Writing in water using an ion-exchange bead as a pen

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 ...

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. An ion-exchange bead serving as a pen The team of researchers decided to adopt a completely new strategy to overcome this inherent problem: "We have put the ink directly in the water and use a microbead made of ion-exchange material with a diameter of 20 to 50 microns as a writing instrument," explained Professor Thomas Palberg of JGU. This bead is so small that it generates no vortices at all. The clever bit is that the bead exchanges residual cations in the water for protons, thus altering the local pH value of the water.

A selection of images drawn in water (linear scales: 250 µm) . Credit: Thomas Palberg, Benno Liebchen Read more

This diagram shows how 'writing in water' works. The 'pen' attracts ink while superfluous ink remains behind to mark the trail it has followed. Credit: Thomas Palberg Read more

Writing of straight lines. a) Sketch of the experimental situation. The sample is placed on a stage allowing for tilting by ϑ and rotation around the optical axis (OA) by φ . The IEX (large sphere) rolls under gravity (large red arrow) through settled tracer particles (small spheres), rolling at much smaller speed (short red arrow). Tracers are swept towards the IEX by the co-moving solvent flow field (green arrows) and are assembled in the back of the IEX into a line of positive density contrast, which is left behind and disperses slowly by diffusion. b) Dark field image of C-IEX45 rolling on an inclined glass substrate ( ϑ = 3.1°, substrate ζ-potential ζ S = −105 mV, v IEX = 7.7 µm s −1 ) in a suspension of Si832 at c = 0.1 wt.% (tracer ζ-potential ζ T = −68 mV). Scale bar: 200 µm. c) The same but in bright field b/w and at c = 0.02 wt.%. The red arrow denotes the location of the line focus. d) Height averaged pH field in lab coordinates as determined by 3-channel micro-photometry for the experiment shown in panel (b). The pH-values are color coded as indicated in the key. Arrows denote the local gradient direction. e) Ground pH-field as derived from panel (d). f) Map of tracer approach trajectories in IEX-relative coordinates for C-IEX45 rolling at v IEX = 1.6 µm s −1 . The red arrow denotes the rolling direction of the IEX. g) Tracer velocity field in lab coordinates as determined from Particle Image Velocimetry for the situation in panel (c). Local velocities are color coded according to the key. Credit: Small (2023). DOI: 10.1002/smll.202303741 Read more