Writing on Standing Water

A reader writes "Engadget is reporting on Japanese scientists who have found a way to 'write' characters on the surface of water using waves. This looks very cool - but the time required to change character seems very high (15-30 seconds). From the article: 'Liquid-based displays are nothing new -- in a vertical orientation, at least -- but apparently it's a lot more difficult to coax a standing pool of water into forming recognizable shapes and characters.'"

Comments

I thought they meant the opposite

[oskard]

For example, writing a word in water and having the perimeter of the pool recognize the waves, and convert it to digital text. Microscopically, that could actually have a use with a liquid enclosed touchscreen.

John Keats

[j1m 5n0w]

I believe that phrase likely originated with the poet John Keats:

He died on February 23, 1821 and was buried in the Protestant Cemetery, Rome. His last request was followed, and thus he was buried under a tomb stone reading, "Here lies one whose name was writ in water."

Re:Processing time?

[gardyloo]

Part of the trouble is that these things are NOT stationary. There's only once in a long time that all of the waves produced by those actuators end up forming the characters they want them to, AND all the rest of the surface is smooth. And that can only be done with some sophisticated feedback as to what waves are present. I haven't read their paper, but I suspect they either sense some inductances at the edge of the tank, or do some fancy laser-scanning of the surface. I can easily see incorporating the continually-changing conditions into the calculations as taking a long time. And cylindrical Bessel functions are not so easily precomputed if you need 50 of them at a particular time. I'd think the easiest way to do that is to set up 50 analog circuits with the appropriate parameters and continuously feed in the water heights along the edge of the tank.

      For applications... I can't answer this in full, since part of my research is sort of related. But for detecting things buried in the seafloor, ripples on the seafloor do some amazing things to signals. Having a reliable way to set up such ripples in the laboratory is very useful.

CAT Scan in Reverse

[Mignon]

I think this is somewhat related to CAT scans, but applied in reverse. My crude understanding of a CAT scan is that sensors are placed in a ring shape around the object to be scanned and a series of "slices" are scanned. For each "slice", this gives a set of axial views through the object (the 'A' in CAT). Bessel functions are the mathematical tool that let you convert the axial data - which is a kind of sum across a diameter - into data at each point in the circular cross section.

It seems they've reversed this process and solved for the axial data given the point-by-point data - e.g. the rasterized character.

By the way, CAT scans and Bessel functions are one of the examples of "abstract" math that later turns out to have practical application.