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Ion-exchange bead writes under water – Physics World

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A selection of images drawn in water, including shapes and letters
Water writing: As the bead moves, it trades residual cations in the water for protons, and thus traces out an invisible track of lower pH in the liquid. Linear scales: 250 µm. (Courtesy: ill./©: Thomas Palberg, Benno Liebchen)

Writing requires a substrate, like clay or paper, to fix the written lines and letters in place. Doing the same thing in a liquid such as water is not possible because the movement of the pen creates turbulence that quickly eradicates ink trails. In principle, you could remove this turbulence by using a very tiny pen, since smaller moving objects create fewer vortices, but even a tiny pen would require a substantial reservoir of ink, cancelling out any size advantage. Water-writing, it seems, is doomed to fail.

Or is it? Researchers led by Thomas Palberg of Germany’s Johannes University Mainz (JGU) have now developed a completely new water-writing technique that involves placing the “ink” directly in the water and using a bead 20-50 microns in diameter as the “pen”. This bead is too small to generate any vortices, Palberg explains, and it is made of an ion-exchange resin that alters the local pH value of the water, thereby attracting sedimented colloidal particles – the ink – to its tracks. The new technique could be used for drawing and patterning fluids right down to the microscale.

No swirls

In their approach, which is detailed in Small, the researchers rolled the bead across the base of a water bath. As the bead moves, it trades residual cations in the water for protons, and thus traces out an invisible track of lower pH in the liquid. This track attracts the (finely dispersed) ink particles thanks to a phenomenon known as diffusion-osmotic flow, or phoresis. The particles thus build up in the path marked out by the bead. The result: a fine line measuring just a few tens of microns wide marking out the area of the lowest pH value.

Though the lines thus produced are not permanent, Palberg says they are durable. “Since no swirls are generated, ink-particle dispersion is purely diffusive and thus very slow,” he explains.

To create spaces between lines, the team simply switched the ion-exchange process on and off using laser light. Creating curved shapes such as letters is somewhat trickier, as the water bath must be tilted to make the bead moves under the effect of gravity. “During our first attempts, we moved the water bath by hand, but we have since constructed a programmable stage to do this,” Palberg says.

“No other such technique to produce freely suspended and reconfigurable lines exists,” he adds. “All known methods today rely on solid substrates to fix the ink deposited from a reservoir.”

According to the researchers’ mathematical simulations, the approach is generic and could thus be employed in a variety of forms. “In addition to beads made of ion-exchange resins, ‘pens’ consisting of particles that can be heated by lasers could be employed, or even individually steerable microswimmers,” says team member Benno Liebchen, a soft-matter physicist at TU Darmstadt, Germany. “This could even allow extensive parallel writing of structures in water. Hence, the mechanism could also be used to generate highly complex density patterns in fluids.”

The team says it is now busy refining its technique and exploring ways to create patterns over larger-scale, centimetre-sized, areas.

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