pattern the size of an “I” in 18-point font, which we then viewed under a microscope.”
Pahlberg stresses that this work is primarily proof of principle and that their research is still quite preliminary. But the team believes their method should be able to reproduce any type of writing that uses continuous lines. It may also be possible to have breaks between separate letters by turning the ion exchange process on and off as needed, or erasing and/or correcting what has been ‘written’. Using UV-sensitive “sticky” inks may help hold lines and letters in place longer.
“In addition to beads made of ion-exchange resins, ‘pens’ consisting of molecules that can be heated by a laser or even small, individually steerable swimmers could be used,” said co-author Beno Lipschen from the University of Darmstadt. “This could also allow large-scale parallel writing of structures in water. Hence, the mechanism could also be used to generate very complex density patterns in liquids.”
Small, 2023. DOI: 10.1002/smll.202303741 (about DOIs).